OF CONCIIOLOQY. 57 



It appears from the foregoing, if the origin of Families only 

 be considered, that the origin and distribution of the opercu- 

 lated species in America and the West Indies, is as follows : — 



Origin. No. of Sp. America. 



Asiatic, 66 



West Indian, 84 



150 100 599 100 



This would lead to the inference that the Asiatic and "West 

 Indian influences are not far from equal in the Islands, and 

 that the latter predominates on the Continent. 



If, however, the origin of Families and Subfamilies (in those 

 cases in which the former are subdivided) be considered, — by 

 no means an unreasonable way of treating the question, — the 

 results are very striking. 



150 100 599 100 



The greater African influence in the Islands, and Asiatic on 

 the Continents, is rather to be expected, on the supposition 

 that America would receive African colonists from the West 

 Indies, and the latter Asiatic colonists from the former. 



The following table, prepared on the same basis as the one 

 immediately preceding, shows, in a very interesting manner, 

 the various degrees in which the African, Asiatic and West 

 Indian influences appear to have been exerted in different sec- 

 tions of the American Continent: — 



37 100 39 100 63 100 

 Total on the Continents, 150 species. 



It will be noticed (from the last table), that the African in- 

 fluence is manifest only in Mexico, where the Asiatic influence 

 is at its minimum, and the West Indian (the rest of North 

 America excepted) at its maximum ; that the Asiatic influence 

 is greatest in South America, diminishing towards the North, 

 while the West Indian influence has its maximum in the North, 

 and becomes gradually less towards the South. 



It is remarkable that on the Continent of America there 

 is no peculiar genus of operculated shells, save Bourciera 

 (Cyclostomacea), of which 2 species only are known, and 

 Schasicheila (Helicinacea), embracing 5 species, while in the 

 Islands there are very many strictly West Indian genera re- 

 presented by a multiplicity of species. 



