132 EOCENE MOLLUSCA. 



carry our opinions respecting what amounts to simple variation and what constitutes 

 specific distinction will probably long be matter for dispute. The differences in America, 

 considered sufficient for specific isolation according to this author, appear to be more 

 clearly defined than they are with us. We have nothing in other sections of the Mollusca, 

 either marine or terrestrial, that will bear a comparison with such an enormous difference 

 in the number of species as is alleged to be exhibited in this family, and if correct (and 

 I am not able to say that it is otherwise) it is an anomaly in the history of the Mollusca. 



The remains of fresh-water deposits of any past period give comparatively a very limited 

 number of species in this family, bearing in that respect a resemblance to the dissemination 

 of these animals on the continent of Europe. I have here figured seven species from the 

 Eocene deposits of England, but these are not at all well defined ; and there are nine or 

 ten in those of France. The Upper Tertiary species are, I believe, still existing. We might, 

 perhaps, expect that the limited number of living species should have descended from a 

 limited number of their predecessors ; but the fresh-water Tertiary deposits of America 

 appear also to have been but sparingly supplied by these animals, while the specific deve- 

 lopment in this family at the present day in America is out of all proportion when the 

 present is compared with the past, as is here attempted to be done with the Tertiaries of 

 Europe and their descendants. 



M. Deshayes considers one of the French Eocene shells U. Michaudi, to be very closely 

 related, probably the same, as a living species in North America, U. cicatricosus ; but I 

 have not been able to identify any one of our own. This may arise from a want of 

 acquaintance with the numerous existing species of the American waters, where almost 

 every conceivable form of the genus is represented. On a comparison with the figures 

 and descriptions of the existing shells, given by Mr. Lea, there may be pointed out two or 

 three which very closely resemble those of British Eocene species ; and when each comes 

 to be better known, and the specimens themselves compared, it is possible that one or 

 more may be found to have retired from England in the direction of America after the 

 Eocene period, through land, or rather rivers, that probably existed at that time on the 

 western side of the Eocene sea of Europe, such as has been the case with peculiar genera 

 of fresh-water fish and reptiles now confined to the American continent. Our own fossils 

 in this genus, from the older Tertiaries, are generally far from being in a perfect state of 

 preservation, so that no fair comparison can be made or relied upon. The specific separa- 

 tions here proposed must for the present be considered merely as provisional ; for with 

 the fate of an existing European species before our eyes, with its ninety-eight synonyms, 

 it would be hazardous in the extreme to pronounce decisively upon the few and in most 

 instances imperfect specimens hitherto obtained from our Eocene deposits. 



