16 



Geographical Distrilution 



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 each 

 of the continents. 





North 

 America. 



South 

 America. 



North 

 and South 

 America. 



Islands 

 West of 

 Portorico. 



Portorico. 



Islanda 

 E. and S. of 

 Portorico. 



Helix 



.... 

 







* 



» 



* 



* 



* 

 * 

 * 

 * 



* 



* 



* 

 * 

 * 



* 

 .... 



* 

 * 

 * 



* 



* 



* 



.... 



* 



* 



Streptaxis 



Ennea 



Bulimus 



Spiraxis 



* 



Orthalicus 



Achatina 



* 



Oleacina 



Tornatellina 



Pupa 



* 



Macroceramus 



Cylindrella 



* 



Balea. 





Clausilia 





Vitrina 





Simpulopsis 



Succinea 



.... 



Proserpina 



.... 



In 1855 Pfeiffer published a catalogue of the H elicese 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 arbitrar}^, and it is not my present 

 intention to enter more than incidentally upon the subject of the 

 geographical distribution of such subgenera, although they 

 afford 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- 



