Part 6

This species, of which only the hypostoma is known, found at Lansa, Gotland, is likely, to judge by that, to be nearly related to Br. polyactin. The shape of this hypostoma is exactly the same and the maculæ are placed in the same position, but they are larger, a little sunk on the blind surface and the group of the lenses is different. Its superior margin forms an ingoing arch and the lenses themselves are not convex, but rounded or slightly polyedric, and separated through thick interspaces.

=Bumastus= MURCHISON.

The only recorded hypostoma with maculæ belongs to B. insignis HALL as described by SALTER Monogr. p. 208 pl. 27 fig. 7. He says: »A pair of compressed tubercles occurs at the lower third: they are transverse-ovate, and more than their own diameter apart.» This is nearly in concordance with what is seen on the species to be described below.

Although it can in reality be said that the species of Bumastus are the most common of all trilobites in the Silurium of Gotland, in so far that their head pieces and pygidia are met with everywhere in the limestone rocks of the island, the find of entire specimens or of detached hypostomas is amongst the rarest events when collecting there and amongst thousands of fragments, in several places entirely filling large portions of the limestone rock, there have in the Swedish State museum been acquired only three or four hypostomas, which we are now going to describe.

At first we must point out a certain resemblance which prevails between the hypostomas and maculæ in Bumastus and Bronteus. Compare for instance Bum. sulcatus with Br. platyactin and Br. laticauda. The general form is nearly the same in both and the shape of the maculæ almost identical but for the complete want of granulations on those of Bumastus. To this resemblance must be added that there are, as known, forms of trilobites which by some authors have been regarded as Brontei and by others, again, ranked amongst the Illænidæ.

Three species of Bumastus have been recorded as found in the Gotland strata, but of one of these no hypostoma is known. Besides, to judge by the head shields and hypostoma, there are a few new species. To begin with the most common

=B. sulcatus= LDM.

Pl. II figs. 33-40.

The hypostoma is broadly shield-formed and the anterior margin is most characteristic being in its central part elongated in a short evenly rounded projection. The side wings are triangular acuminate, bent a little backwards and towards the interior surface. There is only a single rounded field passing into an evenly bent border, in front of which the projecting, elliptic maculæ lie. A second pair of wings stretches backwards near the posterior margin. We have not obtained any sections of the maculæ.

The cephalic eyes figs. 36-38 are covered with an uncommonly thick stratum of homogenous shell, one and a half times as thick as the subjacent lenses. These are comparatively broad, above passing indistinctly in the covering stratum, below, at their basis, convex. In a horizontal section figs. 33, 34 they exhibit the common hexaedral or pentaedric aspect, their exterior sides being white and confluent without any separating lines. Their interior is dark and from the polygonal sides white lines go in giving the whole the appearance as of a starry, composite coral with its septa. The correspondence in structure with Bronteus laticauda pl. II fig. 7 is striking. A narrow space separates the surface of the eye from a nearly similar patch below the eve on the cheek 35 _b_, 38 _b_. But there the position of the composing strata is singularly reversed and the spongious or pseudo-prismatic stratum lies close to the outside and the homogenous stratum on the inside. The white longitudinal lines are well marked out and regular in the vertical section, fig. 38 _b_, indicating the prismatic structure most clearly.

=B. barriensis= MURCH.

Pl. II figs. 31, 32.

The hypostoma fig. 31 is of so transverse a shape that the breadth much surpasses the height, the former being 6 millim., while the latter attains only 3,5 millim. The anterior margin is so much arcuated and sloping backwards that it forms an obtuse angle. The side wings are triangular and acuminated and from them a thick pad runs backwards and forms the posterior margin. Below the largest field of the exterior surface the maculæ rest on a ridge having a much elongated, vermiform shape and sunk in the surface (fig. 32) a little more than three times longer than broad. In its shape it approaches much to the hypostoma of Bum. insignis HALL as delineated by SALTER pl. 27 fig. 7.

In another, undetermined Bumastus (pl. II figs. 47, 48)[40] the hypostoma (from Klints, Othem, isle of Gotland) is of a more elongate obovate form, the anterior margin elevated, rounded, the terrace lines faint, the tubercles are larger than in the former Bumasti, oblong, curved, beanshaped, evenly rounded and smooth.

[Footnote 40: Probably the same as Bum. sulcatus pl. XII fig. 12 in my paper on the Trilobites of Gotland 1885.]

Another of the new species is one that LILJEVALL has found at Korpklint near Wisby (Pl. II figs. 41-46). The hypostoma is more elongate than transverse with the anterior margin regularly arcuated, by far not so prominent as in B. sulcatus and the former. By a shallow groove the exterior surface is divided in two fields, the anterior one the largest and covered with a few, distantiated, concentric terrace lines. The posterior field is smooth, nearly even and the maculæ lie just in the groove, with their axis oblique to the longitudinal axis of the hypostoma, beanshaped and smooth. The cephalic eyes show in a horizontal section hexaedral white walls enclosing a dark space in which some indistinct radii emanate, fig. 41 _a_. In the vertical sections the prisms are narrow and elongate and the integument by far not so thick as in Bumastus sulcatus, fig. 42, 43. The border zone of the eye in this (fig. 41) as well as in B. sulcatus, as seen in horizontal sections, agrees with that of the Asaphidæ.

=Bumastus= sp. inlet.

Pl. III fig. 1, 2.

In a detached piece of limestone, found in Gotland, at Norderstrand, near Wisby, two hypostomas and a free cheek lay embedded along with a coral of the genus Acantholithus and a Bronteus probably belonging to a new species. As the coral has not been found above the uppermost beds of the Lower Silurian it is probable that the trilobites also are derived from the same horizon.

The hypostoma, figured pl. III fig. 1, is broadly tongueshaped, with a short, blunt, triangular wing on each side of the anterior margin. The exterior side is nearly completely occupied by a single large field, which is convex and decorated by sparse terrace lines. Below this a smooth, nearly even plane, reaching to the inferior, elevated border of the hypostoma. The two tubercular macula' are situated exactly on the boundary line between the convex and the plane field. They are elongately ovate, pointed outwards, rounded inwards, with smooth and glossy surface. That portion, on which in the species of Bronteus the granular spot rests, is transversally wrinkled by some faintly elevated, sigmoid lines (fig. 2). It would have been of great interest to investigate the interior structure of these curious maculæ, but the scarcity of the material has prevented our doing so.

The other hypostoma, pl. III fig. 3-5, is larger and has the terrace lines more distantiated and forming more open curves, but for the rest it is of the same shape as the smaller, so that we may not ascribe these small deviations to a specific difference. The maculæ are more elongated, narrow, curved tubercles, thus differing from the next preceding. A section running through its left tubercle does not show any structure, but the shell of the maculæ is extremely thin, thus contrasting strongly with the surrounding thicker shell, fig. 4 _a_. The cephalic eye, again, remaining on the free cheek found in the same piece of limestone and probably belonging to the same specimen, has its surface well preserved and exhibits semiglobular lenses in low relief, fig. 5.

=Calymmene= BRONGN.

In the following species of this genus hypostomic tubercles have formerly been recorded.

_Cal. Blumenbachi_ SALTER Monogr. pl. 8, fig. 9. The identity of the species is dubious.

_Cal. duplicata_ SALTER Mon. pl. 9, fig. 22. The tubercles rather more linear and approximated than in the subsequent. SALTER calls them »two small transverse lobes ... forming a nearly continuous ridge».

_Cal. tuberculata_ ANGEL. Tab. XIX fig. 5 c from the inside.

It is remarkable that the eyes or that part of them where the cornea and the lenses should be found, has been destroyed and entirely lost in all specimens of the Calymmene known to us, many hundreds having been searched and every one with a lacuna on that spot. It must have been of an excessive thinness. We know only of one instance, the Bohemian Calym. Arago, in which BARRANDE has seen the eye intact and provided with a small number of about eleven sphærical lenses.

=Calymmene intermedia= LDM.

Pl. III figs. 6, 7.

The clypeiform hypostoma is divided in two fields, one anterior larger, in the centre elevated in a short, blunt knob, and a posterior, bearing the two elevated rounded maculæ, just below the semicircular groove which separates it from the anterior field. These fields are surrounded by a large border which is posteriorly emarginated through a short rounded sinus. The surface is entirely covered by small semiglobular wartlets perforated by a straight pore or duct, which being filled with a black mineral, probably iron pyrites, gives a peculiar and characteristic aspect to the sections. In the same manner the whole mass of the hypostoma is pierced by straight, black lines, also filled up pores, starting from the outside and ending before attaining the inner surface.

The maculæ are prominent, separated from the hypostoma through narrow, distinct grooves, ovate, with the pointed end directed obliquely outwards, against the margins of the hypostoma. They are covered with wartlets and leaving only a little oblong spot free, quite smooth, this being consequently the macula proper. But in a longitudinal section there is not the least distinction between this macula and the other hypostoma, only, that it is free of pores.

=Calymmene tuberculata= BRÜNNICH.

Pl. III figs. 8, 9.

We have succeeded to prepare a horizontal section of the macula. This is oval, perfectly homogenous and without any structure proper, showing only indistinctly a mottled medley of pale, brownish spots in the clear mass. There is in both maculæ a small dark, angular spot placed in their inferior part, fig. 9.

=Centropleura= ANGELIN.

This author has delineated most distinct maculæ in Centr. Steenstrupi pl. III fig. 4, b. two, small elongate darkish spots, near the posterior margin. I have not seen any specimen of this trilobite and have consequently not been able to follow up ANGELIN'S observation.

=Chasmops= M'COY.

Pl. III fig. 10.

In his work on the Russian Phacopidæ FRIEDR. SCHMIDT has given the figures of Chasmops Eichwaldi and Ch. Wesenbergensis showing, as it were, patches of black maculæ. But as there is no mention of them, and as they are quite unlike the macula? which are described here, I am in doubt of their real nature. We have figured a macula from the hypostoma of a Chasmops, probably Ch. macroura, from a drift block near Rostock. It is elongate and narrow, more than four times longer than broad. Its smooth surface is a little scooped out and no structure is observable.

=Chirurus= BEYRICH.

We find the hypostomic maculæ annotated and delineated as occurring in the following species.

_Ch. bimucronatus_ SALTER pl. 5, fig. 5 as two narrow elongated pits, surface seems intact. No mention of them in the letter-press.

_Ch. exsul_ SCHMIDT pl. VI fig. 9. Indications of the oblong, narrow maculæ.

_Ch. macrophthalmus_ SCHM. pl. VII, figs. 1 c, 2. Faint indications of narrow pits.

_Ch. Quenstedti_ BARR. Tab. 42 fig. 3. Maculæ near lower border as two narrow slits.

_Ch. spinulosus_ SCHM. pl. VII fig. 10. The lateral pits well visible.

_Ch. tumidus_ SCHM. pl. VIII, fig. 22. The pits are short, but present.

The exterior surface of the hypostoma is entirely covered with small wartlets, closely set, of different size in the various species, and spread between them lie a great number of elongate smooth spots dispersed, on the interior side of the hypostoma showing an excavated surface (pl. III fig. 16).

=Chirurus (Cyrtometopus) clavifrons= DALM.

Pl. III figs. 17-21.

The shape of the hypostoma is shown in fig. 21 pl. III. It is regularly shield-formed with posterior margin rounded. A shallow groove running parallel with the margins forms a large, faintly elevated border. The anterior margin projects on both sides in a short, backwards directed, broad and flat wing, hollowed out with a pit on the front side, and there is a posterior lateral wing, midways between the anterior and posterior margin short, blunt, directed obliquely backwards. The whole exterior surface is covered with diminutive tubercles, the above mentioned smooth spots interspersed on the central disk. A little below the middle there are two lengthened pits, so shallow that they often are not discernible. In a specimen of Chirurus spinulosus NIECZKOWSKI from Estland there are two, almost 3 mm. long dots, one on each side above the maculæ as the fig. 21 shows and in several others of this generic group there are also indications of similar. I cannot compare these dots with anything more than the lengthened spots visible in Acidaspis crenata.

The maculæ are wanting in this species, but the cephalic eyes are well developed. A section of them fig. 19 shows the enormous difference of the shell in the eye and the surroundings where the shell surpasses it many times in thickness. The eye consists in fact only of the ovate, beadlike lenses of which a string is seen sectioned in the figure mentioned. More enlarged (fig. 20) a nucleus is visible in each lens, and in a horizontal section (fig. 18) a little below the surface, where they are more pressed against themselves, they have a polygonal shape. The figure 17 seems to represent lenses that have been much changed interiorly, having only a narrow zone left of the primary structure.

=Chirurus glaber= ANGELIN.

Pl. III fig. 11.

As a sample of the shape of the surface of the eyes in this genus, we have given a figure, showing the small rounded lenses.

=Chirurus ornatus= DALMAN.

Pl. III figs. 12-14.

The cephalic eyes have globular lenses, of more than double the size of those in Chir. clavifrons. The difference between the lenses and the surrounding shell amounts at the highest to thrice the former, while in Ch. clavifrons it is at least seven times as much. In a horizontal section the lenses have a little, darkish irregular nucleus surrounded by a radiated structure (fig. 12).

The maculæ (fig. 14) are slightly concave, oblong, smooth, surrounded by comparatively large granulations.

=Chirurus speciosus= DALM.

Pl. IV fig. 1.

The exterior surface of the hypostoma is sparingly covered by granules of larger size than in other Chiruri. The maculæ as seen in a cast are prominent near the posterior border, sunk in their centre and surrounded by a narrow distinct border line. In a horizontal section it as not been possible to detect any peculiar structure, only a dark border surrounding the interior clear surface.

_Chirurus spinulosus_ NIECZKOWSKI.

Pl. III fig. 15.

We have had the interior surface of a hypostoma at our disposal from the lower Siluria, of Esthonia (Kuckers C^2) belonging to Dr. G. HOLM. The macula are well preserved, standing out black on the white surface, oblong, 1,5 millim. in greatest length (fig. 15). There are indications as of a great number of small lenses on these maculæ. Remarkable are the two elongated spots, almost 3 mm. in length, above these maculæ, of which already mention has been made under Ch. clavifrons.

=Chirurus= sp. indet.

Pl. III fig. 16.

The hypostoma of an undescribed species from Öland. The maculæ are lengthened, ellipsoid and smooth tubercles lying in a groove. The finely granulated surface of the hypostoma shows rare smooth spots interspersed, being on the inside smooth pits, from which again in casts of the hypostoma larger tubercles are moulded. Of the same nature are those that are visible on the nucleus of Chir. speciosus and others. There are still some undescribed Lower Silurian species, for instance one nearly related to Ch. conformis with tubercular maculæ. A large hypostoma from the Leptæna limestone of Dalecarlia, 33 millim. in length, has a longitudinal macula, 2 millim. in length, as seen on the inside of the fragment.

On pl. VI fig. 10 a little hypostoma is delineated that shows some resemblance with that of the Chirurus, but probably belongs to some other, unknown genus. It is Upper Silurian, found at Mulde in Fröjel, Gotland. It is of an elongated ovate shape, with the anterior border faintly arched, the anterior wings broad, truncate. A narrow elevated border surrounds the lateral and posterior margins. The somewhat vaulted surface is covered by fine granulations, and the macula, situated on equal distance from the anterior and posterior margins are ovate, smooth and directed obliquely inwards. There are two minute pointed processes on each side of the lateral borders and one on each side of the pointed posterior margin.

=Ctenopyge= LINSN.

=Ct. spectabilis= BR. (I, pl. XII, fig. 12 a).

A small lengthened, apparently smooth hypostoma with two globular tubercular maculæ near the posterior border. No mention made of them in the description of BRÖGGER. Cambrian. I am not, however, quite sure whether this really belongs to a Ctenopyge, as it rather more resembles the type of a Peltura as shown in Pelt. scarabæoides. As Ctenopyge is so nearly related to Sphærophthalmus it could be expected to see its hypostoma of the same type and the maculæ entirely wanting. The eyes of this genus have been described above at page 29.

=Cybele= LOVÉN.

=Cyb. bellatula= DALMAN.

Pl. IV fig. 2.

We give a new figure of the hypostoma to complete that of FR. SCHMIDT, pl. XIII fig. 9. The anterior wings are larger and obtuse, excavated below and the border rim is not so distinct all round as he has figured. There are two pair of oblique, lengthened grooves above each other. The opposite do not, however, join to form a coherent groove across the median field of the hypostoma, which is level between them. No maculæ. Nearer the anterior border close below the wings, there is a small roundish, dark spot. The surface of the hypostoma is granulated.

=Cyphaspis= ANG.

=Cyph. elegantula= ANG.

Pl. III figs. 22-25.

The hypostoma is elongated, anterior margin evenly arched, anterior wings triangular, lateral margins faintly curved as well as the posterior margin, at both sides of which there is one diminutive spine. The posterior wings are short forming a spiny process from the lateral margin. The surface of the hypostoma is smooth with a shallow transverse groove on the inferior moiety and two corresponding lateral impressions below it, one on each side. The elevated lateral borders are longitudinally striated by a few terrace lines. There are no maculæ visible. The eyes consist f short hexagonal prisms (figs. 22, 23) nearly resembling those of Proetus.

=Dalmanites= EMMRICH.

In this large genus the hypostoma has been figured in a great number of species. But it is only in the following that we, at least in the figures, if not in the descriptions, are able to detect the maculæ.

_D. atavus_ BARR. Suppl. pl. 5 fig. 14.

_D. Calypso_ J. HALL, vol. VII, pl. XI A. fig. 21 p. 66 »Postero-lateral pits, moderately strong and elongate». There are casts of the two well distinct maculæ.

_D. caudatus_ BRÜNN.

a. FORBES and SALTER in British Organic remains, Dec. II pl. 1, fig. 3 give a good figure, showing the oblique pitlike maculæ. On page 2 is told that »a pair of lateral strong indentations indicate a second furrow above» (»the transverse furrow between the tip»).

b. SALTER, pl. 3, figs. 7, 8, bad figures, the description verbally the same as above. What ANGELIN pl. VIII, fig. 2 c gives as the hypostoma of »Ph. caudata» is in fact the hypostoma of a species of Lichas.

_D. Mac Coyi_ BARR. Supl., pl. 13 f. 32. This magnificent hypostoma belongs to the same group as D. micrurus HALL as to which we may hesitate whether there not be a double pair of maculæ above each other near the posterior border. In this species, however, there is the transverse lower groove and above probably the two maculæ united by a groove so closely as to resemble the lower groove.

_D. micrurus_ J. HALL, Pal. N. Y. III pl. 74 fig. 20. Shows, as it were, a double pair of maculæ, very like D. spinifer BARR.

_D. rugosus_ BARR. pl. 24 f. 23. The two maculæ in the ordinary place, without being united by a groove.

_D. socialis_ BARR. pl. 26 f. 21. Rather of an uncommon shape, not so triangular as in the other species, the two maculæ distinct as narrow slits. P. 553 »Vers l'extrémité du corps central, on aperçoit de chaque côté, près du bord, une impression oblique, alongée et arquée».

_D. spinifer_ BARR. Pl. 25, fig. 20. A large beautifully preserved hypostoma, shows what I think we positively must interpret as a double pair of maculæ close above each other. As stated above there is reason to believe that in other species of Dalmanites there also are indications of four maculæ, though not so evident as in this species. BARRANDE says nothing about this remarkable feature.

=Dalmanites imbricatulus= ANGELIN.

Pl. III figs. 43, 44.

The interesting image, as given by the agglomerated eyes, is represented in the magnified figure 43 Plate III. The lenses lie there separated from each other at much varying distance, some in close contact. The presence of the delicate covering membrane is clearly seen, being of white colour contrasting with the black, glossy lens. It is much lacerated and only preserved around the periphery. The size of the ocelli is on an average 0,5 mm., a little one has 0,3 mm. The surface between the ocelli is most finely granulated. The granules scarcely attaining the fourth of the granules in Dalm. vulgaris.

In a vertical section the regular biconvex lenses are seen to be covered with the extremely thin integument, which is a direct continuation from the test. The test between the lenses is perforated by some longitudinal tubes, as usual in the skeleton of the trilobites. We have not found any hypostoma with the maculæ.

=Dalmanites obtusus= LDM.

Pl. III figs. 45, 46.

We have not succeeded in finding any hypostoma, but as the structure of the cephalic eyes is sufficiently well preserved, we here describe it. The lenses seen from the surface look globular, and in a vertical section they are ovate and in a horizontal circular. They are covered by a very thin and delicate membrane, that envelops their superior moiety completely and between the eyes it is enclosed by the surrounding test and growing out from it. In the horizontal section, fig. 45, it looks as a circular frame around the lenses and the membranes of two contiguous lenses sometimes lie so close, that they entirely fill up the interspace between the lenses, leaving only a narrow slit between them marking their boundaries. The space between the lenses, fig. 46, is porous, being perforated by tubes, which continue vertically down through the shell of the cheek.

_Dalm. sclerops_ DALMAN.

Pl. IV fig. 3.