176 SUPPLEMENT 
itself into a spire, and often in a very regular manner, and 
sometimes forming a great number of turbinations, is a ques- 
tion to which it is extremely difficult to reply, and more espe- 
cially so as the subject has been by no means sufficiently 
studied. From this, however, may be derived good characters 
for genera and families, for the opercle differs not only in its 
point of attraction, in its size, relatively with that of the orifice 
of the shell, but also in its form, its chemical nature, and its 
mode of adherence. | 
We have already observed its principal differences with 
respect to its point of attachment. 
As to its size, it is often sufficiently developed to close the 
aperture of the shell even atits orifice, as in all the cyclostomata, 
being applied almost on the edges. But sometimes it is much 
less so, and does not close it but when it has been more or less 
sunk in the spiral cavity. This is the case with almost all the 
siphonobranches. Finally, it also happens that it is almost 
rudimental ; that is to say, that it can only close a very small 
part of the aperture of the shell, as in some purpure, in strom- 
bus, and more particularly in the cones. 
This facility with which the opercle can enter more or less 
into the aperture of a univalve shell, has a necessary influence 
on its general form. In fact, when it remains at the orifice 
itself, applied in the little widening formed by the peristoma, 
it has constantly the form of its aperture ; accordingly, almost 
circular in the cyclostomata, it is elliptical in the ellipsosto- 
mata, semi-circular in the hemi-cyclostomata, or nerites, &c. 
In the species where it sinks into the spiral cavity it again 
pretty nearly presents the form of its orifice, but it is much 
smaller. Finally, in those where it is only rudimentary, there 
are no longer any relations between its form and that of the 
aperture of the shell. 
As to its specific form, it also varies in a manner determi- 
nate for each very natural group: oval or rounded in the shells 
SS 
