PART IV.

DESCRIPTION OF THE APPENDAGES OF INDIVIDUAL SPECIMENS.

Triarthrus becki Green.

In order to make easily available the evidence on which the present knowledge of the appendages of Triarthrus and _Cryptolithus_ rests, it has seemed wise to publish brief descriptions and photographic figures of some of the better specimens preserved in the Yale University Museum. These specimens are pyritic replacements, and while they do not as yet show any signs of decomposition, it should be realized that it is only a matter of time when either they will be self-destroyed through oxidation, or else embedded for safe keeping in such a fashion that they will not be readily available for study. It is therefore essential to keep a photographic record of the more important individuals.

Specimen No. 220 (pl. 3, fig. 2).

Illustrated: Amer. Geol., vol. 15, 1895, pl. 4 (drawing); Amer. Jour. Sci., vol. 13, 1902, pl. 3 (photograph).

This is one of the largest specimens showing appendages, and is developed from the ventral side. It shows some appendages on all parts of the body, but its special features are the exhibition of the shafts on the proximal ends of the antennules, the rather well preserved appendages of the cephalon and anterior part of the thorax, and the preservation of the anal opening. In the drawing in the American Geologist, the right and left sides are reversed as in a mirror, a point which should be borne in mind when comparing that figure with a photograph or description.

The shaft of the left antennule is best preserved and is short, cylindrical, somewhat enlarged and ball-shaped at the proximal end. It is 1.5 mm. long. The posterior part of the hypostoma is present, but crushed, and the metastoma is not visible, the pieces so indicated in Beecher's figure being the rim of the hypostoma. Back of the hypostoma may be seen four (not three as in Beecher's figure) pairs of gnathites, the first three pairs broad and greatly overlapping, the fourth pair more slender, but poorly preserved. The inner edges of the gnathites on the right side are distinctly nodulose, and roughened for mastication.

The outer ends of one endopodite and three exopodites project beyond the margin on the right side. The dactylopodite of the endopodite is especially well preserved. It is cylindrical, the end rounded but not enlarged or pointed, and bears three small sharp spines, all in a horizontal plane, one anterior, one central, and one posterior. The outer ends of the exopodites show about ten segments each (in 2.5 mm.) beyond the margin of the test, and from three to five setæ attached to the posterior side of each segment. These hairs are attached in a groove, well shown in this specimen. On the anterior margin of the exopodite there is a minute spine at each joint.

_Measurements:_ Length, 38 mm.; width at back of cephalon, 19 mm.

Specimen No. 210 (pl. 2, fig. 3).

Illustrated: Amer. Jour. Sci., vol. 46, 1893, p. 469, fig. 1 (head and right side); Amer. Geol., vol. 13, 1894, pl. 3, fig. 7 (same figure as the last); Amer. Jour. Sci., vol. 13, 1902, pl. 2, fig. 1 (photograph).

This individual supplied the main basis for Professor Beecher's first figure showing the appendages of the thorax, the head and appendages of the right side having been taken from it, and the appendages of the left side from No. 206. Such of the endopodites as are well preserved show from three to four segments projecting beyond the test, and the dactylopodites have one or two terminal spines. The antennules are unusually well preserved and have about forty segments each in front of the cephalon, or an average of five to one millimeter.

Specimens 209 and 210 are on a slab about 7 × 5.5 inches, and with them are twelve other more or less well preserved individuals, all but one of which are smaller than these. Two of the fourteen are ventral side up on the slab, which means dorsal side up in the rock. Nine are oriented in one direction, two at exactly right angles to this, and three at an angle of 45 with the others. If the majority of the specimens are considered to be headed northward, then seven are so oriented, two northeast, one east, two south, one southwest, and one west.

Nine of the specimens show antennules. Five of these are specimens headed north, and in all of them the antennules are in or very near the normal position. The antennules of two, one headed east and the other west, are imperfectly preserved, but the parts remaining diverge much more than do the antennules of those in the normal position. The individual headed southwest has one antennule broken off, while the other is curved back so that its tip is directed northward. Another one, headed south, has the antennules in the normal position. These observations indicate that the specimens were oriented by currents of water, rather than in life attitudes, and that the distal portions of the antennules were relatively flexible.

_Measurements:_ The specimen (No. 210) is 20 mm. long, 9.5 mm. wide at the back of the cephalon, and the antennules project 8 mm. in front of the head. The smallest specimen on the slab is 6.5 mm. long. A specimen 7.5 mm. long has antennules which project 2.5 mm. in front of the cephalon.

Specimen No. 201 (pl. 2, fig. 1; pl. 3, fig. 4).

Illustrated: Amer. Jour. Sci., vol. 46, 1893, p. 469, figs. 2, 3; Amer. Geol., vol. 13, 1894, pl. 3, figs. 8, 9.

An entire specimen 17 mm. long, exposed from the dorsal side. It shows only traces of the appendages of the head, but displays well those of the anterior part of the thorax, and a number of appendages emerge from under the abdominal shield. This specimen is of particular interest as it is the subject of the first of Professor Beecher's papers on appendages of trilobites. On the right side the pleura have been removed, so as to expose the appendages of the second, third, and fourth segments from above. The first two of the appendages on the right are best preserved, and these are the ones figured. They belong to the second and third segments. The endopodites of each are ahead of the exopodites, and the proximal portion of each exopodite overlies portions of the first two segments (second and third) of the corresponding endopodite. The coxopodites are not visible, but very nearly the full length of the first segment of the endopodite (the basipodite) is exposed. The first two visible segments (the first and second) extend just to the margin of the pleural lobe, while the other four extend beyond the dorsal cover. The segments decrease in length outward, but not regularly, the meropodite being generally longer than the ischiopodite or the carpopodite. The terminal segment (dactylopodite) is short and bears short sharp hair-like spines which articulate in sockets at the distal end. On this specimen the anterior limb on the right side shows one terminal spine, the second endopodite on that side has two, and two of the endopodites on the left-hand side preserve two each. The segments of the limbs are nearly cylindrical, but the ischiopodites and meropodites of several of the endopodites show rather deep longitudinal grooves which appear to be rather the result of the shrinkage of the thin test than natural conformations.

The endopodites on the left-hand side have a number of short, sharp, movable, hair-like spines, and cup-shaped depressions which are the points of insertion of others. On the distal end of the carpopodite of the first thoracic segment there seems to have been a spine, whose place is now shown by a pit. This same endopodite shows, rather indistinctly, three pits in the groove of the carpopodite, and the propodite has two. On the endopodite of the second appendage on this side, both the carpopodite and propodite possess a fine hair-like articulated spine at the distal end, that of the propodite arising on the dorsal and that of the carpopodite on the posterior side. On the dorsal side of the carpopodite there are three pits for the articulation of spines, and on the propodite, one.

The exopodites belonging to the thoracic segments are of equal length with the endopodites, and while the proximal portion of each is stouter than that of the corresponding endopodite, the exopodites taper to a hair-like termination, while the endopodites remain fairly stout to the distal segment. Most of the setæ of the exopodites have been removed, so that each remains as a curving, many-segmented organ, transversely striated, with a continuous groove along the posterior side. The setæ appear to be set in this groove, one for each of the transverse ridges on the shaft.

A good deal of the test has been cut away on the left-hand side from the thorax and pygidium, and the appendages exposed from above. Enough of the dorsal shell has been cut away so that the anal opening is exposed, and directly behind the pygidium, on the median line, is a bilaterally symmetrical plate with serrated edges which appears to be the appendage of the anal segment. (See pl. 3, fig. 4.)

_Measurements:_ The specimen is 17 mm. long, and 8 mm. in greatest width (at the back of the cephalon). From the median tubercle to the outer edge of the pleuron of the second thoracic segment the distance is 3.7 mm. From the point of articulation to the distal end of the spines on the dactylopodite of the second endopodite on the right-hand side is 4.3 mm. The basipodite of this appendage is 1.5 mm. long, the ischiopodite 1 mm. long, the meropodite 1.2 mm. long, the carpopodite 0.5 mm. long, the propodite 0.35 long, and the dactylopodite 0.15 mm. long. On the left-hand side the endopodite of the first segment projects 3 mm. beyond the pleuron, the second, 3.2 mm. At the back the appendages extend a maximum distance of 2.5 mm. behind the pygidium. The median spinose process of the anal segment extends 0.75 mm. behind the pygidium, and is 1.6 mm. in greatest width.

Specimen No. 204 (pl. 3, fig. 1; pl. 4, fig. 6; text fig. 42).

Illustrated: Amer. Jour. Sci., vol. 13, 1902, pl. 2, figs. 4, 5 (reproduced from photographs).

This specimen, which is developed from the dorsal surface, shows especially well nine appendages of the left side. The first represent the last segment of the cephalon; the remainder belong to the thorax. As is usual, the exopodites of these appendages overlie and curve behind the endopodites. All the exopodites have lost their setæ and the segments of the endopodites are flattened by crushing. The endopodites, while retaining only one or two of the movable spines, have the cup-like bases of from two to four on each of the visible segments, namely, the meropodite, carpopodite, propodite, and, in one case, the dactylopodite. These appendages, although really marvellous in preservation, are of such small size and react so badly to light that their study is very difficult, and Professor Beecher, who had observed hundreds of specimens through all stages of the laborious process of cleaning the matrix from them, undoubtedly was much better equipped to interpret them than any other person.

The drawing is made on the assumption that the appendages are displaced and all moved uniformly outward so that the distal ends of the coxopodites emerge from under the pleural lobe, whereas these ends would normally be under the dorsal furrow, and the distal end of the ischiopodite should reach the margin of the pleural lobe. While it seems very remarkable that it should happen, that all the appendages should be so moved that they would lie symmetrically a few millimeters from their normal position, nevertheless it is found on measuring that they bear the same proportion to the length and width that the appendages of other specimens do, thus indicating that Professor Beecher's interpretation of them was correct. I am unable, however, to see the coxopodites which he has drawn as articulating with the two branches of the limb.

This individual shows, better than any other, the connection of the exopodite with the endopodite. Even though the coxopodites are gone, the two branches of each appendage remain together, showing that the basipodite as well as the coxopodite is involved in the articulation with the exopodite. Just what the connection is can not be observed, but there seems to be a firm union between the upper surface of the basipodite and the lower side of the proximal end of the exopodite, as indicated diagrammatically in text figure 33.

_Measurements:_ The specimen is 20 mm. long and 9 mm. wide at the back of the cephalon. From the tubercle on the middle of the first segment of the thorax to the tip of the corresponding appendage the distance is 8 mm. The entire length of the exopodite of the first thoracic segment is 4.6 mm. The exopodite of the appendage belonging to the seventh segment is only 3.5 mm. long. The pleural lobe is 2.5 mm. wide at the front of the thorax.

Specimen No. 205 (pl. 2, fig. 4).

Illustrated: Amer. Jour. Sci., vol. 13, 1902, pl. 5, figs. 2, 3 (photographs).

This is a small imperfect specimen, developed from the ventral side. It retains the best preserved metastoma in the collection, but was used by Professor Beecher especially to illustrate the convergent ridges on the inside of the ventral membrane in the axial region of the thorax. These ridges are very low, and on each segment of the thorax there is a central one, outside of which is a pair which are convergent forward, making angles of 35 to 45 with the axis.

The metastoma is shaped much like the hypostoma of an _Illænus_. It is convex, nearly semicircular, with the straight side forward, and there is a continuous raised border around the curved sides and back. This border is separated from the central convex body by a deep linear depression.

The hypostoma is also rather well preserved and has a narrow, slightly elevated border at the sides and back.

_Measurements:_ The incomplete specimen, from which only a very small portion of the length is missing, is 9 mm. long. The metastoma is 0.45 mm. long and 0.58 mm. wide.

Specimen No. 214 (pl. 1, fig. 2; pl. 3, fig. 6).

This is a large specimen, developed from the ventral side. It shows the antennules and some other appendages of the head, but derives its special interest from the excellent preservation of a few of the exopodites, which are turned back parallel to the axis of the body and lie within the axial lobe.

The shaft of the exopodite is made up of numerous short segments which at their anterior outer angles are produced into spines, and which also bear movable spines along the anterior border. As shown in several other specimens, the exopodite ends in a more or less long spoon-shaped segment bearing on its lower surface a broad groove. No setæ appear to be attached to this, but both anterior and posterior margins bear numerous small, apparently movable spines. From the groove along the ventral side of the remainder of the exopodite arise numerous long slender filaments which become progressively shorter toward the tip. This specimen shows that they are not cylindrical, but are flattened along opposite faces, at least at their distal ends. While no connection can be seen between adjacent setæ, they seem to stay together like the barbs on a feather.

_Measurements:_ Length, 33 mm., width at back of cephalon, 16 mm.; from front of cephalon to back of hypostoma, 6 mm.

Specimen No. 219 (pl. 2, fig. 6; pl. 4, fig. 4).

Illustrated: Amer. Jour. Sci., vol. 13, 1902, pl. 4, fig. 1, pl. 5, fig. 4 (photograph and drawing).

The endopodites of most of the appendages of the thorax are well shown, and occasional portions of exopodites. The coxopodites are long, flattened, and do not taper much. The anterior and posterior edges of the basipodites of the endopodites of the first two segments are approximately parallel, but on the succeeding endopodites the basipodites and ischiopodites are triangular in form, with the apex backward. In successive endopodites toward the posterior end, the angle made by the backward-directed sides of the basipodites becomes increasingly acute, so that in some of the posterior appendages this segment is wider than long. The ischiopodite shows a similar increase of width and angularity on successive segments, and the meropodites and carpopodites also become wider on the posterior segments, and even triangular in outline toward the back of the thorax and on the pygidium.

Along the median portion of the axial lobe the specimen has been cleaned until the inner side of the ventral membrane was reached. Here the test shows on the inner surface at each segment of the thorax a series of low ridges which are roughly parallel to the axial line, but which really converge in an anterior direction. Between the ridges are shallow canoe-shaped depressions, which have the appearance of areas for the insertion of muscles.

_Measurements:_ Length, 31 mm.; width at back of head, 15 mm.; distance, in a straight line, from point of insertion of the right antennule to its tip, 14.25 mm.; it projects 12 mm. beyond the cephalon.

Specimen No. 218 (pl. 6, fig. 3; text fig. 43).

This specimen is a large one, developed from the lower side, but retains only the endopodites of a few appendages. The cephalon and anterior portion of the thorax are missing.

Professor Beecher had a drawing made to show the appendages on the right-hand side of the last two segments of the thorax, seen of course from the ventral side. This drawing shows well the broadening of the basipodite, ischiopodite, and meropodite, while the coxopodite is thick and heavy, and the inner end of the gnathobase somewhat rugose. Almost every segment of the endopodites has one or more pits for insertion of spines, these being along the anterior or posterior margins. The exopodites lack the setæ, but show no unusual features.

Specimen No. 222 (pl. 4, fig. 5).

Illustrated: Amer. Jour. Sci., vol. 47, 1894, pl. 7, fig. 3 (drawing).

A small specimen, developed from the lower side, and used by Professor Beecher to illustrate the form of the segments of the endopodites of the pygidium. In addition to this, it shows very well the form of the endopodites of the thorax. All of the appendages on the specimen are shifted to the left of their normal position. This specimen differs from most of the others in that the segments of the endopodites do not lie with their greatest width in the horizontal plane, but were embedded vertically, with the posterior edge downward. From this circumstance they retain their natural shape, and it is seen that they are naturally flattened, with about the same thickness in proportion to length and width as in some of the modern isopods (Serolis, for instance). In even the most anterior of these endopodites (that of the second segment) the ischiopodite, meropodite, and carpopodite are triangular in shape, with the point backward, but in all the endopodites at the anterior end of the thorax, the triangle has a very obtuse angle at the apex, and the base is much longer than the perpendicular. On the other hand, those of the pygidium, which were figured by Beecher, have a number of short wide segments, all wider than long, and, excepting the dactylopodites, triangular in form.

_Measurements:_ Length, 8.75 mm.; width at back of cephalon, about 4 mm.

Specimen No. 230 (pl. 5, fig. 3; text fig. 44).

Illustrated: Amer. Jour. Sci., vol. 47, 1894, pl. 7, fig. 2 (drawing); Ibid., vol. 13, 1902, pl. 2, fig. 2.

An entire specimen of medium size, developed from the ventral side. It seems to have been the first one to yield to Professor Beecher any satisfactory knowledge of the appendages of the pygidium. There are five endopodites, all on one side, which appear to belong here. The segments in this region are characterized by their short, wide, triangular form. At the apex of each is a small tuft of spines or short hairs, and the ventral surfaces of some of the endopodites show pits for the insertion of spines.

_Measurements:_ Length, 21 mm.; width at back of cephalon, 10 mm.

Cryptolithus tessellatus Green.

Specimen No. 233 (pl. 7, fig. 1; text fig. 45).

This is the best preserved entire specimen. It is developed from the lower side, and shows the hypostoma, antennules, and a few fragmentary appendages of the cephalon, the outer portions of the exopodites of thorax and pygidium on both sides, and the endopodites on the left side.

The hypostoma is imperfectly preserved and is turned completely around, so that the anterior margin is directed backward, and the posterior one is so much in the shadow that it does not show well in any of the photographs. The form is, however, essentially like that of _Trinucleoides reussi_ (Barrande), the only other trinucleid of which the hypostoma is known, except that the border does not extend so far forward along the sides, and it is much smaller.

The antennules are not inserted close to the hypostoma, as in Triarthrus, but at some distance from it, and, as nearly as can be determined, directly beneath the antennal pits which are seen near the front of the glabella in many species of trinucleids.

The antennules are long, and are composed of far fewer and longer segments than those of Triarthrus. In this specimen they converge backward, cross each other and at the distal end are more or less intertwined.

As is shown in the drawing and photograph, very little can be learned from this individual about the other appendages of the cephalon. A few fragments of exopodites on either side suggest that these members pointed forward and were much like those in Triarthrus, but nothing conclusive is shown.

The exopodites and endopodites of the left side of the thorax are best preserved. The exopodites are above the endopodites, and only that portion exposed from the ventral side which projects beyond the line at which the endopodites bend backward. The endopodite on the left side of the first thoracic segment is the best preserved. It shows seven segments, the outer ones best. The coxopodite is short and narrow, the basipodite somewhat heavier and longer, while the carpopodite and propodite are the widest and strongest segments. The propodite is triangular and flattened, like the segments on the middle and posterior part of the thorax of Triarthrus. At the inner end of the ischiopodite and meropodite are tufts of spines pointing inward and backward. These are not shown on any of the photographs, but may be seen with the light striking the specimen at the proper angle.

It is not possible to count the exact number of limbs, but one gets the impression that on the left side of this specimen there are twenty-one sets of appendages, six of which of course belong to the thorax. On the thorax and anterior part of the pygidium, successive endopodites show the propodites and dactylopodites becoming progressively more slender and shorter, while the ischiopodites, meropodites and carpopodites become shorter and more triangular, and with increasingly large numbers of short spines on their posterior borders. Back of the fourth endopodite on the pygidium it is not possible to make out the detail, but the appearance is of an endopodite consisting of short broad segments fringed at the back with short spines, the ones at the very posterior end appearing to be exceedingly short and rudimentary.

The exopodites are not so well shown as in some others but the setæ are flattened and blade-shaped, and often bear numerous small spines.

_Measurements:_ Length (lacking most of the fringe), 10.5 mm. Width of thorax, 10.5 mm. Length of hypostome, 1.41 mm., width at front, 1.46 mm. The distance from back of fringe to end of antennules is 5.4 mm. If straightened out, the left antennule would be about 6.1 mm. long. In the first 3.1 mm., there are only ten segments, so that the average length of a segment is 0.31 mm. The distance from the inner end of the endobase of the first segment of the thorax to the outer end of the meropodite is 2.43 mm., and from that point to the end of the dactylopodite 2.47 mm. making the total length 4.90 mm. These measurements are taken from the photograph. Measurements taken from Professor Beecher's drawing, which was made with the camera-lucida, give a total length of 4.57 mm., the distance to the outer end of the meropodite being 2.3 mm. and thence to the tip of the dactylopodite 2.27 mm. Detailed measurements of the segments, on the photograph, are as follows: coxopodite, 0.321 mm.; basipodite, 0.78 mm.; ischiopodite, 0.68 mm.; meropodite, 0.642 mm.; carpopodite, 0.642 mm.; propodite, 1.01 mm., dactylopodite, 0.825 mm.

Specimen No. 235 (pl. 7, fig. 2; pl. 8, fig. 3; pl. 9, figs. 1, 2).

Illustrated: Amer. Jour. Sci., vol. 49, 1895, pl. 3, figs. 5, 6.

Specimens 235 and 236 were originally parts of an entire _Cryptolithus_, but, as Professor Beecher has explained, the specimen was cut in two longitudinally on the median line, and the halves transversely just back of the cephalon, so that each now represents one half of a thorax and pygidium. Both halves have been cleaned from both upper and lower side, a perfectly marvelous piece of work, for the thickness is no greater than that of a thin sheet of paper, and the soft shale of the matrix has a very slight cohesive power.

Both sides of specimen 235 were figured, but the dorsal side was apparently then somewhat less fully developed than at present. On plate 9 are two figures in which specimens 235 and 236 are brought together again, and both dorsal and ventral sides illustrated.

On the dorsal side, specimen 235 shows portions of three exopodites which lie in a direction roughly parallel to the outer portions of the endopodites on the lower side, that is, their direction if projected would reach the axis in an acute angle back of the end of the pygidium. The setæ stand at right angles to the shaft, and on a portion of it 0.5 mm. long there are seven of them. This is a fragment of an exopodite near the front of the thorax, and the setæ, which are flattened, are about 1.63 mm. long.

On the ventral side this same specimen shows incomplete endopodites and exopodites of about seventeen segments, six of which would belong to the thorax and the remainder to the pygidium. The greater part of the appendages belonging to the pygidium are exceedingly small (about 0.15 mm. long) and so incompletely exposed that the structure can not be definitely made out.

The endopodites of the thoracic segments all lack the greater part of their proximal segments and are all of practically the same form. They turn abruptly backward at the outer end of the meropodite, and the carpopodite of each is greatly widened, projects inward and is armed with tufts of spines. The propodite and dactylopodite are wide, flattened, and taper but slightly outward, the dactylopodite bearing on its distal end a tuft of spines. On several of the endopodites, the meropodites are visible and they bear on their inner ends fringes of spines pointing inward. Behind these well preserved appendages the proximal segments of several endopodites are visible, and a regular succession of flattened, oval bodies armed with numerous forward-pointing spines. These latter bodies Professor Beecher took to be leaf-like exopodites, which they certainly resemble, and as they lie beyond the line of endopodites they probably do belong to the outer halves of the appendages.

The exopodites under the thorax are long, the shaft shows numerous short segments, and is in each case bent backward, though not through a right angle. They extend considerably beyond the endopodites. The setæ do not diverge from the shaft at a right angle as on the dorsal side of this same specimen, but at an acute angle, indicating that they were not rigid. The individual hairs are broad and blade-shaped, frequently with a linear depression along the median line, perhaps due to collapse of the internal tube.

_Measurements:_ The greatest length of the fragment in its present state is 5 mm. The dactylopodite of the second endopodite (without terminal spines) is 0.18 mm. long, the propodite 0.23 mm. long and 0.15 mm. wide; the carpopodite is 0.24 mm. long and 0.38 mm. wide. All measurements were made on the photographs.

Specimen No. 236 (pl. 7, figs. 3-5; pl. 9, figs. 1, 2; text fig. 45).

The right half of the same thorax and pygidium as specimen No. 235.

The specimen is cleaned from both upper and lower sides and, the dorsal test being removed, reveals the long blade-like setæ of the exopodites, each blade being concave along its median line. They are long on the exopodites of the thoracic segments, but become shorter, without, however, any visible change of form on the pygidium. Although the posterior end is not well preserved, one gets no suggestion from a study of this side of the specimens that the exopodites of the posterior end are in any striking way different from those of segments further forward. The tips of some of the setæ show minute spines, one to each blade.

On the ventral side are a number of endopodites, but they are more fragmentary than those of the other half of the specimen. Some of the exopodites are well shown, the blades being in all cases broken from the shaft. Two of the endopodites of this specimen are of especial interest, as they have interarticular membranes between the last three segments. Professor Beecher made a drawing of one of these which he placed under his pen drawing (text fig. 45).

_Measurements:_ The specimen is 5 mm. long from the front of the second thoracic segment to the end of the pygidium. The setæ on the exopodites of the anterior thoracic segments are 1.7 mm. long, as exposed from the dorsal side. Some of those on the posterior part of the pygidium, only incompletely exposed, are 0.31 mm. long.

The dactylopodite of the first endopodite showing the articular membranes is 0.23 mm. long and 0.13 mm. wide. The propodite is of the same length and 0.17 mm. wide. The interarticular membrane between them is 0.066 mm. thick. The spines on the dactylopodite of this appendage are 0.15 mm. long. All measurements were made on photographs.

Specimen No. 238 (pl. 8, fig. 4; text fig. 46).

A triangular specimen consisting of the greater part of a pygidium and parts of all the thoracic segments. Under the thorax the specimen has been so cleaned that the outer portions of the endopodites are well shown, while under the pygidium the greater part of the endopodites seem to have been removed, disclosing the setæ of the exopodites. As in other specimens, the endopodites of the thorax turn backward at the distal end of the carpopodite, which is broad and curved, and bears a tuft of spines on the posterior margin. The dactylopodites seem to preserve their natural shape, and are very nearly cylindrical in form. Under the pygidium are several sets of overlapping fringes of setæ of exopodites, and along the edge of the dorsal furrow, a number of fragments of segments of what may be coxopodites while with them are a number of fragmentary shaft of exopodites.

_Measurements:_ The pygidium is 3.3 mm. long, the thorax 3 mm.

BIBLIOGRAPHY.

Agassiz, L.

1873.--Discovery of the basal joint of legs of trilobites. Amer. Nat., vol. 7, pp. 741-742.

Angelina N. P.

1854.--Palæontologia Scandinavica, pars 1, Crustacea formationis transitionis.

Audouin, J. V.

1821.--Recherches sur les rapports naturels qui existent entre les trilobites et les animaux articulés. Ann. Gen. Sci. Phys. Nat. Bruxelles, vol. 8, p. 233, pl. 26. 1822. Isis (Encycl. Zeitung), Oken., vol. 10, p. 87, pl. 1, No. 4, figs. 1-5.

Barrande, J.

1852.--Systême Silurien du centre de la Bohême, vol. 1, pp. 226-230, and 629, pl. 30, figs. 38, 39.

1872.--Ibid., vol. 1, Suppl., p. 180, pl. 4.

Barth, Hermann von.

1875.--Die Stellung der Trilobiten in zoologischen Systeme. Das Ausland, 26. Jahrg., p. 2 5.

Beecher, C. E.

1893.--On the thoracic legs of _Triarthrus_. Amer. Jour. Sci. (3), vol. 46, pp. 367-370, 467-470, text figs. 1-3.

1894 A.--On the mode of occurrence, and the structure and development of _Triarthrus becki_. Amer. Geol., vol. 13, pp. 38-43, pl. 3.

1894 B.--The appendages of the pygidium of _Triarthrus_. Amer. Jour. Sci. (3), vol. 47, pp. 298-300, pl. 7, text fig, 1.

1895 A.--Further observations on the ventral structure of _Triarthrus_. Amer. Geol., vol. 15, pp. 91-100, pls. 4-5.

1895 B.--Structure and appendages of _Trinucleus_. Amer. Jour. Sci. (3), vol. 49, pp. 307-311, pl. 3.

1895 C.--The larval stages of trilobites. Amer. Geol., vol. 16, pp. 166-197, pls. 8-10.

1896 A.--The morphology of Triarthrus. Amer. Jour. Sci. (4), vol. 1, pp. 251-256, pl. 8; Geol. Mag., dec. 4, vol. 3, pp. 193-197, pl. 9.

1896 B.--On a supposed discovery of the antennas of trilobites by Linnæus in 1759. Amer. Geol., vol. 17, pp. 303-306, text figs. 1-3.

1897 A.--Outline of a natural classification of trilobites. Amer. Jour. Sci. (4), vol. 3, pp. 89-106, 181-207, pl. 3.

1897 B.--Remarks on Kingsley's "Systematic position of the trilobites." Amer. Geol., vol. 20, pp. 38-40.

1900.--Trilobita. Eastman-Zittel Text-book of Paleontology, vol. 1, pp. 607-638, text figs. 1261-1331; ed. 2, 1913, p. 700. London.

1901.--Structure and development of trilobites. In "Studies in Evolution," pp. 109-225. New York and London.

1902.--The ventral integument of trilobites. Amer. Jour. Sci. (4), vol. 13, pp. 165-174, pls. 2-5, text fig. 1; Geol. Mag., dec. 4, vol. 9, pp. 152-162, pls. 9-11, text figs. 1-8.

Bernard, H. M.

1892.--The Apodidæ.

1893.--Trilobites with antennæ at last! Nature, vol. 48, p. 582.

1894.--The systematic position of the trilobites. Quart. Jour. Geol. Soc., London, vol. 50, pp. 411-434, text figs. 1-17.

1895 A.--The zoological position of the trilobites. Science Prog., vol. 4, pp. 33-49.

1895 B.--Supplementary notes on the systematic position of the trilobites. Quart. Jour. Geol. Soc., London, vol. 51, pp. 352-360, figs. A-C.

Beyrich, E.

1846.--Untersuchungen ueber Trilobiten. 2. Stück, p. 30, pl. 4, fig. 1c.

Billings, E.

1870.--Notes on some specimens of Lower Silurian trilobites. Quart. Jour. Geol. Soc., London, vol. 26, pp. 479-486, pls. 31-32. Abstract in Geol. Mag., vol. 7, p 291, and Nature, vol. 2, p. 94.

Brongniart, A.

1822.--Histoire naturelle des crustacés fossiles. Paris.

Brünnich, F. E.

1781.--Beskrivelse over trilobiten, en dyreslaegt og dens arter, med en ney arts aftegning. Nye Samlig of det Kong. Danske Vidensk. Selskabs. Skriften, Copenhagen.

Burling, L. D.

1916.--Pædeumias and the Mesonacidæ, with description of a new species, having at least 44 segments, from the Lower Cambrian of British Columbia. Ottawa Nat., vol. 30, pp. 53-58, pl. 1.

1917.--Was the lower Cambrian trilobite supreme? Ibid., vol. 31, pp. 77-79, text figs. 1-2.

Burmeister, H.

1843.--Die Organisation der Trilobiten. Berlin.

1846.--The organization of trilobites, deduced from their living affinities. Eng. translation, Ray Society, London.

Calman, W. T.

1909.--Crustacea, in "A treatise on zoology," edited by Sir Ray Lankester. London.

1919. Dr. C. D. Walcott's researches on the appendages of trilobites. Geol. Mag., dec. 6, vol. 6, pp. 359-363, pl. 8, text fig. 1.

Carpenter, G. H.

1903.--On the relationships between the classes of Arthropoda. Proc. Roy. Irish Acad., vol. 24, pp. 320-360, pl. 6.

Castelnau, F. DE.

1843.--Systeme Silurien de l'Amérique Septentrionale, p. 15, pl. 2, figs. 1, 4.

Clarke, J. M.

1888.--The structure and development of the visual area in the trilobite, _Phacops rana_ Green. Jour. Morph., vol. 2, pp. 253-270, pl. 1.

Crampton, G. C.

1916.--The phylogenetic origin and the nature of the wings of insects, according to the paranotal theory. Jour. New York Entomol. Soc., vol. 24, pp. 1-39, pls. 1, 2.

1919.--The evolution of the arthropods and their relatives, with especial reference to insects. Amer. Nat, vol. 53, pp. 143-179.

Dalman, J. W.

1826.--Om Palæaderna eller de så kallade Trilobiterna. Stockholm, Acad. Handl., pp. 113-152, 226-294.

1828.--Ueber die Palæaden, oder die sogennanten Trilobiten. Nuremberg.

Dana, J. D.

1871.--On the supposed legs of the trilobite, _Asaphus platycephalus_. Amer. Jour. Sci. (3), vol. 1, pp. 320-321, 386; Ibid. (3), vol. 3, 1872, pp. 221-222. Also printed in Ann. Mag. Nat. Hist, vol. 7, 1871, pp. 366, 451.

Dekay, J. E.

1824.--Observations on the structure of trilobites, and description of an apparently new genus. Ann. Lye. Nat. Hist. New York, vol. I, p. 174, 2 pls.; Isis (Encycl. Zeit.), Oken, 1825 and 1832.

Dollo, L.

1910.--La paléontologie éthologique. Bull. Soc. Beige de Geol., Pal., et d'Hydrol., vol. 23, pp. 377-421, figs. 1-13, pls. 7-11.

Eichwald, E. VON.

1825.--Geognostico-zoologicæ per Ingriam Marisque Baltici Provincias nee non de trilobitis observationes. Section 45.

1858.--Beiträge zur geographischen Verbreitung der fossilen Thiere Russlands. Bull. Soc. Imp. des Natural, de Moscou, vol. 30, 1855-1857, p. 204.

1860.--Lethæa Rossica, pl. 21.

1863.--Beiträge zur nähern Kenntniss der in meiner Lethæa Rossica beschriebenen Ilænen. Bull. Soc. Imp. des Natural, de Moscou, vol. 36, p. 408.

Emmrich, H. F.

1839.--De trilobitis dissertatio petrefactologica, etc. Berlin.

Exner, S.

1891.--Die Physiologic der facettirten Augen von Krebsen und Insecten. Leipzig and Vienna. Pp. 33-35, pl. 2, figs. 18-19.

Finch, G. E.

1904.--Notes on the position of the individuals in a group of _Nileus vigilans_ found at Elgin, Iowa. Proc. Iowa Acad. Sci. for 1903, vol. 11, pp. 179-181, pl. 14.

Gegenbaur, C.

1878.--Elements of comparative anatomy. Eng. ed. (Bell and Lankester). London.

Goldfuss, A.

1828.--Observation sur le place qu'occupent les trilobites dans le règne animal. Ann. Sci. Nat., Zoologie, vol. 15, p. 83, pl. 2, figs. 5, 7, 9, 10.

Green, J.

1839 A.--The inferior surface of the trilobite discovered. The Friend, Philadelphia, March 16.

1839 B.--The inferior surface of the trilobite discovered. Illustrated, with colored models. Philadelphia.

1839 C.--Remarks on the trilobites. Amer. Jour. Sci. (1), vol. 37, p. 25 _et seq._

1840.--An additional fact, illustrating the inferior surface of _Calymene bufo_. Ibid., vol. 38, p. 410.

Handlirsch, A.

1906.--Ueber Phylogenie der Arthropoden. Verhandl. d. k. k. zool.-bot. Gesell., Vienna, Jahrg. 1906, pp. 88-103.

1907.--Functionswechsel einiger Organe bei Arthropoden. Ibid., Jahrg. 1907, pp. 153-158.

1908.--Die fossilen Insekten. Leipzig.

1914.--Eine interessante Crustaceenform aus der Trias der Vogesen. Verhandl. d. k. k. zool.-bot. Gesell., Vienna, Jahrg. 1914, pp. 1-7, pls. 1, 2.

Hawle, I., and Corda, A. J. C.

1847.--Prodrom einer Monographie der boehmischen Trilobiten, pp. 9, 24, 56, pl. 2, fig. 10; pl. 3, fig. 15; pl. 4, fig. 33b-g.

Jaekel, O.

1901.--Beiträge zur Beurtheilung der Trilobiten, Theil I. Zeits. d. deutsch. geol. Gesell., Bd. 53, pp. 133-171. Pis. 4-6, text figs. 1-30.

Kingsley, J. S.

1897.--The systematic position of the trilobites. Amer. Geol., vol. 20, pp. 33-38.

Koenen, A. von.

1872.--Ueber die Organisation der Trilobiten. Verhandl. d. naturhist. Ver. d. preuss. Rheinl. u. Westphalen, vol. 29, C, pp. 93-95.

1880.--Ueber die Unterseite der Trilobiten. Neues Jahrb. f. Min.., Geol., u. Pal,, Bd. 1, pp. 430-432. pl. 8.

Lang, A.

1891.--Text-book of comparative anatomy, Eng. ed. (Bernard). London.

Lankester, E. R.

1881.--Observations and reflections on the appendages and on the nervous system of _Apus cancriformis_. Quart. Jour. Micros. Soc., vol. 21, pp. 343-376.

Laurie, M.

1911.--A reconstructed trilobite. Nature, vol. 88, p. 26.

Lindstroem, G.

1901.--Researches on the visual organs of the trilobites. K. svenska Vet.-Akad. Handl., new ser., vol. 34, pp. 1-86, pls. 1-6.

Linné, K.

1759.--Petrificatet entomolithus paradoxus sådant, som det finnes uti Hans Excellence Riks. Rådets Högoälborne Herr Grefve C. G. Tessins Samling. K. svenska Vet.-Akad. Handl., vol. 20, pp. 21, 22, pl. 1, fig. 1.

Matthew, W. D.

1893.--On antennæ and other appendages of _Triarthrus becki_. Amer. Jour. Sci. (3), vol. 46, pp. 121-125, pl. 1; Trans. New York Acad. Sci., vol. 12, pp. 237-241, pl. a.

McCoy, F.

1846.--A synopsis of the Silurian fossils of Ireland, p. 42.

Mickleborough, J.

1883.--Locomotory appendages of trilobites. Jour. Cincinnati Soc. Nat. Hist., vol. 6, pp. 200-204; Geol. Mag., dec. 3, vol. 1, 1884, pp. 80-84; Amer. Jour. Sci. (3), vol. 27, 1884, p. 409. Reviewed by Dames, Neues Jahrb. f. Min., Geol., u. Pal., Bd. 1, 1885, p. 477.

Miller, S. A.

1880.--Silurian ichnolites, with definitions of new genera and species. Jour. Cincinnati Soc. Nat. Hist, vol. 2, pp. 217-218, fig.

Milne-Edwards, H.

1881.--Compte rendu des nouvelles recherches de M. Walcott relatives à la structure des trilobites, suivi de quelques considérations sur l'interprétation des faits ainsi constatés. Ann. Sci. Nat, Zoologie, ser. 6, vol. 12, pp. 1-33, pls. 10-12. Paris.

Moberg, J. C.

1902.--Bidrag till Kännedomen om trilobiternas byggnad. Geol. Fören Förhandl., Bd. 24, pp. 295-302; pl. 3, text fig. 1.

1907.--Om ett gätfultt fossil frän sveriges olenidskiffer samt en kort ofversigt af viktigase data rorande trilobiternas ventrala skelettdelar. Ibid., Bd. 29, Heft 5, pp. 265-272, pl. 4, fig. 2; pl. 5, fig. 1.

Oehlert, D. P.

1896.--Résumé des derniers travaux sur l'organisation et le developpement des trilobites. Bull. Soc. Géol. France, ser. 3, vol. 24, pp. 97-116, text figs. 1-34.

Packard, A. H.

1872.--On the development of _Limulus polyphemus_. Mem. Boston Soc. Nat. Hist., vol. 2, pp. 155-202, pls. 3-5.

1880.--The structure of the eye of trilobites. Amer. Nat., vol. 14, pp. 503-508.

1882.--On the homologies of the crustacean limb. Ibid., vol. 16, pp. 785-799, figs. 11, 12.

Pander, C.

1830.--Beiträge zur Geognosie des russischen Reiches. St. Petersburg.

Peach, B. N.

1882.--On some fossil myriopods from the Lower Old Red Sandstone of Forfarshire. Proc. Roy. Physical Soc., Edinburgh, vol. 7, pp. 177-187, pl. 2.

1899.--O some new myriopods from the Palæozoic rocks of Scotland. Ibid., vol. 14, pp. 113-126, pl. 4.

Quenstedt, A.

1837.--Beitrag zur Kenntniss der Trilobiten, mit besonderer Rücksicht auf ihre bestimmte Gliederzahl. Archiv f. Naturg., Berlin, 3. Jahrg., 1 Bd., pp. 337-352.

Raymond, P. E.

1910.--On two new trilobites from the Chazy near Ottawa, Ontario. Ottawa Nat., vol. 24, pp. 129-134, pl. 2.

1917.--Beecher's classification of trilobites, after twenty years. Amer. Jour. Sci. (4), vol. 43, pp. 196-210, text figs. 1-3.

Raymond, P. E., and Barton, D. C.

1913.--A revision of the American species of _Ceraurus_. Bull. Mus. Comp. Zool., vol. 54, pp. 525-543. pls. 1, 2, 3 text figs. 1-3.

Reed, F. R. C.

1916.--The genus _Trinucleus_. Pt. 4. Geol. Mag., dec. 6, vol. 3, pp. 121, 122.

Richter, R.

1848.--Bitrag zur Palæeontologie des Thüringer Waldes. Dresden and Leipzig.

Ringueberg, E. N. S.

1886.--A trilobite track illustrating one mode of progression of the trilobites. Proc. Amer. Assoc. Adv. Sci., vol. 35, p. 228 (abstract only).

Ruedemann, R.

1916.--The presence of a median eye in trilobites. Bull. New York State Mus., No. 189. pp. 127-143, pls. 34-36.

Schlotheim, E. F. von.

1823.--Nachträge zur Petrefactenkunde, II. Gotha.

Six, Achille.

1884.--Les appendices des trilobites d'après M. Ch. D. Walcott. Ann. Soc. Geol. du Nord, vol. 11, pp. 228-236.

Spencer, W. K.

1903.--The hypostomic eyes of trilobites. Geol. Mag., dec. 4, vol. 10, pp. 489-492.

Staff, Hans v., and Reck, Hans.

1911.--Ueber die Lebensweise der Trilobiten. Eine entwicklungsmechanische Studie. Gesell. naturforsch. Freunde, Sitzb., pp. 130-146, figs. 1-20.

Sternberg, K. M.

1830.--Ueber die Gliederung und die Füsse der Trilobiten. Isis (Encycl. Zeitung), Oken, p. 516, pl. 5, figs. 1-3.

Stokes, C.

1823.--On a trilobite from Lake Huron. Trans. Geol. Soc., London, ser. 2, vol. 1, p. 208, pl. 27.

Swinnerton, H. H.

1919.--The facial suture of the trilobite. Geol. Mag., dec. 6, vol. 6, pp. 103-110.

Törnquist, S. L.

1896 A.--On the appendages of trilobites. Ibid., dec. 4, vol. 3, p. 142.

1896 B.--Linnæus on the appendages of trilobites. Ibid., pp. 567-569.

Tothill, J. D.

1916.--The ancestry of insects, with particular reference to chilopods and trilobites. Amer. Jour. Sci. (4), vol. 42, pp. 373-383. text figs. 1-8.

Troedsson, G. T.

1918.--Om skanes Brachiopodskiffer. Lunds Universitets Arsskrift, n. f., Avd. 2, Bd. 15, Nr. 3. pp. 57-67, pl. 1, figs. 19-24.

Valiant, W. S.

1901.--Appendaged trilobites. The Mineral Collector, vol. 8, No. 7, pp. 105-112.

Volborth, A. von.

1858.--Ueber die Bewegungs-Organe der Trilobiten. Verhandl. russ. k. mineral. Gesell. zu St Petersburg, 1857-1858, p. 168.

1863.--Ueber die mit glatten Rumpfgliedern versehenen russischen Trilobiten, nebst einem Anhange ueber die Bewegungs-organe und ueber das Herz derselben. Mem. Acad. Imp. Sci. St. Petersburg, ser. 7, vol. 6, No. 2, pp. 44-47, pl. 1, fig. 12.

1866.--Ueber Herrn von Eichwald's Beitrag zu näheren Kenntniss der Illænen. Bull. Soc. Imp. des Natural, de Moscou, vol. 39, p. 40.

Wahlenberg, G.

1821.--Petrificata telluris Suecana examinata a Georgio Wahlenberg. Nova Acta Reg. Soc. Scient. Upsala, vol. 8.

Walcott, C. D.

1875.--Description of the interior surface of the dorsal shell of _Ceraurus pleurexanthemus_ Green. Ann. Lye. Nat. Hist. New York, vol. II, pp. 159-162, pl. 11.

1876.--Preliminary notice of the discovery of the natatory and branchial appendages of trilobites. 28th Rept. New York State Mus. Nat. Hist., adv. sheets, pp. 89-92; published as full report in 1879.

1877.--Notes on some sections of trilobites. 31st Rept. New York State Mus. Nat. Hist., adv. sheets, pp. 61-63, pl. 1; published as full report in 1879. Reviewed by Dames, Neues Jahrb. f. Min., Geol., u. Pal., Bd. 1, 1880, p. 428.

1879.--Notes upon the legs of trilobites. 31st Rept. New York State Mus. Nat. Hist., adv. sheets, p. 64.

1881.--The trilobite: New and old evidence relating to its organization. Bull. Mus. Comp. Zool., vol. 8, pp. 192-224, pls. 1-6.

1884.--The appendages of the trilobite. Science, vol. 3, pp. 276-279, figs. 1-3. Reviewed by Dames, Neues Jahrb. f. Min., Geol., u. Pal.., Bd. 1, 1885, Referate, p. 102.

1894.--Note of some appendages of the trilobites. Proc. Biol. Soc. Washington, vol. 9, pp. 89-97, pl. 1; Geol. Mag., dec. 4, vol. 1, pp. 246-251, pl. 8.

1911.--Middle Cambrian Merostomata. Smithson. Misc. Coll., vol. 57, No. 2, pp. 17-40, pls. 2-7.

1912 A.--Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and Merostomata. Ibid., No. 6, pp. 145-228, pls. 24-34, text figs. 8-10.

1912 B.--New York Potsdam-Hoyt fauna. Ibid., No. 9, pp. 251-304, pls. 37-49.

1913.--Eastman-Zittel Text-book of Paleontology, ed. 2, vol. 1, figs. 1343, 1376, 1377.

1916.--Ann Rept., Secretary Smithsonian Inst, for 1915, pl. 9.

1918.--Appendages of trilobites. Smithsonian Misc. Coll., vol. 67, No. 4, pp. 115-226, pls. 14-42.

Watase, S.

1890.--On the morphology of the compound eyes of arthropods. Johns Hopkins Univ., Studies from Biol. Lab., vol. 4, no. 6, p. 290 (footnote).

Woodward, H.

1870.--Note on the palpus and other appendages of _Asaphus_, from the Trenton limestone, in the British Museum. Quart. Jour. Geol. Soc., London, vol. 26, pp. 486-488, fig. 1. Abstract in Geol. Mag., dec. 1, vol. 7, p. 292, also in Nature, vol. 2, p. 94.

1871.--On the structure of trilobites. Geol. Mag., dec. 1, vol. 8, pp. 289-294, pl. 8.

1884.--Notes on the appendages of trilobites. Geol. Mag., dec. 3, vol. 1, pp. 162-165, 2 text figs.

1895.--Some points in the life history of the Crustacea in early Palæozoic times. Quart. Jour. Geol. Soc., London, vol. 51, pp. lxx-lxxxviii, 1 pl.

* * * * *

PLATE 1.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 213. The dorsal test has been removed from the glabella, revealing the outline of the posterior end of the hypostoma, the proximal ends of the antennules, the gnathites, and incomplete endopodites of some appendages, × 5.43.

Fig. 2. Specimen 214. The head of a complete large specimen. Part of the thorax is shown on pl. 3, fig. 6. Note especially the form of the segments of the endopodites and of the anterior coxopodite on the right side, × 7.33.

Fig. 3. Specimen 217. This specimen shows better than any other the form of the gnathites of the cephalon. Note also the setæ of the exopodites under the cheek at the right. The appearance of a hook on the posterior gnathite on the right may be accidental, but it does not show broken edges, × 6.85.

Fig. 4. Specimen 215. The ventral side of the cephalon of a small entire specimen. Shows well the form of some of the gnathites and a few of the endopodites. Note the unusual position of the antennules. × 7.63.

Fig. 5. Specimen 226. This specimen did not photograph well, but is important as showing the exopodites and endopodites emerging from under the cephalon. × about 6.

PLATE I.

HELIOTYPE CO. BOSTON

* * * * *

PLATE 2.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 201. The entire specimen, details of which are shown in pl. 3, fig. 4 and pl. 4, figs. 1, 2. The dorsal test has been removed from the anterior segments on the right side. × 4.12.

Fig. 2. Specimen 206. A small individual with the endopodites, and the exopodites minus their setæ; well preserved on the left side. Note the position of the antennules. The course of the facial suture is unusually well shown. × 10.

Fig. 3. Specimen 210. The specimen which served as the main basis for Professor Beecher's first figure of the appendages of the thorax, specimen 206 (fig. 2, this plate) having supplemented it. Note the "normal" position of the antennules and the extension of the appendages from beneath the pleural lobe. Specimens with the antennules in this position may possibly be males. × 4.

Fig. 4. Specimen 205. A small specimen with some of the appendages preserved, especially toward the posterior end, but particularly valuable for the unusually well preserved metastoma. × 11.

Fig. 5. Specimen 211. A small cephalon, cleaned from the ventral side, and showing well the gnathites which approach each other unusually closely on the median line. × 10.5.

Fig. 6. Specimen 219. An entire specimen of medium size, developed from the ventral side. It shows particularly well the "normal" curvature of the antennules, the change in form of the segments of the endopodites from cephalon to pygidium, and, along the axial lobe, the apodemes of the ventral integument. See also pl. 4, fig. 4. × 3.6.

PLATE II.

HELIOTYPE CO. BOSTON

* * * * *

PLATE 3.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 204. See also text fig. 42 and pl. 4, fig. 6. The exopodites and endopodites of the first few segments of this specimen are better preserved than those of any other revealing them from the dorsal side, × 9.5.

Fig. 2. Specimen 220. A large individual exposed from the lower side. It shows well the endopodites and part of the exopodites, and, rather better than any other specimen, the endobases of the coxopodites. × 2.4.

Fig. 3. Specimen 216. A small entire specimen showing considerable of the detail of the appendages of the cephalon, and some of those of the remainder of the body, × 7.4.

Fig. 4. Specimen 201. This figure shows the details of the appendages of the left side and of the pygidium. Note the plate on the median line back of the pygidium, the sockets for spines, and the terminal spines on the anterior endopodites. See also pl. 2, fig. 1 and pl. 4, figs. 1, 2. × 7.1.

Fig. 5. Specimen 207. One half of the posterior part of the thorax and pygidium, showing exopodites and endopodites as seen from the dorsal side, × 7.6.

Fig. 6. Specimen 214. The exopodites have been turned back nearly parallel to the axis of the shell. Notice particularly the long flattened setæ and the spinose spatula-shaped terminal portion of each shaft. See also pl. 1, fig. 2. × 7.

PLATE III.

HELIOTYPE CO. BOSTON

* * * * *

PLATE 4.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 201. Another photograph, similar to fig. 4, pl. 3, but showing more clearly some details of spines on the endopodites. × 12.66.

Fig. 2. Specimen 201. Three appendages on the right side of the thorax. See also pl. 2, fig. 1 and pl. 3, fig. 4. × 12.66.

Fig. 3. Specimen 223. A small crushed specimen which nevertheless shows well the appendages of the right side of the thorax, developed from the ventral side. Note coxopodites, exopodites, and endopodites, and that all appendages are moved equally laterally from their original position. × 11.4.

Fig. 4. Specimen 219. Another photograph, with different lighting, of the individual shown in pl. 2, fig. 6. This print brings out better the coxopodites and the folds of the ventral membrane. × 3.23.

Fig. 5. Specimen 222. This specimen is interesting, because it shows the endopodites in what is probably their natural position, that is, in a plane nearly vertical to the plane of the body, instead of being flattened down, as is usually the case. The appendages under the pygidium are unusually well preserved. × 12.

Fig. 6. Specimen 204. Photograph of the entire specimen of which a part is shown in text fig. 42 and pl. 3, fig. 1. × 4.5.

PLATE IV.

HELIOTYPE CO. BOSTON

* * * * *

PLATE 5.

Photographs of _Triarthrus becki_, made by C. E. Beecher.

Fig. 1. Specimen 209. Photograph of the pygidium shown in pl. 6, fig. 2. This specimen shows especially well the way in which the exopodites of the pygidium decrease in length backward, × 11.5.

Fig. 2. Specimen 229. The under side of the posterior end of a medium-sized specimen, showing the appendages, especially the endopodites. On and among the limbs are scattered numerous minute spheres of pyrite, of the kind usually known as "trilobite eggs." They do not show very well in the photograph, but can be made out much more clearly with a hand lens, × 12.

Fig. 3. Specimen 230. A specimen showing the appendages of the posterior part of the thorax and the pygidium. The same individual is also shown in text fig. 44. Note particularly the form of the segments of the endopodites, and the spines on them, × 13.

Fig. 4. Specimen 227. The small doubly curved bodies shown in this figure lie under the axial portion of the cephalon and anterior part of the thorax. The specimen still has a very thin coating of matrix between it and the shell. Whether the curved bodies have anything to do with the trilobite is not known, × about 12.

Fig. 5. Specimen 221. A small individual which shows well the exopodites of the posterior part of the thorax. Note the spatulate terminations and the spines of the shaft, × 11.

Fig. 6. Specimen 202. Posterior part of the thorax and pygidium, showing endopodites and exopodites projecting under the dorsal test. Note the spiniferous plate on the median line, and the large opening in the anterior portion of it. × 9.75

PLATE V.

HELIOTYPE CO. BOSTON

* * * * *

PLATE 6.

All figures except 4 and 5, from photographs by C. E. Beecher.

Fig. 1. _Triarthrus becki_. Specimen 203. A well preserved small individual, showing the appendages of the right side of the thorax. × 11.46.

Fig. 2. _Triarthrus becki_. Specimen 209. A well preserved individual, showing the antennules and some appendages of thorax and pygidium. For detail of the pygidium, see pl. 5, fig. 1. × 4.

Fig. 3. _Triarthrus becki_. Specimen 218. Ventral side of the pygidium and greater part of the thorax of an individual of medium size. Note especially the relation of exopodites to endopodites of the last two thoracic segments. A drawing of these appendages is shown on text fig. 43. × 4,3.

Figs. 4 and 5. Endopodites, probably from a species of _Calymene_. These specimens, with several others, are on a small slab of limestone from the Point Pleasant (Trenton) beds opposite Cincinnati, Ohio. Specimen in the U. S. National Museum. Photographs by R. S. Bassler.

Fig. 6. _Acidaspis trentonensis_ Walcott. Both the specimen, No. 245, and the photograph are poor, but show that in this genus the endopodites are like those of Triarthrus. × 8.5.

Fig. 7. _Cryptolithus tessellatus_ Green. Specimen 234. This specimen shows well the backward directed antennules and also the outer segments of some of the cephalic endopodites. × 11.

PLATE VI.

HELIOTYPE CO. BOSTON

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PLATE 7.

Photographs of _Cryptolithus tessellatus_ Green, made by C. E. Beecher.

Fig. 1. Specimen 233. The best preserved individual, the one from which Professor Beecher's drawing (text fig. 45) was made, and which served as the principal basis for the restoration (text fig. 20). Note the long, backward directed antennules, the abrupt backward turn of the outer portions of the endopodites, the way in which the exopodites extend beyond the endopodites, and the fact that alt are beneath the cover of the dorsal shield. The hypostoma is turned entirely around. × 10.9.

Fig. 2. Specimen 235. Half of the thorax and pygidium, with the appendages revealed from the ventral side. Note the abrupt manner in which the outer portions of the endopodites are turned backward. See also pl. 8, fig. 3, and pl. 9, fig. 1 (right half). × 14.45.

Fig. 3. Specimen 236. Detail from fig. 4, to show the blade-like setæ of the exopodites and the numerous terminal spines of the endopodites. × 30.

Fig. 4. Specimen 236. The appendages of the thorax and pygidium, seen from the lower side. Specimen 236 is the right half of the same individual from which specimen 235 was obtained. Note the interarticular membranes between the segments of the endopodites and the blade-like setæ of the exopodites. See also pl. 9, fig. 1 (left side). × 19.

Fig. 5. Specimen 236. The same specimen, seen from the dorsal side, showing, when the test is removed, the long blade-like setæ of the exopodites. See also pl. 9, fig. 2 (right half). × 19.

PLATE VII.

HELIOTYPE CO. BOSTON

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PLATE 8.

Photographs of _Cryptolithus tessellatus_ Green, made by C. E. Beecher.

Fig. 1. Specimen 231. A nearly complete individual, cleaned from the ventral side and showing obscurely the hypostoma and fragments of numerous appendages. Note the lines of appendifers along the sides of the axial lobe. × 11.

Fig. 2. Specimen 232. Although this is not very well preserved, it shows more of the cephalic appendages than any other. Even so, only just enough is shown to indicate that they were similar to those on the thorax. × 12.

Fig. 3. Specimen 235. Dorsal side of the appendages of the thorax and pygidium. See pl. 7, fig. 2 for the ventral view. On pl. 9, fig. 2 (left side) is a drawing taken from the same specimen. × 11.

Fig. 4. Specimen 238. Part of a thorax and pygidium, seen from the ventral side. The series of heavy segments shown in the upper part do not belong to one appendage, but are the distal ends of several endopodites. See also text fig. 46 for a drawing of this specimen. × 18.

Fig. 5. Specimen 237. Pygidium and part of the thorax, with some of the appendages. × 11.

PLATE VIII.

HELIOTYPE CO. BOSTON

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PLATE 9.

_Cryptolithus tessellatus_ Green. Upper drawing by C. E. Beecher; lower drawing by Miss F. E. Isham, under the direction of C. E. Beecher.

Fig. 1. Appendages of the thorax and pygidium, seen from the ventral side. These are not restorations, but drawings from the halved individual numbered 236 (right side of drawing) and 235. For photographs of these specimens, see pl. 7, figs. 2, 4. × 20.

Fig. 2. Appendages of the thorax and pygidium, seen from the dorsal side. Same specimen as in fig. 1. For photographs, see pl. 7, fig. 5, and pl. 8, fig. 3. × 20.

PLATE IX.

HELIOTYPE CO. BOSTON

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PLATE 10.

From photographs made by C. E. Beecher.

Fig. 1. _Isotelus latus_ Raymond. Ventral surface of the specimen in the Victoria Memorial Museum at Ottawa, Canada. Note the large, club-shaped coxopodites and the more slender endopodites. The first large coxopodite back of the hypostoma belongs to the last pair of cephalic appendages. The coxopodite of the appendage in front of it is seen turning in beneath the tip of the hypostoma. × 2.

Fig. 2. _Isotelus maximus_ Locke. The ventral side of the specimen described by Mickleborough and now in the U. S. National Museum. The tips of the hypostoma may be seen at the front, and the first two pairs of coxopodites behind them belong to the last two pairs of appendages of the cephalon. Note how much stronger the coxopodites are than the endopodites. The appendages of the pygidium show but poorly, × 1.45.

PLATE X.

HELIOTYPE CO. BOSTON

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PLATE 11.

_Ceraurus pleurexanthemus_ Green. A restoration of the ventral surface and appendages, made by Doctor Elvira Wood, under the supervision of the writer, from data obtained from the translucent slices prepared and described by Doctor Walcott. × 5.

PLATE XI.

HELIOTYPE CO. BOSTON

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Transcriber's Notes

Small captioned text was not converted to ALL CAPS. The numer 1 and capital I both look alike in the printed version. Therefore, some of the volume, plate and other roman numerals may have been incorrectly converted to 1. Some tables were reformatted due to space considerations.