OSTEOSTRACI. 



D 



Cephalaspis (figs. 710). In this genus the body is elevated, trian- 

 gular in transverse section, and none of the scales are fused into a plate 

 behind the head-shield. An outline of the shield viewed from above is 

 given in fig. 7. The postero-lateral angles are produced into short cornua 

 (p.c.), and there is a more or less conspicuous median spine (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, not improbably for the reception of 

 a pineal body. Immediately in advance of this, on the visceral aspect 

 of the shield, is a short, narrow, median longitudinal septum, between 



Cephalaspis lyelli ; diagram of dorsal shield, about one-half nat. size. L. Old 

 Bed Sandstone ; Forfarshire, and Herefordshire district, a./., antorbital 

 fossae; a.p., antorbital prominences; i.g., interorbital ridge or groove; 

 i.p., interorbital prominence ; m.c., marginal cells ; o.r., orbital rim ; p.a., 

 posterior angle ; p.c., posterior cornu ; p.o.v., postorbital depression, 

 occupied by separate small plate ; p.r., posterior ridge ; p.s., posterior 

 spine; r, rim. (After Lankester.) 



which and the antorbital prominences a pair of small antorbital fossae 

 (a./.) are exposed when the substance of the shield is removed. Behind 

 a transverse ridge or groove (i.g.) joining the posterior borders of the 

 orbits, a large 'postorbital valley' (p.o.v.) is also conspicuous, and this 

 is always filled by a plate different in texture from the rest of the 

 shield. Inexplicable concavities immediately beneath the cranial roof 

 near the rostrum are named " marginal cells " (m.c.} ; and when the 

 fossil is so preserved as to show the contour of some of the originally 

 soft parts, the cast of a pair of great rounded lobes, meeting in the 



