166 On ihe Gnathodon beds around the head of Mobile Bay. 
orographic changes have rendered the water permanently fresh; 
but it is evident, the few hardy individuals that still live there, 
cannot long abide the effects of their unnatural situation. 
uta more remarkable event in the history of the species is 
that of their entire destruction north of Florida. This phenome- 
non, considered as a rare effect of that more remarkable part 
of the eco i 
stibject ness oe It likewise suggests the impor- 
tance of more extended observation respecting what relates to the 
species in this aati of their former geographical range—such as 
the condition of their remains, and the various other paper 
peculiar to these localities ; also the other organisms that n 
found imbedded with them ;—together with those indiestigns that 
may be discovered explanatory of the cause of their extinction; : 
whether it be an alteration of climate, or some orographic change 
affecting the thermal influence of the gulf stream along the coast ; 
or any other agency yet undiscovered. 
What has already been ascertained concer une the adaptations 
of the animal may serve to throw light upon some of the circum- 
stances in which the species is found, aiaions living or fossil. 
Their natural adaptation to an exposed situation in the loose mud, 
furnishes the reason why they are so generally found, either in the 
sinuosities of the shore, or the cavities of river channels, where the 
wind and the current may not disturb them. In addition to these 
ee their limited locomotive power, somewhat as in 
e case of the oyster, will, without the necessity of referring to 
any shi special agency, natural or artificial, sufficiently indicate 
the cause why such large masses are found precisely in those cit- 
cumstances, where they are actually presented to our observation. 
These deposit will now claim our attention in a geological 
point of v 
he fit eal ddccata regards their position, which occurs in 
three very distinct varieties of ee answering to as 
many successive periods in which they were formed. First, the 
submerged beds; next, those which are one partially submerged ; 
finally, ‘such as are entirely elevated above the water, at various 
distances from it, and at different elevations. 
e submerged beds are not unfrequent around the head of 
Mobile bay. They have been observed more particularly in Raft 
river, where they occur in a very regular succession for miles, at 
a depth of fifteen or twenty feet. The various channels that 
intersect the extensive morass between the Mobile and Tensaw 
rs a seem for the most part to be weil adapted to the prosperity 
the species in question; at least, this was eminently the case — 
in a more remote period, for here are two of the most remarka-— 
ble deposits, within the limits of our observation, one for its eX- 
traordinary superficial extent, the other for its st 
