ON MOLLUSCA., 265 
Dorsal, or external, when it is pierced near the external 
edge. 
Internal, ventral, or against the spire, when it is towards 
the internal edge. 
3. As to its form, which may be round, oval, or triangular. 
In the unilocular shells the cavity is rarely divided into two 
only, and incompletely, by a straight lamina, more or less 
extended, which is named diaphragm, as in the septaria. 
At other times this lamina is more or less curved, thus forming 
a little tongue or horn, ex. crepidula, calyptrex, &c. 
The aperture, or mouth of the univalve shells, is the en- 
trance of their cavity. It is really formed or circumscribed 
by the edges, which are only the union of the interior surface 
of the shell with the exterior. Linnzus gives the name of 
faux, or throat, to all that part as far as we can see into the 
interior of the shell, that is, pretty nearly to the last semi- 
whorl. 
Some authors give the name peristoma to the entire thick- 
ness of the shell at its aperture, but most generally it is di- 
vided into two parts, designated as edges or lips, distin- 
guished into the internal and external, or the right and left, or 
columellary lip. 
Considered in totality, and along with a portion of the last 
whorl, which it terminates, the aperture is said to be falling, 
or inclined, when, not following the direction of the spire, it 
falls suddenly; inverted, when, on the contrary, it curves to- 
wards the spire. 
If we consider the aperture as to its regularity or irregu- 
larity, it is symmetrical when it can be divided into two parts 
perfectly equal and similar, and mon-symmetrical in the con- 
trary case ; thus it may be formed by the excavation, more or 
less considerable, of one of its edges, which ought to be taken 
into consideration. 
As to its proportional size to the rest of the shell, it may 
