136 COOKE: GEOGRAPHICAL DISTRIBUTION OF MOLLUSCA. 



Streptoslcle. This genus is thus quite peculiar to the Mascarenes 

 proper, not even occurring, according to our present information, in 

 Madagascar, and must therefore in all probability, together with 

 Pachystyla, have been developed when these three islands became 

 finally separated from Madagascar, and were still, if not intimately, 

 united, at all events much larger and nearer together than they 

 now are. 



(ii) Madagascan Element. The principal link between the 

 Mascarene subregion and Madagascar (which is by no means as 

 strong as we should expect), is found in a part of the operculate 

 land fauna. Cydostoma, so richly represented in Madagascar, is 

 present (with Otopomd) in several fine living forms, and the number 

 of subfossil species is a clear indication that this group was, not long 

 ago, much more abundant, for, of the 16 Cydostoma known from 

 Mauritius as many as 10 are subfossil. Bourbon has 3 sp., 

 Rodriguez 4, and the Seychelles 1. The operculates, as a whole, 

 form a decided feature of the land fauna, thus in Mauritius there 

 are 32 species, or more than 28 per cent, of the whole. 



(iii) Indian and Australasian affinities are unmistakably present 

 in the Mascarene fauna. From some points of view, the group 

 looks like a fragment of Polynesia transplanted to the western shores 

 of the Indian Ocean. Thus we have Omphalotropis profusely 

 represented, a genus especially characteristic of small islands, which 

 does not occur in Madagascar or Africa, Ceylon or India, but first 

 appears in the Andamans and Nicobars, is sparingly distributed in 

 the Malay Archipelago, and becomes abundant in the New 

 Hebrides, the Viti and Society Islands. The two Helicinoz (Mauritius 

 and Seychelles) represent a genus whose distribution is to some 

 extent identical with that of Omphalotropis, while the single 

 Leptopoma (possibly a Leptopomoides) is also of strongly eastern 

 relationship. Cyelotopsis, Cyathopoma, and Geostilbia are markedly 

 Indian genera. Microcystis is Indian and Polynesian. Patula and 

 Tornatcllina are Polynesian only, the nearest recorded species of 

 the latter being a straggler from the Philippines. Hyalimax —and 

 this is a very striking fact — occurs nowhere else but in the Andamans 

 and Nicobars, and on the Aracan coast. The nearest relation to 

 the Seychelles Mariaella appears to be the Cingalese Tennentia. 

 Not a single representative of these eleven genera has been found 

 even in Madagascar. 



The fresh-water mollusca (omitting the Neritidce) are : Mauritius 

 9 species, Bourbon 5, Rodriguez 4, Seychelles 6, with only 15 species 

 in all. The one Planorbis is probably identical with an Indian 



