30 FOSSILS AND NON-MARINE MOLLUSKS 



admitted as of specific rank; fully as many more could be added if all those 

 hitherto described were enumerated. I have no doubt that a large number 

 of well-characterized new forms will be added to the catalogue of those now 

 known, when the various islets are all thoroughly and systematically ex- 

 plored. 



Of the well-established species of the existing fauna a certain number 

 seem to have been lately introduced from Cuba or the adjacent region. These 

 naturalized members of the fauna include the species of Oleacina, Pleurodonte, 

 Orthostyla and Urocoptis, and one of the species of Bulimulus. The chief and 

 most conspicuous elements of the strictly indigenous landshell population are 

 the species of the genus Cepolis and of the genus Cerion, which together 

 make up nearly half the existing fauna. Eight Helicinidce and nine Cyclo- 

 stomatidce are next in importance, in the census. Remaining types are rep- 

 resented usually by only one or two species each. 



The genera Cepolis and Cerion take their origin from the Oligocene period. 

 During the Middle Oligocene the peninsular part of Florida was represented 

 by a group of islets which must have greatly resembled the Bahamas of to-day. 

 They were low with occasional lakes of fresh water, and generalized forms of 

 Cepolis and Cerion made up the bulk of their landshell fauna, in which 

 llelicina, Bulimulus, Urocoptis and Polygyra were represented, as well as 

 rianorbis and Lioplax. So far as the scanty remains in the Bowden marl 

 of nearly the same geological age afford evidence, the characteristics of the 

 Floridian island fauna and that of Jamaica were as different as they are 

 to-day, the fossils found having nothing in common. 



The present landshell fauna of south Florida is apparently not directly 

 derived from this Oligocene assemblage, of which the more tropical types may 

 have been eliminated during the comparatively cold Miocene epoch; but the 

 similarity to the present Bahama fauna is sufficiently striking to suggest 

 that the latter is the recent representative of the former. 



This representation does not proceed from a lineal succession on the spot, 

 for it is probable that the entire archipelago of the Bahamas may have been 

 submerged during comparatively recent geological time. The islands of 

 which it is composed are so low that their submergence during the changes of 

 level which are known to have taken place in the adjacent high islands of 

 Cuba, Haiti, etc., must have been almost inevitable. We are led to believe 

 that the types which existed in Florida also were represented in Cuba, which 



