324 Annals of the Carnegie Museum. 



part of Cuba as well as the Isle of Pines are species of generally wide 

 distribution throughout the larger island. It is clear, therefore, that 

 the molluscan fauna of the Isle of Pines is a composite of the Western 

 Cuban forms which migrated at the time of a land-connection between 

 the two islands. Since the separation in comparatively recent time 

 there has been an isolation sufficient to develop on the lesser island a 

 faunal or separate element. In the case of almost every species this 

 relationship is obvious and is often very close. The case of the two 

 Priotrochatellas is not clear. This is an example of remarkable devel- 

 opment along a particular line, or these two species are of exotic 

 origin. The former is likely the case. 



