CEPHALOPODOUS MOLLUSCA. 255 



cretaceous periods, and characterise the beds by their specific 

 forms. 



In taking a general review of this class, we observe the 

 TENTACULIFEBA among the first animal forms of our globe. 

 They attained their greatest development in the seas of the 

 Silurian period. We count twenty-two genera in the 

 palaeozoic rocks ; seven in the triasic ; and the same number 

 in the oolitic ; fourteen in the cretaceous period ; and only 

 two in the tertiary strata. Of this once extensive group, a 

 single genus lives in our modern seas. 



The ACETABTILIFEBA commenced in the muschelkalk ; in 

 the oolitic rocks are twelve genera ; and in the cretaceous 

 and tertiary strata, there are four. Of all these genera 

 five only now survive. Comparative calculations on this 

 order must be made with much caution, as their rudimentary 

 internal skeletons do not afford such positive data as the 

 external shells of the tentaculifera. 



Of the tentaculifera one conclusion may be drawn from 

 the physiology of this class. The nautilus now only inhabits 

 the Indian ocean, whilst extinct species of this genus of large 

 size are found in the tertiary strata of our island, and in 

 beds of the same age in Prance and Belgium ; thus affording 

 evidence that the European seas of the tertiary epoch had a 

 much higher temperature than they now possess. 



