ON MOLLUSCA. 255 
employed, which, though vague enough, are nevertheless 
necessary to be known. 
The first distinction is that which relates to the equality or 
inequality of the two sides of a shell, of whatsoever form, 
separated by a fictitious axis, taken from the head to the base, 
or from one extremity to the other. A shell is named symme- 
trical, the two sides of which are perfectly equal, and non- 
symmetrical when such is not the case. Thus the bone of the 
sepia, the shell of the argonaut, that of the patelle, &c. are 
symmetrical. ‘The Chinese patelle, the sigaret, and many 
others are not symmetrical. 
The flat shells are those which have no cavity, as the bone 
of the sepia, the Chinese patella, &c. 
The tubular, those whose diameter is considerably less than 
the length. 
Covering or sheathing, those which are conical, and with- 
out any spire, properly so called, as in the patelle. 
Spiral, those which are more or less turned, and in differ- 
ent directions, as we shall presently explain. But it is first 
necessary to define some terms, which belong to the shell con- 
sidered in the mass. We name, 
Discoid, those which more or less resemble a disk, and 
which, considering the manner in which the spire is rolled, 
are termed rolled, as in the ammonites. 
Depressed, the species oval or rounded, whose form is very 
much flatted, and the spire very short—for example, the 
sigaret. 
Globular, those of which all the diameters are obviously 
equal, in consequence of the great development of the last 
turbination of the spire, which is much larger than that which 
precedes, as in the ampullariz, &c. 
Oval, or ovoid, the species whose longitudinal diameter is. 
a little longer than the transverse, as in the porcelaines, and a 
considerable number of the helices. 
