CHONDRENCHEL YS 



85 



The final form of Palaeozoic shark whose structural char- 

 acters have in any way been described is Chondrenchelys. 

 It appears to have somewhat resembled the Pleuracanthid 

 in its elongate form and tapering tail ; but as yet the 

 details of its structure have not been discovered. In its 

 vertebral characters it had certainly made a marked ad- 

 vance ; the notochord had become greatly constricted ; 

 and well-marked centra and arches were present. These 

 appear to have been highly calcified, and show a peculiar 



Fig. 91. — Port Jackson shark, Cestracionphilippi (9). X J^. (After GarmaN.) 

 Australia. A. Ventral. B. Anterior, and C. Dorsal aspect of head. 



beaded or fretted structure which in this form is appar- 

 ently unique. 



Other ancient sharks, as far as can be inferred from 

 fragmental structures, appear to have closely resembled 

 forms that are still extant. 



Such unquestionably were the Cestracionts, a group 

 of sharks especially abundant in the early Palaeozoic 

 seas, judging from the numbers of their fin spines and 



