172 On the Gnathodon beds around the head of Mobile Bay. 
loss of their epidermis, retain all the marks of recent. shells. 
Their normal characteristics are generally . be sought where 
they are imbedded in earthy mixtures; the determinations of the 
relative age of those beds that are differently constituted, must 
depend upon other criteria. 
An instance of the former kind is the interesting deposit a st 
‘Twenty-one Mile Bluff. These remains have continued with- 
out disturbance in their alluvial deposit, as they were originally 
formed, until the present period. They have lost their animal 
matter, their external surface has become somewhat worn by the 
wasting influence of time, and a silicious incrustation has been 
formed in their cavities, exhibiting the bas rant of an approxi- 
mation to a petrified state. From these older deposits an evident 
gradation in effects of a similar ends may be traced in other 
masses conn hp to the date of their formation as it approaches 
the present tim 
There are mee other organisms associated with the former, and 
though not numerous, they are more or less important as means 
to illustrate the history of these formations. Among these, the 
Cyrena of Carolina claims a particular notice; it is an estuary 
shell, and is usually associated with G. cuneatus, both living and 
fo ssil : and in both of these states it occurs in the same apparent 
numerical proportion. ‘There are, however, some few deposits, 
especially such as are more elevated, (the one near the junction 
of T’ensaw and Raft rivers is an instance,) in which the Cyrene 
are limited to particular sections; perhaps they are not adapted 
to every variety of cireumstance in which the G. cuneatus may 
exist. 
The Neritina reclivata is another species found in these de- 
posits, but in various proportions in different beds, and in some 
they are entirely absent. In the one at Twenty-one Mile Bluff, 
the sak occur in about the same numerical proportion as 
those the G. cuneatus itself, while there are very few to be 
didi in es large deposit, near the junction of Raft and Tensaw 
rivers. But it isa striking fact that at the present time, living 
ones abound in the vicinity of the latter, while the former is far 
beyond the limits of their present habitation. They are usually 
found living in calm, stagnant situations, attached to water plants 
and old sunken logs. In a locality of this kind, near Choctaw 
Point, they may be seen crawling over the bottom where the — 
G. cuneatus still exists. 
The few remains of other testacea found in these deposits 
such as the Unio and Oyster, must have been apace tg | 
methods more indirect. The former were probably tran 
from their more interior abode by means of the fresh ies eure 
rents. But there is evidence that the latter perhaps in some 
accidental way were to a small extent generated upon the } 
