112 University of 3Iichigan 



These peculiarities of distribution may be explained by the 

 hypothesis that the molluscan species of the Dutch Leeward 

 Islands are considerably changed remnants of the ancient 

 Antillean fauna of Archiguiana {cf. Yon Ihering, 1907; 

 Archhelenis and Archinotis, p. 111). On the mainland, this 

 fauna is largely replaced by Brazilian and northern, conti- 

 nental elements, although it may still be represented by 

 Brachypodella, various Pomatiasidae, probably such groups 

 as Serieea, Analcadia and Tamsiana among the Helicinidae, 

 and the Lesser Antillean group of Amphicyclotus among the 

 Cyclophoridae. The Oleacinid group Ravenia from Los 

 Roques is another Antillean group of this same general region. 

 In addition, there are a number of doubtful species, such as 

 Eutrochatella semilirata, Helicina kieneri and Oiondropoma 

 suhaiiriculatum, described as from Venezuela (of. no. 137 of 

 this series), and Cerion antonii, cited above, which arouse the 

 suspicion that a more thorough study of the northern coast 

 of South America may bring a greater number of these 

 ''relics" to light. 



However, it is somewhat doubtful if the Dutch Leeward 

 Islands could have retained their species as the direct and con- 

 tinuous descendants of the faunas of such a land mass. The 

 present altitude of such late Tertiary coral formations as 

 the cap of the Tafelberg of Sint Hyronimus (C20) seem to 

 indicate a possibility that the islands were completely sub- 

 merged in comparatively recent times. Much of the present 

 fauna of the Dutch Leeward Islands and the presence of An- 

 tillean elements in northern South America is also explicable 

 on the basis that these are all comparatively recent arrivals 

 by drift or other occasional means of dispersal, and that some 

 of them are only able to gain a foothold in places where they 

 do not come into competition with the continental species. 

 This is especially true of the fresh water species; their pres- 

 ence in artificial ponds and reservoirs certainly indicate ex- 

 traordinary means of dispersal, such as carriage by birds. 

 But, such endemic groups as Cistulops, Stoastomops, Bonairea 

 and Cerion s. s. can scarcely be explained by this alternative 



