Occasional Papers of the 3Iuseum of Zoology 111 



to the known South American fauna, can only be explained 

 on this basis. 



As already indicated, one of the most notable features of 

 this island fauna is the total absence of typically South Amer- 

 ican groups, such as the Streptaxidae and the Strophocheili- 

 nae, and the paucity of others, such as the Achat inidae and 

 Bulimulidae. In fact, not a single, characteristically South 

 American genus reaches these islands, although four (Dry- 

 maeus, Oxystyla, Thysanophora and Guppya) might be con- 

 sidered as widely distributed, tropical or subtropical groups 

 which penetrate South America to a considerable extent. 



On the other hand, Cerion is practically limited to the 

 northern Antilles and Florida, although a single species, Ce no n 

 antonii (Kuester), has been described from Guiana. ]Micro- 

 ceramus is another Antillean genus, which reaches the main- 

 land of North America but is apparenth^ lacking from the 

 southern Antilles and South America. Stoastomops is, I be- 

 lieve, most closely related to Stoastoma from Jamaica, while 

 Cistulops appears to have distant affinities with Troschelvin- 

 dex from central Cuba. The Chondropominae are also a char- 

 acteristically Antillean group, although they reach the main- 

 land in many places around the Caribbean and the Gulf of 

 Mexico. In addition, the two freshwater species, Potamopyr- 

 gus parvulus and Planorhis pallidiis, are widely distributed 

 in the Antilles, although they are represented by closely re- 

 lated species in northern South America. 



Although the moUuscan fauna of these islands is thus 

 mainly Antillean in its general affinities, some of the individual 

 groups are most closely related to those of northern South 

 America. For example, Tudora rupis, T. mxiskusi and T. 

 aurantia are placed in the section Tudorata, along with Tudora 

 plicatula from Venezuela. Also, the genus Neosubulina is 

 known only from the Dutch Leeward Islands and northern 

 South America, while Bradiypodella raveni and gihhonsi be- 

 long to the mainland group of their genus. Finally, Oxystyla 

 maracaibensis imitator and TliysanopUora crinita have already 

 been listed as South American species that also occur in Aruba. 



