ON MOLLUSCA. P47 
of the latter, and in the bivalves the proportion of the two 
sides of each valve, the direction of the furrows with which 
the superficies is worked, the system of coloration, of epi- 
dermic covering, &c. 
After this rapid sketch of the degree of relative importance 
of the characters which the different parts of the organization 
of the mollusca may present, and after the observation that it 
is often useful to consider their spoils, or protecting bodies 
isolatedly, we must add, that though this last be of real im- 
portance, the true basis of classification is, as much as pos- 
sible, in the general form of the body, the distinction more or 
less complete or non-existent, of head and body, and the 
organ which afterwards has most influence in modifying the 
shell, namely, that of respiration; at the same time observing 
that the same form of the shell may sometimes, though rarely, 
be represented in genera sufficiently different; such, for ex- 
ample, is the form of the haliotides, which exists in the pul- 
mobranches, in the chismobranches, and in the otides; it is 
the same with the patelloid and turriculate form, &c. 
Such are the principles followed by the most recent natu- 
ralists, for the divisions and higher subdivisions of the mol- 
lusca; let us now briefly consider those which regard the 
distinction of species, by far the most difficult part of science 
im all the types of the animal series, but still more in this 
than in any other, in consequence of the employment which 
has been made of the shell alone for this distinction. 
If the sex and age have an evident influence in determining 
the differences in the molluscous animal, and consequently 
in its shell, it is not less evident that circumstances not easily 
explained may act less profoundly no doubt, that is, little on 
the animal, but much upon the colour, size, and proportion 
of the parts of the shell. Of this we have certain proofs in 
species a great number of whose individuals may be seen at 
once, as the helix nemoralis, the limnza stagnalis, the buc 
