ON MOLLUSCA. 257 
shells, for the purpose of studying and naming their different 
parts. Linneus, Bruguiéres, Da Costa, Lamarck, &c. place 
the shell which they study standing on the extremity opposite 
to the head, and the aperture in front of the observer. M.de 
Blainville, on the contrary, imitating Draparnaud, and many 
other authors, supposes it placed obliquely on the back of the 
animal, or what comes pretty nearly to the same thing, applied 
on a table, on the side of the aperture, and consequently the 
head, or highest point, being backwards, and upmost, while 
the opposite extremity is in front,and below. From this it re- 
sults, that the names of right and /eft are applied to the same 
sides, according to the two different points of view ; but those 
of ¢nferior and superior, in the description of the aperture and 
its edges, are replaced by the words anterior for the first, and 
by that of posterior for the second. 
The summit or head (apex), which is the part where the 
shell has begun, may be altogether flat or very projecting, 
straight or vertical, or inclined directly backwards, to right, or 
left, but it does not appear to be ever directed forwards. It 
may be pointed or nippled, entire or carious, and even some- 
times hollow, as in the bulle. 
The base (basis), or the part usually opposed to the summit, 
is that in which the aperture, of which we shall presently 
speak, is invariably pierced. Under this name, however (for 
we follow the system of M. de Blainville), we do not under- 
stand what Linnzus and the majority of conchologists so desig- 
nate. In fact, with them it is the extremity, pointed or not, 
opposite the head, and they name it thus, because, in their 
manner of designating the different parts of a shell, they placed 
it vertically, the head being above, and the aperture in front; but 
in the system which we follow, the base is all that part which 
rests more or less obliquely on the back of the animal. Some- 
times this base is very broad and round, as in the trochi, 
which gives them the form of an inverted top; at other times 
VOL. XII. S 
