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) a large posterior dorsal plate, 

 supporting behind a stout spine ; (/>) 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 



FIG. 316. Restored outline of Pt era apis 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 (e) a large ventral plate. There 

 is probably, also, a small median " parietal," or " pineal," plate, 

 with a pit on its inner surface, sitiiated between the rostral and 

 posterior dorsal plates. Externally the plates are sculptured into 

 fine ridges, w r hich 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 Eed Sandstone of Scotland and 

 England, and the Lower Devonian of Galicia) ; Cyatliaspis (Upper 

 Silurian and Lower Old Eed Sandstone), known only by its dorsal 

 and ventral shields ; and Holaspis (Lower Old Eed 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. Sac. 1868, 1870 ; Geol. Mag. x. 1873, p. 241 ; 

 Smith Woodward, Brit. Mus. Cat. Foss. Fishes, ii. 1891, p. 159. 



