SKULL 



171 



facial nerve, it would appear more probable that the hyomandibular 

 is indistinguishably fused both with the auditory capsule and with 

 the quadrate, the spiracle being suppressed (Fig. 135). If this 

 interpretation is correct, the autostylism of the Holocephali differs 

 radically from that of the Dipnoi, and has probably been derived 

 from a hyostylic structure. Cartilaginous branchial rays are 

 present in single series, as in the Selachii (p. 123), and are much 

 developed on the hyoid arch to support the operculum (Fig. 134). 

 One of the most characteristic organs of the Holocephali is the 



vr. 



Fio. 135. 



Callorhynchus antarcticus, Lac. A, skeleton and nerves of the head of a young specimen 

 (after Schauinsland's figures). OH, auditory capsule ; b, buccal branch of facial nerve ; cbr, 5th 

 ceratobranchial; ch, ceratohyal ; dr, dorsal median rostral cartilage; eh, epihyal ; gp, glosso- 

 pharyngeal nerve ; ml; Meckel's cartilage ; p, palato-quadrate region ; ph, pharyngohyal ; q, 

 quadrate, or probably hyomandibular, region ; y, interorbital septum ; sp, spinal nerve-roots ; 

 vb, vi, rl, branchial, intestinal, and lateral -line branches of vagus nerve ; v.r, ventral paired 

 rostral cartilage; 2, optic, 3, oculomotor, 4, pathetic, 5, trigeminal, (3, abducent, and 7, facial 

 nerves. B, calcified skeletal supports of the lateral line. 



frontal clasper, or tenaculum, found in all undoubted members of 

 the group from the Jurassic time onwards, but in the male sex 

 only. It is a. movable median process, supported by a cartilaginous 

 axis articulated to the skull above and in front of the orbit 

 (Figs. 141, 144). Long and pointed in the Myriacanthidae and 

 Squalorajidae, short and with a swollen tip in the later forms, it 

 is always armed with denticles. The origin of the tenaculum is 

 unknown ; its derivation from the front end of the dorsal fin has 

 been suggested by Dean ([11 Off], Reis [35 Iff]). 



The cartilaginous auditory capsule, unlike that of the Selachian, 

 is widely open to the cranial cavity internally, and the basicranial 

 axis is strongly bent upwards. The skull extends backwards so as 





