XVII ELASMOBRANCHII—SELACHII 445 
principally on Molluscs, the shells of which are crushed by 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 Hybodus 
ranges from the Middle Trias to the Lower Cretaceous (Wealden); 
an allied genus, Acrodus, from the Middle Trias to the Upper 
Cretaceous (Gault). Palaeospinaz occurs in the Lias and 
possibly in the Upper Trias. Synechodus is a Cretaceous genus, 
and Asteracanthus, which has large hooked spines on the head, is 
Fig. 253.—Port Jackson Shark (Heterodontus philippi). A, lateral view ; B, mouth 
and nostrils. dd, 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 Orodus, Sphen- 
acanthus, Tristyehius (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, Deltopty- 
chius, Helodus, and Menaspis (Permian) are characteristic genera. 
1 Smith Woodward, Nat. Science, i. 1892, p. 671. 
