CEPHALASPIDOMORPHI 



201 



The orbits are placed close together on the top of the head 

 (Figs. 174, 175), roofed over by a thin bony plate, and separated 

 only by a narrow bridge. The orbits are partially floored by bone 

 in Tremataspis. The interorbital space js occupied by a plate, which 



' cf. 



FIG. 1V3. 



Restoration of Ceplialaspis Lyelli. a, lateral area with polygonal plates ; c, cormi of cephalic 

 shield ; c/, caudal fin ; cl, position of cloaca ; c.s, cephalic shield ; d.f, dorsal fin ; d.sc, dorsal 

 ridge scales ; e, orbit : m.a, median area with polygonal plates ; p, position of antorbital fossa. 



la. 



may be quite free (Tremataspis) and bears on its inner surface a 

 median depression possibly for a pineal body. In front of the 

 orbits is an internal cavity opening, at all events in some genera 

 (Tremataspis), by a median 

 pore to the exterior. It A 



has been suggested that - 



this aperture represents 

 an olfactory organ, 1 or a 

 median eye. A series of 

 pits in the cephalic shield 

 of Tremataspis indicates 

 the presence of a lateral- 

 line system of sense-organs 

 (Patten [325]). 



The Cephalaspido- 

 morphi are, for the most 

 part, entirely covered 

 over with plates and 

 scales consisting of three 

 layers. The outer layer 



f\i rlpriQP rlpntinp lilrp <anV through the cephalic shield 



oi dense aentme-iiKe SUD- trun ^ d dorsa 1 1 ridge gcale . e> ocular scale . ic> in - tur ned 



Stance Dears SpineletS Or margin ; La, lateral area of small scutes ; v, ventral scales. 



tubercles. The middle 



layer is pierced with vascular spaces ; the inner layer is lamellated. 



True bone-cells are found in all three (Huxley [224], Lankester 



[276]). 



1 Some authors believe the Cephalaspids to be monorhinal, and allied to the 

 Cyclostomes. 



FIG. 174. 



Transverse sections of Cephalaspis. (After Patten.) A, 

 and eyes; B, through the 



