144 



GEOLOGICAL BIOLOGY. 



specialized in their form, developing elongations, spines, and 

 processes giving them peculiar shapes. It is probable, there- 

 fore, that in their life they are not capable of much change of 

 local habitation. 



The Strombus is confined to warm seas, the Pacific, the 

 Indian, and the mid-Atlantic, including the Caribbean seas 

 and Mexican Gulf. The other recent genera of Strombidae 

 are from the Indian and Pacific warm seas. The genus 

 Chenopus of the second family is a North Atlantic form, and 

 is not associated with the Strombidae in habitat. 



But representatives of both families are as old as the 

 Jurassic, and there are also several genera in each family from 

 Cretaceous rocks. 



It is evident from this set of facts that the distinction be- 

 tween the two family types of structure was initiated in the 

 Mesozoic, and that there was adjustment to particular condi- 

 tions of environment very early an adjustment which change 

 of time did not modify. 



The following table will graphically illustrate this fact : 



TABLE OF THE GEOLOGICAL RANGE OF THE FAMILIES 

 STROMBID^E AND CHENOPODID^. 



The Relation of Antiquity to Distribution. The distribution 

 of the genera in the family of Cerithiidae illustrates another 



