CHONDRENCHELYS 85 
The final form of Palzeozoic 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, Cestracion philippi (2). X Ys. (After GARMAN.) 
Australia. A, Ventral. &. 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 Palzozoic 
seas, judging from the numbers of their fin spines and 
