242 SUPPLEMENT 
while the animal was living, begin to incline by degrees, 
become horizontal, are filled with the substance in which 
they were sunk, resist the pressure of the accumulated strata, 
so aS sometimes to remain perfectly entire, with all their 
asperities, or if not so, they are broken and crushed, and 
disposed in beds more or less free from every other shell, or 
even from every other foreign body. This is very perceptible 
in the alluvions formed at the present mouths of our great 
rivers, or in the creeks of our sea-coasts, where currents pre- 
vail, which justifies our presuming by analogy that at those 
parts of our continents where similar accumulations are to be 
found, there was formerly the mouth of ariver, or some gorge 
in which the waters formed an eddy. The other mollusca, 
living freely at the bottom of fresh and salt water, without 
sinking in the sand or mud which constitutes this bottom, or 
sunk only in the moveable part, at their death abandon their 
shells. These shells, rolled, overturned, cast against the 
rocks and the projections of the soil by the movements of the 
waves, are broken, reduced to a fragmentary state, more or 
less fine, and are then drawn along in the habitual direction 
of the currents and the winds, and accumulated along the 
shores, especially in bays, over an extent and to a height 
often very considerable. The strata which result from these 
are thus entirely composed of fragments, more or less bulky, of 
shells, often rolled, which consequently have lost their aspe- 
rities, and are often of very different genera, which depends a 
little upon the localities. It is also remarked, that in the 
structure of these strata the fragments are in general deposited 
according to the laws of specific gravity, and that they are 
little or not at all intermingled with mud or other foreign 
substances, the entire shells which have escaped the de- 
structive action of the currents, being filled even to the bottom 
with detritus, or coquillaceous sand. Many specimens of this 
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