GEOLOGICAL RELATIONS. 183 
climate than the present, though they do not now exist 
in southern latitudes, and therefore a very considerable 
change in their habits must have since taken place. 
Notwithstanding the facility with which the terrestrial 
mollusks accommodate themselves to the physical influ- 
ences which act upon them, such a change is not consist- 
ent with what we know of their history, and hence the 
most reasonable conclusion is, that the climate in which 
they have lived, from the days when the multitudes 
which now compose the mass of the fossil beds were in 
the enjoyment of life upon the surface of the earth, to 
the present time, has remained essentially the same. 
The question of the identity of this fossil, with any 
living species of Melicina is also interesting, as upon its 
solution, perhaps, may depend the opinion we may form 
as to the comparative remoteness of the period when all 
the fossil species of the formation flourished. If it 
should be considered to be specifically distinct from any 
other known living form, or in other words to be an ex- 
tinct species, we should refer its existence to a more 
ancient date in the tertiary period than would otherwise 
be assigned to it. If on the other hand it should prove 
to be identical with an existing species, it would date 
back only to the most recent epoch. This point we 
have established to our own satisfaction by carefully 
comparing specimens of the fossils of the Wabash de- 
posit with the few specimens we have seen of the only 
species of Melicina which inhabits the country north of 
the tertiary section of the coast of the Gulf of Mexico, 
