222 



FISHES 



CHAP. 



been remarked that it " dominates the whole body, everything 

 else yields to it." Meckel's cartilages are represented either 

 by the cornual cartilages, as seems most probable, or by the 

 dental plate (Fig. 125, c.c. and D). 



In the generality of Elasmobranchs the skull resembles that 

 of the Dog -Fish in essential structure. The more important 

 modifications within the limits of the group relate to differences 

 in the mode of attachment of the primitive upper jaw to the 

 skull, and the number of branchial arches. In most Elasmo- 

 branchs the skull is hyostylic, as in Scyllium, but there are 



ftt. orb 



FIG. 128. Lateral view of the skull of Notidanus (Heptanchus) cinereus ; mck, Meckel's 

 cartilage, or primitive lower jaw; pai.qu, palato - quadrate cartilage or primitive 

 upper jaw ; pt.orb, post-orbital process of the cranium with which the post-orbital 

 process of the palato - quadrate articulates. (From Parker and Haswell, alter 

 Gegenbaur.) 



two genera which, in different ways, are exceptions to this rule. 

 In Notidanus the hinder part of each palato-quadrate cartilage 

 grows upwards into a strong post-orbital process, which articulates 

 with the suitably modified post-orbital process of the periotic 

 capsule (Fig. 128); hence the primitive upper jaw acquires a direct 

 dorsal connexion with the cranium, and, as the hyoid arch is now 

 relieved from taking any part in its support, the hyomandibular 

 is reduced to the condition of a relatively slender rod of cartilage. 

 By this arrangement both the mandibular and hyoid arches have 

 their own separate and independent connexions with the cranium, 

 and the skull is said to be ampJiistylic} The Port Jackson Shark 



1 Huxley, P.Z.S. 1876, p. 40, et seq. 



