I12 



beak-like jaws of the Devonian Rhynchodus (Fig. ill), 

 of the Devonian Ptyctodus, or of the Mesozoic genera, 

 e.g. Ischyodus (Fig. 112), differ little in their structures 

 from those of their living kindred (Figs. 109, 109 A, 1 10). 

 The tritors accordingly are only doubtfully to be derived 

 from the fusion of the primitive basal substance of the teeth 

 with the tissue of the jaws. But the history of Chimae- 

 roids tells of their ancient importance and of the diversity 

 of their forms, and demonstrates that they cannot be con- 

 nected with other existing forms of fishes. In Liassic 

 times their specialized members bore the same relation to 

 Chimaera as did the aberrant Cestracionts of the Coal 

 Measures to the simpler sharks. In their dental evolution 

 they had even reached a more specialized condition than 

 the Cochliodonts (Cestracionts ?). Thus in Myriacanthus 

 and Squaloraja, "all anterior prehensile teeth have disap- 

 peared, and the growth of the dental plates, instead of 

 taking place exclusively at the inner border, seems to have 

 gradually extended to the whole of the attached surface. 

 The Chimaeridae exhibit an advance in the circumstance 

 that all the dental plates are thickened, while the hinder 

 upper pair are both closely apposed in the median line and 

 much extended backward " (Smith Woodward).* Squaloraja 

 had certainly attained a high degree of evolution in the 

 calcified vertebral rings, and in its specialized girdles, fins, 

 and clasping organs. Myriacanthus, on the other hand, 

 while retaining its ancient vertebral characters, had evolved 

 a well-marked series of membrane bones. 



One cannot deny that the study of Chimaeroids as a 

 group emphasizes many of their structural affinities to 

 the sharks. They resemble them in their cartilaginous 

 skeleton, fins and girdles, " claspers," integument, and 



* Cat. Fossil Fishes II, xvi. 



