182 INTRODUCTION. 
ing to the opinions at present received among geologists, 
that the epoch of their deposition corresponded with the 
time when the surface of the earth in that region was 
diversified with lakes of considerable extent, and that it 
was antecedent to the period when, by the lifting of their 
beds, the surface attained its present position, or when 
by some relative change in the level of the land, the 
lakes were drained of their waters. We have said that 
these deposits contain the species of terrestrial and 
fluviatile shells inhabiting the surrounding country. Of 
the species indigenous to that section, nearly two thirds 
have already been found in a fossil state (although but 
little attention has been given to them) and their identity 
is beyond all doubt. There is, however, a single ap- 
parent exception to the general remark, in a species of 
Helicina, which Mr. Say, supposing it to be a recent 
species, described under the specific name occulta, and 
which is one of the most common species among the 
fossils. As the genus Helicina belongs mostly to inter- 
tropical regions, and has rarely been met with in a re- 
cent state in so high a latitude as that occupied by these 
fossils, a good deal of importance has been attached to 
its occurrence here as indicating such a change of cli- 
mate as has been alluded to. But this supposition 
creates more difficulties than it obviates, for the numer- 
ous species of other genera found in company with the 
species in question, and which live at this time in the 
same distinct in which the fossils are situated, must, ac- 
cording to this view, have also been adapted to a warmer 
