PART I.

THE APPENDAGES OF TRILOBITES.

Terminology.

The terminology employed in the succeeding pages is essentially the same as that used by Beecher, with two new terms added. Beecher assigned to the various segments of the limbs the names suggested by Huxley, but sometimes used the name protopodite instead of coxopodite for the proximal one. It is obvious that he did not use protopodite in the correct sense, as indicating a segment formed by the fusion of the coxopodite and basipodite. The usage employed here is shown in figure 1.

The investigation of _Ceraurus_ showed that the appendages were supported by processes extending downward from the dorsal test, and on comparison with other trilobites it appeared that the same was true in _Calymene_, _Cryptolithus_, _Neolenus_, and other genera. Thin sections showed that these processes were formed by invagination of the test beneath the dorsal and glabellar furrows. While these processes are entirely homologous with the entopophyses of _Limulus_, I have chosen to apply the name _appendifer_ to them in the trilobites.

The only other new term employed is the substitution of _endobase_ for gnathobase in speaking of the inner prolongation of a coxopodite of the trunk region. The term gnathobase implies a function which can not in all cases be proved.

The individual portions of which the limbs are made up are called _segments_, and the articulations between them, _joints_. Such a procedure is unusual, but promotes clearness.

The Appendages of Neolenus.

HISTORICAL.

The first mention of _Neolenus_ with appendages preserved was in Doctor Walcott's paper of 1911, in which two figures were given to show the form of the exopodites in comparison with the branchiæ of the eurypterid-like _Sidneyia_. In 1912, two more figures were presented, showing the antennules, exopodites, and cerci. The specimens were found in the Burgess shale (Middle Cambrian) near Field, in British Columbia. This shale is exceedingly fine-grained, and has yielded a very large fauna of beautifully preserved fossils, either unknown or extraordinarily rare elsewhere. It was stated in this paper (1912 A) that trilobites, with the exception of _Agnostus_ and _Microdiscus_, were not abundant in the shale.

In discussing the origin of the tracks known as _Protichnites_, Walcott presented four figures of _Neolenus_ with appendages, and described the three claw-like spines at the tip of each endopodite.

Three new figures of the appendages were also contributed to the second edition of the Eastman-Zittel "Text-book of Paleontology" (1913, p. 701). Later (1916, pl. 9) there was published a photograph of a wonderful slab, bearing on its surface numerous Middle Cambrian Crustacea. Several of the specimens of _Neolenus_ showed appendages.

Finally, in 1918, appeared the "Appendages of Trilobites," in which the limbs of _Neolenus_ were fully described and figured (p. 126), and a restoration presented. Organs previously unknown in trilobites, epipodites and exites, attached to the coxopodites, were found.

=Neolenus serratus= (Rominger).

(Text fig. 2-8.)

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 20, pl. 6, figs. 1, 2 (exopodites of thorax and cephalon);--Ibid., vol. 57, 1912, p. 191, pl. 24, figs. 1, la (antennules, caudal rami, and endopodites of thorax);--Ibid., vol. 57, 1912, p. 277, pl. 45, figs. 1-4 (antennules, endopodites of cephalon and thorax, caudal rami);--Text-book of Paleontology, edited by C. R. Eastman, 2d ed., vol. 1. 1913, p. 701, fig. 1343 (exopodites), p. 716, fig. 1376 (abdominal appendages), fig. 1377 (appendages of thorax and pygidium);--Ann. Rept. Smithson. Inst. for 1915, 1916, pl. 9;--Smithson. Misc. Coll., vol. 67, 1918, pp. 126-131 et al., pl. 14, fig. 1; pls. 15-20; pl. 21, fig. 6; pls. 22, 23; pl. 31 (restoration); pl. 34, fig. 3 (restored section); pl. 35, fig. 4; pl. 36, fig. 3 (hypostoma).

The following description of the appendages of _Neolenus_ is summarized from Walcott's paper of 1918, and from a study of the eight specimens mentioned below.

_Cephalon._

The antennules are long, slender, and flexible, and lack the formal double curvature so characteristic of those of _Triarthrus_. There are short fine spines on the distal rims of the segments of the proximal half of each, thus giving great sensitiveness to these organs. In the proximal portion of each, the individual segments are short and wider than long, and in the distal region they are narrow and longer than wide.

There are four pairs of biramous cephalic appendages, which differ only very slightly from the appendages of the thorax. All are of course excessively flattened, and they are here described as they appear.

The coxopodites, shown for the first time in Walcott's paper of 1918, are broad, longer than wide, and truncated on the inner ends, where they bear short, stout, unequal spines similar to those along the anterior margin. The gnathobases are but slightly modified to serve as mouth parts, much less so than in _Triarthrus_, but the coxopodites of the cephalon are shorter and wider than those of the thorax.

At the distal end of the coxopodite arise the endopodite and exopodite. The endopodite consists of six segments, the distal ones, propodite and dactylopodite, more slender than the others, the last bearing three terminal spines. The first endopodite is shorter than the others and slightly more slender (pl. 16, fig. 1)[1] and the anterior appendages turn forward more or less parallel to the sides of the hypostoma (pl. 22). The basipodite, ischiopodite, meropodite, and carpopodite are, in their flattened condition, roughly rectangular, only a little longer than wide, taper gradually distally, each bears small spines on the outer rim, and some of the proximal ones usually have a row along the margin.

[Footnote 1: _Nota bene!_ All references in this section are to the plates of Doctor Walcott's paper in 1918.]

The exopodites of the cephalon, as of the body of Neolenus, are very different from those of any other trilobite whose appendages were previously known. As shown in the photographs (pl. 20, fig. 2; pl. 22), each exopodite consists of a single long, broad, leaf-like blade, not with many segments as in _Triarthrus_, but consisting of a large basal and small terminal lobe. It bears on its outer margin numerous relatively short, slender, flat setæ. The long axes of the exopodites point forward, and the setæ are directed forward and outward. They stand more nearly at right angles to the shaft on the cephalic exopodites than on those of the thorax. This same type of broad-bladed exopodite is also found on the thorax and pygidium.

The number of functional gnathobases on the cephalon is unknown. That four endopodites were present on one side is shown pretty clearly by specimen 58591 (pl. 16, fig. 3) and while no more than two well preserved exopodites have been seen on a side, there probably were four. Specimen 65513 (pl. 16, fig. 1) shows gnathobases on the second and third appendages of that individual as preserved, but there is no positive evidence that these are really the second and third appendages, for they are obviously displaced. The hypostoma of Neolenus is narrow but long, several specimens showing that it extended back to the horizon of the outer ends of the last pair of glabellar furrows. It is not as wide as the axial lobe, so that, while gnathobases attached beneath the first pair of furrows would probably not reach back to the posterior end of the hypostoma, they might lie parallel to it and not extend beneath. It seems possible, then, that there were four pairs of endobases but that the second rather than the first pair served as mandibles, as seems to be the case in Ceraurus.

_Thorax._

The thorax of _Neolenus_ consists of seven segments, and the appendages are well shown (pl. 17, fig. 1; pl. 18, figs. 1, 2; pl. 20, fig. 1.), The endopodites of successive segments vary but little, all are slender but compact, and consist of a long coxopodite with six short, rather broad segments beyond it. In the figures, the endopodites extend some distance in a horizontal direction beyond the edges of the dorsal test, as many as four segments being in some cases visible, but measurements show that the appendages tended to fall outward on decay of the animal. The dactylopodites are provided with terminal spines as in _Triarthrus_. The coxopodites are long, straight, and slender. They are well shown on only one specimen (pl. 18), where they are seen to be as wide as the basipodite, and the endobases are set with spines on the posterior and inner margins. They are so long that those on opposite sides must have almost met on the median line. The segments of the endopodites are mostly but little, if any, longer than broad, and at the distal end each shows two or more spines. The propodite and dactylopodite are notably more slender than the others. The exopodites of the thorax are broad and flat, and each shaft has two distinct parts with different kinds of setæ. The posterior edge of the proximal lobe is fringed with a slender, flat, overlapping hairs which are a little longer than the width of the lobe, and stand at an angle of about 60 degrees with the direction of the axis of the appendage. The outer lobe is at an angle with the main one, and has short, very fine setæ oh the margin. One or two specimens show some evidence of a joint between the inner and outer lobes, but in the great majority of cases they seem to be continuous; if originally in two segments, they have become firmly united. The exopodites of the thorax, like those of the cephalon, are directed diagonally forward and outward. (pl. 21, fig. 6; pl. 22.)

_Pygidium._

The pygidium of _Neolenus serratus_ is large, and usually shows five rings on the axial lobe and four pairs of ribs on the sides. There are five pairs of biramous appendages belonging to this shield, and behind these a pair of jointed cerci. That the number of abdominal appendages should correspond to the number of divisions of the axial lobe rather than to the number of ribs on the pleural lobes is of interest, and in accord with other trilobites, as first shown by Beecher.

The endopodites of the pygidium have the same form as those of the thorax, are long, and very much less modified than those of any other trilobite whose appendages are known. On some specimens, they extend out far beyond the dorsal test, so that nearly all the segments are visible (pl. 17, fig. 3; pl. 18; pl. 19; pl. 20, fig. 1), but in these cases are probably displaced. The segments are short and wide, the whole endopodite tapering gradually outward. The dactylopodite bears terminal spines, and the individual segments also have outward-directed spines.

The cerci appear to have been long, slender, very spinose organs much like the antennules, but stiff rather than flexible. They are a little longer than the pygidium (pl. 17, figs. 1, 2), and seem to be attached to a plate on the under surface of the posterior end and in front of the very narrow doublure. The precise form of this attachment can not be determined from the published figures. They bear numerous fine spines (pl. 17, fig. 3).

_Epipodites and Exiles._

Doctor Walcott has found on several specimens of _Neolenus_ remains of organs which he interprets as epipodites and exites attached to the coxopodites. A study of the specimens has, however, convinced me that both the large and small epipodites are really exopodites, and that the exites are badly preserved and displaced coxopodites. Detailed explanation of this interpretation is given below in the description of the several specimens involved.

_Description of Individual Specimens._

Doctor Walcott was kind enough to send me eight of the more important specimens of _Neolenus_ figured by him, and since my interpretation of them does not agree in all respects with his, I have thought it fairer to the reader to present here rather full notes explaining the position I have taken. I understand that since I communicated my interpretation of the epipodites and exites to him, Doctor Walcott has submitted the specimens to several palæontologists, who consider that epipodites are really present. Since I am not able to convince myself that their conclusion is based upon sound evidence, I give here my own interpretation. There is of course, no a priori reason why trilobites should not have had epipodites.

Specimen No. 58589.

Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 45, fig. 2;--Zittel-Eastman Text-book of Paleontology, vol. 1, 1913, fig. 1377;--Smithson. Misc. Coll., vol. 67, 1918, pl. 18, fig. 1; pl. 20, fig. 1.

This is one of the most important of the specimens, as it shows the coxopodites of three thoracic limbs and the well preserved endopodites of six thoracic and five pairs of pygidial appendages.

The appendages are all shifted to the left till the articular socket of the coxopodite is about 8 mm. outside of its proper position. The endopodites extend a corresponding amount beyond the edge of the dorsal test and are there so flattened that they are revealed as a mere impression. The coxopodites, which are beneath the test, seem to have been somewhat protected by it, and while hopelessly crushed, are not flattened, but rather conformed to the ridges and grooves of the thorax.

The coxopodite of the appendage of the last thoracic segment is best preserved. It is rectangular, about one third as wide as long, with a slight notch in the posterior margin near the outer end. The inner end is obliquely truncated and shows about ten sharp spines which do not appear to be articulated to the segment, but rather to be direct outgrowths from it. There are similar spines along the posterior margin, but only two or three of what was probably once a continuous series are now preserved. On the opposite margin of the coxopodite from the slight depression mentioned above, there is a slight convexity in the outline, which is better shown and explained by the coxopodite just in front of this. That basal segment has the same form as the one just described, but as its posterior margin is for the greater part of its length pushed under the one behind it, the spines are not shown. On the posterior margin, two-thirds of the length from the proximal end, there is a shallow notch, and corresponding to it, a bulge on the anterior side. From analogy with Ceraurus and _Calymene_ it becomes plain that the notch and bulge represent the position of the socket where the coxopodite articulated with the appendifer. Since these structures have not been shown in previous illustrations, a drawing giving my interpretation of them is here inserted (fig. 2). It is evident from the position of the notch that the row of spines was on the dorsal (inner) side of the coxopodite and that the truncation was obliquely downward and outward.

The endopodite of the last thoracic appendage is well preserved and may be described as typical of such a leg in this part. The basipodite is as wide as the coxopodite, and it and the three succeeding segments, ischiopodite, meropodite, and carpopodite, are all parallel-sided, not expanded at the joints, and decrease regularly in width. The propodite and dactylopodite are also parallel-sided, but more slender than the inner segments, and on the end of the dactylopodite there are four little spines, three of them--one large and two small--articulated at the distal end, and the fourth projecting from the posterior outer angle. Each segment has one or more spines on the outer articular end, and the ischiopodite has several directed obliquely outward on the posterior margin. All of the four proximal segments show a low ridge parallel to and near the anterior margin, and several endopodites of the pygidium have a similar ridge and a row of spines along the posterior margin of some of the segments. These features indicate that the segments in question were not cylindrical in life, but compressed. From the almost universal location of the spines on the posterior side of the limbs as preserved, it seems probable that in the natural position the segments were held in a plane at a high angle with the horizontal, the ridge was dorsal and anterior and the row of spines ventral and posterior. Because the spines on the endobases are dorsal it does not follow that those on the endopodites were, for the position of the coxopodite in a crushed specimen does not indicate the position of the endopodite of even the same appendage.

The endopodites of the pygidium are similar to the one just described, except that some of them have spines on the posterior margin of the segments, and a few on the right side have extremely fine, faintly visible spines on the anterior side. The specimen shows fragments of a few exopodites, but nothing worth describing. In the middle of the right pleural lobe there is a small organ which Walcott has interpreted as a small epipodite. It is oval in form, broken at the end toward the axial lobe, and has exceedingly minute short setæ on the posterior margin. From analogy with other specimens, it appears to me to be the outer end of an exopodite.

_Measurements:_ The entire specimen is about 64 mm. long and 52 mm. wide at the genal angles. The thorax is about 41 mm. wide (disregarding the spines) at the seventh segment, and the axial lobe about 13 mm. wide at the same horizon. The measurements of the individual segments of the seventh left thoracic limb are:

Coxopodite, 9 mm. long, 3 mm. wide, the middle of the notch 8 mm. from the inner end, measured along the bottom, and 6 mm. measured along the top. Basipodite, 5 mm. long, 3 mm. wide Ischiopodite, 4 " " 3 " " Meropodite, 3.5 " " 2.5 " " Carpopodite, 3.5 " " 2 " " Propodite, 3 " " 1.25 " " Dactylopodite, 2 " " 1.25 " "

The five distal segments of the last pygidial endopodite are together 10.5 mm. long. The whole six segments of the endopodite of the third thoracic segments are together 21 mm. long. The distance from the appendifer of the third segment to the outer end of the spine is 17