OP CONCHOLOQY. 137 



The majority of the genera of operculates common to the 

 Continent and Islands have the greater number of species on 

 the Islands, while the reverse is the case with the majority of 

 such genera of inoperculates. It is noticeable, also, that spe- 

 cies of such former genera (speaking generally) attain larger 

 size on the Islands — in Helicinacea, for example — and of the 

 latter on the Continent, especially in Helix, Bulimus, Oleacina, 

 and Gylindrella. 



Looking at the distribution of species in America and the 

 West Indies, and considering each genus as having had its 

 origin where the greatest number of its species is found, (on 

 the Continent or Islands), the following is the result : — 



OPERCULATES. 



Origin of Genera. N.Am. Mex. C. Am. S. Am. Am. Cont, W. Ind. 



per cent. per cent. per cent. per cent. per cent. per cent. 



American, 1U 10 17 13 1| 



West Indian, 100 86| 90 82 87 98^ 



INOPERCULATES. 



American, 96^ 82 87 98J 95 67 



West Indian, 3£ 18 13 l| 5 33 



It would appear that insular conditions have been more 

 favorable to the increase both of genera and species of oper- 

 culates, or it may be said to variation, and continental condi- 

 tions to inoperculates. 



The species of both classes common to the Continent and 

 Islands (4 operculated and 36 inoperculated) are, with few 

 exceptions, of West Indian rather than American forms. The 

 number of operculated species found in more than one island 

 is 18, but of inoperculates 55, the latter having, on the whole, 

 a wider insular distribution than the former. 



Considering the distribution of genera and species both of 

 operculates and inoperculates in the West Indian Islands, the 

 character of the species common to them and the Continent of 

 America, and found in more than one Island, the West Indies, 

 although, in one sense, a single zoological province, may be 

 divided into five sub-provinces, viz : — 



1. Cuba, with the Isle of Pines, the Bahamas, and Bermudas. 



2. Jamaica. 



3. Haiti. 



4. Porto Rico, with Vieque and the Virgin Islands, also in- 

 cluding Anguilla, St. Martin, and St. Bartholomew. 



5. Guadaloupe and Martinique, with Barbuda and the is- 

 lands between it and them, and to the south, to and inclusive 

 of Trinidad. 



