CEPHALASPIDOMORPHI 



2OI 



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 is occupied by a plate, which 



dsc 



ma 



rf. 



Restoration of Crphalaspis Li/Ml, a, lateral area with polygonal plates ; c, cornu of cephalic 

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

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



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 

 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 

 of dense dentine-like sub- 

 stance bears spinelets or 

 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 Cephalaspls. (After Patten.) A, 

 through the cephalic shield and eyes ; B, through the 

 trunk, d, dorsal ridge scale ; e, ocular scale ; ie, inturned 

 margin ; La, lateral area of small scutes ; v, ventral scales. 



