860 M. de Blainville on the Shell of Cephalated Mollusca. 
really astonished at the facility with which certain geological 
conchologists have multiplied the number of genera, and espe- 
cially of species, without its, perhaps, being ever inquired, what 
is a genus, or a species of shells, and upon what characters can 
tire distinction be established. The greater number seem to 
leave themselves, as it were, to the direction of a sort of instinct, 
scarcely studying even the aspect or habit ; and yet the conse- 
quences which they elicit from these preparatory works, if we 
may so call them, are often of great importance in geology ; 
since, from the identity, more or less complete, of the fossils of 
two strata, often situated at considerable distances, they infer 
the identity or difference of those strata, their order of superpo- 
sition, and their relative antiquity. I shall not at present stop 
to show how, in general, the employment of fossils in geology 
is liable to abuse, although it cannot be denied to be of the 
greatest utility, when properly managed ; and how it would be 
of importance, before deciding whether a species of shell is lost, 
or differs from another found at more or less considerable dis- 
tances, to be somewhat better acquainted with the species which 
live in our seas at the present day ; and the differences which 
age and local circumstances produce in the same species, and 
which are much greater than is generally thought. My ob- 
ject is merely to make known, in this notice, an observation 
which I made a considerable time ago, upon the difference of 
individuals of different sexes, in certain species of shells of the 
class of cephalated Mollusca. 
Although little attention has been paid to the subject until of 
late years, we now know that this class of mollusca contains 
three very different combinations of the generative apparatus. 
In the first, the sexes are separated in two different individuals. 
The whole of the species of this section are not operculated, but 
the operculum only occurs in this group. In the second com- 
bination, the two sexes, although really distinct, are united in 
the same individual ; and, lastly, in the third, we can only dis- 
tinguish one sex. It is according to this consideration of the 
generative apparatus, that I propose henceforth to divide the 
first class of molluscous animals into Dioecious, Hermaphrodite 
and Monoecious Cephalata ; a subdivision which does not seem to 
me to destroy any natural relation, and which perfectly corres- 
ponds with the animal degradation, by establishing a passage to' 
