HOLOCEPHALI 



175 



modification of the front end of the pelvic fin, just as the posterior 

 clasper is formed from the hind end ; in Squaloraja its isolation is 

 less pronounced, since intermediate radials are preserved articulat- 

 ing Avith the pelvis (Dean [llOa]). 



The brain, while resembling that of a shark in its general 

 structure, is remarkable for the great development of the restiform 

 bodies, the relatively small olfactory bulbs, and 

 more especially for the great elongation of the 

 diencephalon (thalamencephalon) widely separating 

 the cerebral hemispheres from the optic lobes, a 

 stretching of the mid-brain which is related to the 

 formation of an interorbital septum. The cerebral 

 hemispheres are more distinctly paired than is 

 usual among Selachii (Fig. 140). 



It may be concluded that the Holocephali are 

 the descendants of some primitive form of shark 

 with unconstricted notochord, which diverged from 

 the main stem in pre-Devonian times, and became 

 specialised in the loss of the denticles, spiracle, 

 and cloaca, and in the acquisition of a spine-bearing 

 dorsal fin close behind the head, of a cephalic 

 prehensile organ in the male, of a pronounced 

 rostrum, of peculiar vertebral rings, of an opercular 

 flap, and above all of a fixed upper jaw and grinding 

 plates. It is in the elaboration of these grinding 

 plates that progress has been most marked up to 

 the present day. 



The chief characters of the Holocephali may 

 be enumerated as follows : the gill-openings are ^ tro * L''"/^"^ 

 covered by a hyoid opercular flap, the spiracle Owen,.4*a*. qf rcrte- 

 being closed ; a rostrum is present, the mouth is of Mewn.'taiigmaiM 

 small, and the teeth specialised into permanent i&^&$SS-, 

 grinding plates ; the palato-quadrate is fixed to the c 1 , cerebellum ; ./,/, 

 skull, the hyomandibular reduced, and an inter- mrelencephaioD o, 

 orbital septum developed dorsal to the brain ; the ?!!: $?%. 

 lateral line is incompletely closed ; the notochord | ated di*-ncepiiaion, 

 is unconstricted, the sheath invaded, but no centra 

 are formed ; the first dorsal fin is specialised to support a spine, 

 the second remains unconcentrated ; a cephalic tenaculum, and an 

 anterior and a posterior clasper, are present in the male ; the cloaca 

 is absent (Fig. 139). 



The earliest remains of true Holocephali occur in Jurassic 

 strata. These fossils are in all essentials like the modern forms, 

 yet in some few points they show a more primitive structure. For 

 instance, as already mentioned, the covering of denticles is more 

 complete, the anterior end of the vertebral column less fused up, 



Dorsal viewer the 



