XX OSTRACODERMI—OSTEOSTRACI 527 
Drepanaspis. That Psammosteus is closely allied to Drepanaspis 
seems certain, but for the present the two genera may be retained 
in separate families. 
Fam. 4. Pteraspidae.1~—Until the recent inclusion of the 
three preceding families, the Pteraspidae were the only repre- 
sentatives 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 large posterior dorsal plate, 
supporting behind a stout spine; (4) a conical rostral plate, 
covering the preorbital part of the head; (¢) a pair of small 
marginal orbital plates, each with a small aperture, probably 
for the eye ; (@) a pair of posterior lateral or cornual plates, each of 
which is perforated by a large oblique foramen, conjecturally an 
Fic. 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.) 
external branchial aperture ; and (¢) 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. 
Order II. Osteostraci. 
While agreeing with the more specialised Heterostraci in the 
division of the body into an anterior carapaced portion and a free 
1 Lankester, Monogr. Palaeont. Soc. 1868, 1870; Geol. Mag. x. 1873, p. 241; 
Smith Woodward, Brit. Mus. Cat. Foss. Fishes, ii. 1891, p. 159. 
