Br. laticauda Angel.
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.
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.