SKULL 



223 



(ffeterodontus) exhibits another and quite different modification. 

 In this Fish the dorsal border of each palato-quadrate cartilage 

 fits into a deep groove along the infero- lateral surface of the 

 cranium, and is firmly attached thereto by hgament. Thus the 

 first step is taken towards that more complete fusion of the two 

 structures which is so characteristic a feature in the more typically 

 autostylic Fishes like the Holocephali and the Dipnoi. Auto- 

 stylism, whether incipient, as in Heterodontus, or complete, is to be 

 regarded as a secondary modification, which may be independently 

 acquired in widely different groups of Fishes, and is usually 

 associated with the need of a firm and rigid support for an ex- 

 ceptionally massive dentition.^ 



J-T-.CI 



lb.3 



Mck.C 



p/i..h.y 



op.r 



CL.S.C 



p.s.c 



V.IO 



Fig. 129. — Lateral view 

 of skull of Chivio,era 

 monstrosa, a.s. c, Posi- 

 tion of anterior semi- 

 circular canal ; c.hy^ 

 cerato - hyal ; e.h^/, 

 epi-hyal ; fr. cl, frontal 

 clasper ; h.s.c, posi- 

 tion of liorizontal 

 semicircular canal ; 

 i.o.s, inter - orbital 

 septum ; Ib.l, Ib.S, 

 lb.3, labial cartilages ; 

 Mck.C, mandible ; 

 Nv.S, optic foramen ; 

 -Nv.XO, vagus fora- 

 men ; olf.cp, olfactory- 

 capsule ; op.r, oper- 

 cular rays ; pal.qu, 

 palato - quadrate ; 

 ph.hy, pliaryngo-hyal, 

 or hyomandibular ; 



p.s.c, position of posterior semicircular canal ; qu, quadrate region ; ■/■, rostrum. (From 



Parker and Haswell, after Hubrecht. ) 



In the Holocephali {e.g. Chimaera ^) the cranium retains its 

 primitively cartilaginous condition, and assumes a somewhat 

 peculiar appearance owing to the lateral compression and vertical 

 growth of its inter-orbital and nasal regions (Fig. 129). There 

 is a complicated series of labial cartilages in relation with the 

 ventrally-placed nostrils and the upper and lower jaws. In the_ 

 males of Chimaera and Callorhynchus, but not in Harriotta, a 

 movable cartilage is attached to the cranial roof, and supports 

 the frontal clasper. The skull is typically autostylic. Along 



' DoUo, Bull. Soc. Bulge GM. etc. ix. 1895, p. 110. 



= Hubrecht, Mederland. Archivf. Zool. ill. 1877, p. 255. 



