178 
OSTEOSTRACI. 
« 
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. Tig. 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 (yr. 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. r.), extended in front into a small antorbital 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 
Tig. 19. 
■S ^ £ ]/■ , ^ y 
, f t "E -iTi SV- A* 4 - 
r-rf s/9 ^ 
/9 L {/.««*& 
St-er^ 0 7 - - • ■ 
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 
