230 SUPPLEMENT 
as the testacelle ; but this is rare. More of them live in the 
air, at the surface of the earth, as the limaces, helices, &c. 
Some are to a certain point amphibious; that is to say, they 
can breathe the air, by means of their organ of respiration, and 
yet live in the water, which they quit but seldom, as for 
example, the limnez and planorbis. Finally, most of the 
malacozoaria live constantly in fresh or salt water, current or 
stagnant, which is the case with all the acephalophora with- 
out distinction. The waters of the Dead Sea, though so 
strongly bituminous, contain living conchyliferous mollusca. 
Some are also found in thermal waters; for example, the 
turbo thermalis, a paludine species no doubt, lives in those of 
Abano, the temperature of which is 40° of Reaumur, while the 
Clio borealis appears to be unable to quit the polar seas. 
There are some characters which indicate this difference of 
media inhabited by the mollusca. ‘This is certainly the case 
with the aquatic and terrestrial species, since the organ of 
respiration has a peculiar structure. 
But this cannot be observed of those species which are 
entirely aquatic, whose gills present no difference to explain 
why they should act in fresh water or insalt. Would the shell 
itself alone furnish any characteristic differences which would 
indicate the nature of the sojourn of the animal? Not by con- 
sidering it merely in itself, but up toa certain point it will, by 
comparing the shells of marine animals with the fresh-water 
or terrestrial, as we shall see a little farther on, when we come 
to add a few remarks on the subject of conchology. 
It has been a question, whether the species which are 
found habitually in salt-water can live in the fresh-water, 
and vice versd. ‘This question, to which a great importance 
is attached in geology, would be resolved in the affirmative, if 
we were. content with analogy alone; for we know indubita- 
bly that certain fishes quit the waters of the sea to live in 
those of rivers, and others quit the latter for the former, as for 
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