254 SUPPLEMENT 
green, we pretty frequently find that they are nacreous, or of 
an exceeding whiteness. Never, at least up to the present 
time, have any been found, the aperture of which is really 
emarginated, and its edges are always straight and trenchant. 
As for the univalve marine shells, they are often very diffi- 
cult to be distinguished from the preceding. In general, how- 
ever, they are thicker, and much more frequently provided 
with pads, varices, spines, &c. Their aperture, very fre- 
quently emarginated, or elongated like a tube, more or less long 
anteriorly, is pretty often edged by a thick pad, which may be 
tuberculous, scaly, or laciniated. ‘They are sometimes nacre- 
ous in the interior, when they are covered by an epidermis, 
which is scaly, pilose, and in general of a very different 
aspect from that of the terrestrial, and even of the fresh-water 
shells. 
2. Under the relation of the degree of depth at which they 
are found, the marine mollusca have been separated into 
littoral and pelasgian, according as they are to be met with 
on the sea-shore, or at depths more or less considerable in the | 
high seas. But it must be allowed that this division is still 
worse than the preceding, since no character inherent in the 
shell can be brought in support of it. 
3. Under a relation almost anatomical, a distinction of 
shells has been established into external and internal. The 
internal shells are in general much thinner than the external, 
flat, or but slightly rolled, and without epidermis, of no colour 
but white or yellowish. 
4. The size is next taken into consideration, for the sepa- 
ration of univalve microscopic shells, which are those, as may 
easily be conceived, which are so very small, as not to be 
seen but by the assistance of the microscope. But such a 
division can by no means be clearly exact. 
5. If we consider the general form of univalve shells, with- 
out paying attention to any of their parts, denominations are 
