59 



already 'explained ; but in the calcification of the endoskeleton 

 and dermal armour it is comparatively degenerate. The species 

 of the extinct genera attain a much larger size than those of 

 the existing Chimcera, Callorhynchus, and Harriotta ; and some 

 idea of the variety in the arrangement of the tritors on their 

 dental plates may be formed by an inspection of the accom- 

 panying diagrams (figs. 46, 47). The different types of tritoral 



FIG. 46. 



Diagram of the oral aspect of the left palatine tooth in the principal genera of 

 Chimaeridse, showing the arrangement of tritors. 1, Ganodus; 2, Ischy- 

 odus ; 3, Edaphodon ; 4, Callorhynchus ; 5, Elasmodus ; 6, Chimara. 

 (From Brit. Mus. Catal.) 



arrangement are, indeed, commonly regarded as sufficiently 

 constant to be utilized in defining the genera. Detached dental 

 plates and fin-spines of Ganodus and Ischyodus occur in the 

 Lower Oolites, but the earliest known complete skeletons are 

 those of Ischyodus from the Lithographic Stone or Upper 

 Jurassic (Kimmeridgian) of Bavaria. Edaphodon is typically 

 Cretaceous and Eocene, but ranges into the Miocene. 



Ischyodus (figs. 46, 47, 2). The form and proportions of the fish are 

 much like those of the existing Chimcera. There are no dermal plates, 

 but the canal of the "lateral line" system is strengthened by calcined 

 rings. The head-spine of the male is short, arched, and provided with a 

 terminal cluster of denticles. The spine of the anterior dorsal fin, placed 



