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, bj 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 dramed of their waters. We have said that 

 these deposits contain the species of terrestrial and 

 fiuviatile shells inhabitmg the surrounding country. Of 

 the species indigenous to that section, nearly two thirds 

 have abeady 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 

 Selieina, 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 

 fossUs, 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 district in which the fossils are situated, must, ac- 

 cording to this view, have also been adapted to a warmer 



