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 nimety-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. 
