RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 345 



distinguishing marine from nonmarine formations and the latter from 

 one another it is with their often abundant fossil remains that the geol- 

 ogist has more frequently to deal than with those of any other animals. 



Cephalopoda. — Every known member of all the families of this elass 

 is exclusively marine, therefore the presence of fossil remains referable 

 to any member of the elass in any formation is regarded as sufficient 

 evidence of its marine origin, provided that its character and condition 

 indicate that the animals thus represented were denizens of the waters 

 in which the formation was deposited. 



(jas1ero[)o(h{. — The following table, constructed upon the same plan 

 as that of the fishes, is intended to give a synoptical view of the aquatic 

 habitat of each of the families of living gasteropods which have direct 

 relation to the subject of this review. For obvious reasons all those 

 families are omitted which include only air-breathers as well as those 

 families all the members of which possess no shell, either internal or 

 external. As in the case of the preceding table, this one is followed 

 by explanatory notes setting forth certain facts which are not clearly 

 expressed by means of such a tabular arrangement: 



Diceca. 

 ORDER PECTINIBRANCHIATA. 



Riasoellidse 



.Vasiminiida 



Ampullariidae 

 Valvatidii'. 

 Vi\ iparida?. 



Assiniilliichr. 



Pomatiopsidni 



