XVII 



ELASMOBRANCHII— SELACHII 



445 



principally on Molluscs, the shells of which are crushed hy their 

 massive grinding teeth. The different species vary in size from 

 2 to 5 feet. 



The Heterodontidae were the most characteristic and abundant 

 Sharks of the Mesozoic period. Amongst extinct genera Hylochis 

 ranges from the Jliddle Trias to the Lower Cretaceous (Wealden) ; 

 an allied genus, Acrodus, from the Middle Trias to the Upper 

 Cretaceous (Gault). Palaeospinax occurs in the Lias and 

 possibly in the Upper Trias. Sy^iechoclus is a Cretaceous genus, 

 and Aster acanthus, which has large hooked spines on the head, is 



FlQ. 253.— Port Jackson Shark (Heterodontus philippi). A, lateral view ; B, mouth 

 and nostrils. d, Clasper. (From a specimen in the Cambridge University 

 Museum.) 



characteristic of the Middle and Upper Jurassic. An even 

 greater antiquity may be claimed for the Heterodontidae if, as 

 is not improbable, such Palaeozoic Sharks as Orochis, Sphen- 

 acanthus, Tristychius (Carboniferous), and Wodnika (Permian) 

 belong to this family. Many ichthyodorulites are probably the 

 spines of various extinct Heterodontidae. 



Fam. 4. Cochliodontidae.' — This Palaeozoic family includes 

 a number of Sharks probably related to the Heterodontidae, but 

 of which little is known except their dentition. The teeth are 

 in some respects similar to those of Heterodontus, except that 

 those which appear to correspond to one or both of the middle 

 rows of the latter genus tend to fuse and form a few large, 

 convex, and often scroll-like plates. The typical Cochliodonts 

 are exclusively Carboniferous (Europe and North America). 

 Psephodus, Pleuroplax, Deltodus, Poecilodus, Cochliodus, Beltopty- 

 chius, Helodus, and Menaspis (Permian) are characteristic genera. 



1 Smith "Woodward, Nat. Science, i. 1892, p. 671. 



