172 SUPPLEMENT 
species. As to the sinus, sometimes extending to the length 
of a spur, and forming a sort of gutter, it appears owing to a 
prolongation or fold of the mantle, and perhaps also to the 
organ of generation. 
Another consideration to which the examination of shells 
gives rise, and of which it will be proper to say something, is 
that of the muscular impression produced, as we shall see far- 
ther on, by the communication or adherence of the muscular 
fibre with the shell. This adherence, so strong in the living 
state, is nevertheless very little so after death. Does it con- 
sist in a simple application? This appears very probable. 
Whatever it may be, the traces of it which remain upon the 
shell are always more or less evident, and form very fine strie, 
more or less parallel or concentric. In the univalves there is 
almost always but a single muscular impression produced by 
the dorsal bundle of the columella, and which perfectly well 
indicates its form. Little or not at all visible in the spirivalves, 
in consequence of its depth, it becomes so in the species where 
the last turbination is very large, as in concholepas, haliotides, 
and the argonauts. But it is especially so in the patelloid 
species, or those in which the shell is not rolled: its form is 
then almost always like a horse-shoe, open in front, for the 
passage of the head of the animal, and with branches more or 
less unequal. On some species of the non-symmetrical patelle 
of Linnzus, the right branch of the horse-shoe is divided 
into two by a smooth space, or canal, of ne great depth, 
through which, without doubt, the water proceeds to the 
gills. Some other species of true patelle have their mus- 
cular impression, as it were, lobate, or strangulated from 
space to space; and finally, some species not symmetrical, 
have really two distinct impressions, the horse-shoe being 
interrupted behind. 
The shell of the acephalous mollusca presents, on the con- 
trary, several muscular impressions, much more frequently 
CO 
