Part 9

The enormous hypostoma of this giant resembles in a high degree that of the Asaphi, but is at the same time the most evident verification of the experience that the hypostoma per se cannot be regarded as the sole criterion for determinating the generic affinities of different species. The whole structure of this trilobite in other respects gives it a quite independent position, distinct, from Asaphus.

We have had at our disposal several specimens of the large hypostoma, the dimensions of the largest, fragmentary in its anterior margin, being as follows: breadth 48 mm. length 41 mm., probably 47 mm. when entire, breadth of each of the posterior lobes at their bases 19 mm., length of the same 23 mm. The maculæ which are placed on the flat surface of the hypostoma without being at all elevated, are prominent through their great size and their pale, whitish colour. They are somewhat oblongue having their longest axis directed inwards and downwards. They attain a diameter of four millimeters. Horizontally sectioned, f. 2, they show the spongious texture and vertically the quasi prismatic reticulate structure richly developed, f. 3. The pillars are very distinct and continue without interruption all through the macula and in the interstices there are traverses joining between two pillars or partially filling the darkish interspaces, thus giving the whole the aspect of some »tabulate» coral sectioned. By the inspection of the horizontal section alone, presenting the irregular spongy texture it would have been impossible to imagine the ordinated arrangement which the vertical section reveals to us. The eyes consist of regular hexaedral prisms, and there is a very sharp boundary line between them and the spongious zone, which is very narrow and without distinct separating lines joins with the cephalic test. For the rest there are in all probability at least two different species sent from America under the name of Isotelus gigas, of which only the largest, almost like a Homalonotus, and the hypostoma of which has been partly delineated in this memoir, seems to be the real one. They differ both in the shape of the hypostoma as well as in other respects.

=Barrandia= MAC COY.

According to SALTER Monogr. pl. 19 fig. 9 his B. Portlocki, of which there is a fragmentary and broken hypostoma, shows feeble traces of oblique macula, nearly in the fashion of Asaphus. In the description at page 139 is told about »the usual pair of tubercles».

=Bronteus= GOLDFUSS.

BARRANDE delineates the maculæ on the hypostoma of

B. palifer, Tab. 45 fig. 17,

B. planus, Tab. 48 fig. 7, and

in the supplementar volume of

B. rhinoceros, Tab. 9 f. 16,

B. furcifer, Tab. 11 fig. 16, but there is not the slightest indication of granulation on any of them, nor is there in the descriptions, generic or specific, the least mention made of the tubercular maculæ. It is also remarkable that in the works of ANGELIN and NOVÁK where several hypostomas belonging to species of this genus are delineated, not a single one shows these tubercles. We shall now continue the descriptions of the Swedish Brontei, already begun with Br. polyactin in the introductory part of this memoir.

=Bronteus irradians= LINDSTR.

Pl. II figs. 4-5.

has a hypostoma that much resembles that of Br. polyactin. In its general shape it is similar and the two concentric grooves with the two maculæ placed in the same way, just below the superior groove. These maculæ are much larger than in Br. polyactin, nearly thrice their size. They are also more ovate or rather like a bean, the smooth surface is larger and the granulated spot restricted to a more narrow space forming an oblique patch. The granules or lenses are also individually larger than in the allied species, double their size or 0,06 millimeters. We have not succeeded in making sections of the cephalic eye nor of the maculæ.

=Br. platyactin= ANGELIN.

Pl. II f. 14-19.

The hypostoma has a transversally triangular form, and is divided only in two fields through a shallow semicircular groove near the posterior margin. The two maculæ are situated above the groove near its superior sinuses. They are elongated, fig. 17, elliptic with the narrow pointed end directed outwards and the broad rounded end inwards. The chief surface is scooped out as a shallow depression. The granulated spot is situated on the broader end and covering it completely. The relatively large lenses are arranged in five regular rows, the uppermost one being the longest. On the interior surface of the hypostoma there are the corresponding sockets of both maculæ with smooth surface. The horizontal sections of the granules figs. 18, 19 present the image of white rings in close contact, without, however, to occasion a prismatic structure, a dark interspace lying between each ring. These lenses are filled with a dark mass, and in some the same sort of radiated structure is perceptible as in the lenses of the cephalic eye. In horizontal sections of the cephalic eye the lenses approach the polyedral shape. In another section near to the surface of another specimen the lenses are decidedly hexaedral. The vertical sections, fig. 14, reveal their real nature as lenses where they lie as a string of beads with a dark nucleus enclosed within a thin whitish shell. They are covered by a thin membranous lining. When seen in transmitted light the lenses proper are dark, and the shell white and in reflected light the lenses are lighter than the rest.

=Br. laticauda= ANGEL.

Pl. II figs. 6-13.

Lower Silurian from Dalecarlia. The hypostoma, f. 10, is of a broad clypeate shape, the anterior margin rounded without any large projecting wings. On the exterior surface there are two grooves parallel with the rounded inferior margin. As in Br. polyactin the two elliptic tubercles are situated on the inferior edge or slope of the upper groove and they are deepened by a shallow depression as in Br. platyactin. The granulated spot situated along the posterior margin of the macula deviates much in shape from that of the other species. It consists of a long and narrow stripe ending in a fine point outwards and widening inwards, where it is rounded, forming thus a claviform, curved elevation. Around the granulated area the maculæ are quite smooth, and show in a horizontal section an irregular structure of tiny black dots nearly resembling the structure of the hypostomic shell. We have not been able to obtain any good vertical section, but by casts of the interior side of the tubercles it is found that the granules form polyedric facets like those of the eyes though perhaps not so regular. There is a specimen with the granules intact, and the polyedric shape is then not so distinct.

Sections of the eyes elucidate the structure, which partially is obscure in other species of Bronteus. There is a thin membranous coating covering the subjacent well formed lenses (fig. 8). When this membrane in some instances has been peeled off the lenses lie bare (fig. 9). These have a dark nucleus and in some instances it seems as if there were two. In a vertical section they look spheroidal, in a horizontal section again they are polyedric, especially when taken somewhat below the surface or near the middle line (fig. 6). In fig. 7, some lenses are delineated in a horizontal section, highly magnified and the corallian appearance is evident. The lenses are well separated by distinct lines, and from their inner tubes whitish reticulations radiate towards the centre.

If we now compare these sections, where the lenses are so distinctly seen, with the more obscure sections of other Brontei, it is evident that the dark points in them are nothing but transformed and deteriorated centres of the lenses. In Br. platyactin they are a little more distinct than in the other species. And it is with these that the granulated spots on the maculæ of the hypostoma are most concordant.

=Bronteus= sp. indet.

Pl. II figs. 28-30.

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.