344 ANTILLEAN SUB-REGION chap. 



Southern Brazil, but is not found in the tropical portion of the 

 continent. Of the fresh-water Pelecypoda Myeetopus^ Hyria^ 

 Castalia^ Leila^ and Mulleria are peculiar forms, akin to the 

 Unionidae. 



(1) The Antillean Sub-region surpasses all other districts in 

 the world in respect of (1) extraordinary abundance of species, 

 (2) sharp definition of limits as a whole, (3) extreme localisation 

 of the fauna of the separate islands. The sub-region includes the 

 whole of the half-circle of islands from the Bahamas to Grenada, 

 together with the extreme southern end of the peninsula of 

 Florida, which was once, no doubt, a number of small islands like 

 the Bahamas. Trinidad, and probably Tobago, although contain- 

 ing an Antillean element, belong to the mainland of S. America, 

 from which they are only separated by very shallow water. 

 The sub-region appears to fall into four provinces : — 

 (a) Cuba, the Bahamas, and S. Florida; (5) Jamaica; (c) 

 San Domingo (Haiti), Porto Rico, and the Virgin Is., with the 

 Anguilla and St. Bartholomew group ; (d) the islands from 

 Guadeloupe to Grenada. The first three provinces contain the 

 mass of the characteristic Antillean fauna, the primary feature 

 being the extraordinary development of the land operculates, 

 which here reaches a point unsurpassed in any other quarter 

 of the globe. The relative numbers are as follows : — 





Cuba 



Jamaica 



San Domingo 



Porto Eico 



Inoperculate . 



. 362 



221 



152 



75 



Operculate 



. . 252 



242 



100 



23 



It appears, then, that the proportion of operculate to inoper- 

 culate species, while very high in Cuba (about 41 per cent of the 

 whole), reaches its maximum in Jamaica (where the operculates 

 are actually in a majority), begins to decline in San Domingo 

 (about 40 per cent), and continues to do so in Porto Rico, where 

 they are not more than 24 per cent of the whole. These oper- 

 culates almost all belong to the families Cyclostomatidae and 

 Helicinidae, only two genera (^Aperostoma and Megalomastoma) 

 belonging to the Cyclophorus group. Comparatively few genera 

 are absolutely peculiar to the islands, one or two species of most 

 of them occurring in Central or S. America, but of the several 

 hundreds of operculate species which occur on the islands, not 

 two score are common to the mainland. 



