1902.] AND ANCIENT GEOGEAPHY. 345 



The same zoogeographical question has been investigated by 

 Simpson l with reference to the land and freshwater Mollusks. He 

 points out that among this group in the Greater Antilles we find 

 quite a number of species which are identical with species from 

 Central America and Mexico (list p. 488, /. c), and, besides, there 

 are in both parts numerous and more or less closely allied forms. 

 Simpson docs not distinguish very sharply these two categories, 

 identical and allied forms, but they correspond very likely to the 

 same two groups among our Decapods. 



Now Simpson draws the following conclusions: Sometime during 

 the Eocene the Greater Antilles were elevated and connected with 

 each other and with Central America by way of Jamaica (and pos- 

 sibly across the Yucatan channel). Then a period of subsidence 

 followed, culminating in the Miocene and submerging the Antilles 

 with the exception of their highest parts, which ended the connec- 

 tion with Central America. In Postmiocene times the Greater 

 Antilles were elevated again and attained their present shape. 



For the Lesser Antilles the matter was entirely different. These 

 islands did not exist at all in Eocene times or were submerged sub- 

 sequently, since their Mollusk-fauna, with the exception of a few 

 forms which may have reached them by drift, shows no affinities to 

 that of the Greater Antilles. After the formation of this island 

 chain, during the course of the Tertiary, 2 it was populated chiefly 

 from South America, and, as Simpson believes, by drift. The 

 South American (Venezuelan) origin of the fauna of the Lesser 

 Antilles is also confirmed by our material. Potamocarcinus denta- 

 tus points directly to Trinidad and Venezuela and not to the 

 Greater Antilles. I should doubt, however, that this species has 

 reached these islands by drift, and I am inclined to assume a con- 

 tinental connection of these parts, which may have been of short 

 duration, during the later Tertiary. I am loath to believe that it is 

 possible for these freshwater crabs to be transported across salt 

 water, and the fact that one species is found on the islands of 

 Guadeloupe, Dominica, Martinique, St. Lucia, another in Trini- 



1 Simpson, C. T. : " Di-trihution of the Land and Freshwater Mollusks of the 

 West Indian Region, and their Evidence with regard to Past Changes of Land 

 and Sea" (Pr. U. S. Nat. Mus., Vol. 17, 1895). 



a That these islands were formed during the Tertiary is also the opinion of 

 Hill. See Report by Robert T. Hill on the volcanic disturbances in the West 

 Indies in 'lhe Nation. Geograph. Magaz. Vol. 13, 1902, pp. 229, 240, 265. 



