GEOGRA PHICA L D IS TRIB U TION. 



145 



law: viz., old genera are widely distributed, while younger 

 genera are more closely restricted in distribution. 



Thus Cerithium is a genus of which species are known 

 from the Triassic, Jurassic, Cretaceous, Tertiary, Quaternary 

 and Recent Periods. It is known from all seas, warm and 

 temperate ; and a species of the genus has been dredged from 

 the abysmal zone, and other species are known up to the lit- 

 toral zone of the ocean. Fastigiella, on the other hand, a 

 genus known no farther back than the Tertiary, is confined 

 to the Antilles, as present knowledge goes. 



Rissoa, of the family Rissoidae, has a similar history ; it is 

 known from the Jurassic up, and it is distributed in all seas. 

 In the accompanying table these facts are graphically repre- 

 sented. 



TABLE OF THE GEOLOGICAL RANGE OF THE FAMILIES 

 CERITHIID.E AND RISSOID^E. 



Distribution in Relation to Temperature of the Waters. Two 

 families maybe selected to illustrate this law. In the Lamel- 



