144 



SE LAC HI I 



Early fossil forms are found, such as Hybodus (A. S. Wood- 

 ward [500], Brown [63]), in which the teeth are much less 

 specialised, resembling those of the early Notidanids. The teeth 

 of Hybodus are nearly uniform, with a long cusped crown and a 

 sharp central cusp. Those of Synechodus and Acrodus become 

 blunter at the sides than towards the middle of the jaws (Fig. 104). 

 Finally, the dentition of Asteracanthus closely resembles that of 

 Cestradon and Heterodontus (Fig. 105). In the Cochliodontidae 

 it is still further specialised by fusion. The teeth are very solid, 

 being formed of vasodentine with fine branching pulp-canals. 



The vertebral centra of Palaeospinax and Synechodus were 



Fio. 103. 



The Port Jackson shark, Jlrterodontus (Cestradon) 

 Dean.) A, ventral, B, front, and C, dorsal view of head. 



, Lac. (After Carman, from 



well-developed and calcined, with distinct radial calcifications in the 

 latter genus ; but Hybodus shows no centra at all, and possibly 

 had a primitive unconstricted notochord ; nor have centra been 

 found in Acrodus and Asteracanthus. 



Most important of all, however, is the development of the jaws ; 

 for it is well established [500] that Hybodus and Synechodus 

 had typical amphistylic skulls, with the palato - quadrate and 

 hyomandibular as in the Notidanidae and other primitive 

 Elasmobranchs (p. 97). Now if this series of Heterodonts is 

 correctly associated as a monophyletic group, it must be supposed 

 that the hyostylic arrangement has been developed independently 

 in the Heterodonti and in the members of Division B (p. 148). The 

 Heterodonti must, if we accept this view, be definitely separated 

 off from the other sharks, as a distinct offshoot from a more primitive 



