76 



COMPARATIVE ANATOMY 



those forms in which the liyaline primordial skull, immovably fixed 

 to the vertebral column, is still retained (Fig. 68). These forms- 

 are spoken of as Oartilaginous Ganoids. As in Elasmobranchs, 

 the cranial cavity reaches forwards to the ethmoidal region, but is 

 separated from the latter by cartilage. The appearance of definite 

 hones, however, divides them sharply off from the Elasmobranchs, 

 and proves their skull to be at a much higher stage of develop- 

 ment. These bones have the form of richly sculptured plates and 

 shields, and are developed partly from the mucous membrane 

 lining the mouth and covering the visceral skeleton, and partly 

 from the skin covering the roof of the skull. In the first-named 



J'y-el 



p.s.c 



(From 



op.r 



I'lo. 57. — Skull of Chimin n mon^ttosa, Lateral View. 

 Parker and Haswell's Zooloyy, after Hubreclit. ) 

 a.s.c. position of anterior semicircular canal; e.hy. ceratohyal ; ep.Ay. epi- 

 hyal ; fr.d. frontal olasper ; h.^.c. position of horizontal semicircular canal ; 

 J.o.s interorbital septum; Ih. 1, lb. 2, lb. J, labial cartilages; McLC. 

 mandible; Xv. 2, optic foramen; Xv. 10, vagus foramen; olf.cp. olfactory 

 capsule; op.r. opercular rays; pal.qu. palatoquadrate ; ph.hy. pharyngo- 

 nyal; p.s.<:. position of posterior semicircular canal; qu. quadrate region: 

 r. rostrum. j i o 



region a narrow parasphenoid forms a roof to the oral cavity 

 and extends for some distance along the ventral side of the 

 vertebral column. Ali- and orbito-sphenoids are present in 

 the walls of the brain-case. The operculum is more pronounced 

 thaii m the Holocephali, and is also supported by bones. The 

 whole palato-inandibular apparatus, which is comparatively small 

 and in connection with which bones are formed, is connected very 

 loosely with the skull by means of a hyomandibular and sym- 

 plectic, as well as by ligaments (Fig. 58). 



The dermal skeleton attains a much more considerable develop- 



