342 



Geogra'phical Distribution 



Table I., the islands west of Portorico are more connected with 

 North' America, and those east and south with South America. 

 Portorico itself has in both respects marked affinities with eacli 

 of the continents. 



In 1855 Pfeiffer published a catalogue of the Helicese arranged 

 in numerous subgenera, but his as well as other similar classi- 

 fications have not been generally adopted. They seem indeed 

 in many cases extremely arbitrary, and it is not my present 

 intention to enter more than incidentally upon tlie subject of the 

 geographical distribution of such subgenera, although they 

 afibrd striking examples of the connexion between the conti- 

 nents and islands which I point out in this paper. 



The relative number of species to genera, of Helix and Buli- 

 mus for instance, in different parts of America and the islands, 

 and the increased number of genera in tropical America, as 

 exhibited in Table I., are interesting facts. 



The operculate land shells are divided by Pfeiffer in his 

 Monograph into forty-four genera, of which thirteen are found 

 in America, and twenty in the West Indies. He has since pro- 



