Fam. 4. Pteraspidae.[[621]]—Until the recent inclusion of the three preceding families, the Pteraspidae were the only representatives of the Heterostraci. In the best known genus, Pteraspis, there is a marked reduction in the number of the component plates of the carapace, and only seven can now be distinguished (Fig. 316): (a) a large posterior dorsal plate, supporting behind a stout spine; (b) a conical rostral plate, covering the preorbital part of the head; (c) a pair of small marginal orbital plates, each with a small aperture, probably for the eye; (d) a pair of posterior lateral or cornual plates, each of which is perforated by a large oblique foramen, conjecturally an external branchial aperture; and (e) a large ventral plate. There is probably, also, a small median "parietal," or "pineal," plate, with a pit on its inner surface, situated between the rostral and posterior dorsal plates. Externally the plates are sculptured into fine ridges, which in their minute structure and their crenated free margins are suggestive of linear series of fused denticles. The tail appears to have been invested by imbricated rhombic scales. Pteraspis (Lower Old Red Sandstone of Scotland and England, and the Lower Devonian of Galicia); Cyathaspis (Upper Silurian and Lower Old Red Sandstone), known only by its dorsal and ventral shields; and Holaspis (Lower Old Red Sandstone of Monmouthshire, and the Upper Silurian of Pennsylvania), are the only genera.
Fig. 316.—Restored outline of Pteraspis rostrata, seen from the side. The scales on the hinder part of the tail are omitted. (From Parker and Haswell, after Smith Woodward.)
Order II. Osteostraci.
While agreeing with the more specialised Heterostraci in the division of the body into an anterior carapaced portion and a free hinder part invested by a rhombic squamation, the Osteostraci are distinguished by the presence of bone as a histological component of the dermal hard parts; by the position of the orbits, which, instead of being marginal in position, are close together on the dorsal aspect of the carapace; and by the possession of a median dorsal fin.
Fam. 1. Ateleaspidae.[[622]]—The general shape of the body is much the same as in the Coelolepidae, but the exoskeleton consists of numerous polygonal tuberculated plates in front of the postero-lateral lobes, and of sculptured rhombic scales behind. A pair of crescentic markings, placed close together about the middle of the dorsal surface of the head, probably indicate the outer margins of orbital recesses (Fig. 317). The only species at present known (Ateleaspis tessellata) occurs in the Downtonian beds.
Fig. 317.—Outline sketch of Ateleaspis tessellata. The crescentic markings indicate the position of the supposed orbits. (From Traquair.)
Fig. 318.—Cephalaspis murchisoni. Upper Silurian and Lower Old Red Sandstone, op, Operculum(?). (From Smith Woodward.)