LIFE'ON THE EARTH. 105 



dation of structure, so that while some races may 

 probably be regarded as of the highest type in the 

 class, others (if we include Myxine) can only just 

 claim to be vertebrata. The fossil races belong gene- 

 rally to the higher types. The skeleton being cartila- 

 ginous or mainly so, it often happens that the verte- 

 bral column is not preserved : indeed very frequently 

 only a few scattered teeth and fin-rays remain to 

 attest the existence and magnitude of these ancient 

 mostly shark-like creatures. Singular to say, one 

 group of these fishes, represented in a living state 

 by one Australian species (Cestracion Philippi), has 

 been traced through all these periods (except Cseno- 

 zoic) by its teeth and fin-rays, abounding in Palaeozoic 

 and Mesozoic ages; while, on the other hand, the 

 ordinary sharks of the modern seas are represented 

 in the Tertiary strata plentifully, in the Mesozoic 

 sparingly, in the Palaeozoic not at all. The Cestra- 

 ciontidee have, besides pointed teeth in front, some 

 very large and broad behind, not pointed but suited 

 for crushing and grinding hard and solid substances, 

 such as shell-covered Mollusca, Crustacea, Echino- 

 dermata, or even Ganoid fishes 1 . 



Reptilia according to Owen 2 may be arranged in 

 thirteen orders, of which five, viz. i. Batrachia (Frogs), 



1 Buckland, Br. Tr. PI. 41. 



2 Reports of British Association, 1859. 



