﻿178 OSTEOSTBACI. 



transverse rows equal in number to the series of flank-scales. Scales 

 immediately behind the anterior shield not fused together or repre- 

 sented by a broad plate. 



The shield of this genus has been described in detail by Huxley 

 and Lankester, and the accompanying figures (figs. 18-20) are 

 copied from the latter author. Fig. 18 shows the dorsal contour of 

 the shield, with its parts indicated by the lettering. The several 

 prominences of the hinder border are the lateral cornua {p. c), the 

 median spine (p. s.), and the broad median production of the shield, 

 with its sharp angles (p. «.). Each orbit has a surrounding rim 

 (o. ?'.), extended in front into a small antarbital prominence (a.p*) ; 

 and between the eyes is an elongated interorbital prominence (i.p.\ 

 evidently hollow, and homologous with the pit in the pineal plate 

 of the Antiarcha (see p. 210). Immediately in advance of the latter 

 on the under surface of the shield is a small, short, narrow median 

 septum. Between the antorbital prominences and this septum is 

 the pair of small antorbital fossae (a. /.) exposed only when the 

 substance of the shield is removed ; and another great superficial 

 fossa (p. o. v.) extends from a ridge or groove (i. g.) joining the hinder 

 borders of the orbits to the origin of the median ridge (p. r,) which 



Pig. 19. 



Cephalaspis. — Diagram of inferior aspect of shield, showing 

 inferior rim ; after Lankester. 



terminates in the posterior spine. Inexplicable concavities imme- 

 diately beneath the cranial roof near the rostrum are named marginal 

 cells (m. c), these being more extensively developed round the rim in 

 Eukeraspis (fig. 27, p. 194) ; and when the fossil is so preserved as to 

 show the contour of some of the originally soft parts, the cast of a 



