ON MOLLUSCA. | wi 
of the siphonostomata, where it is always corneous; it is not 
formed into a spiral, but on one side we see the striz of aug- 
mentation, which have commenced towards one of the extre- 
mities, and on the other a space more or less ovaliform, orna- 
mented with subregular striz, at the middle of a border or 
smooth swelling, much broader on one side than the other. 
Another form is that of the calcareous or corneous operculum 
of the anentomostomata. It presents, in fact, constantly, a 
spiral enrolment on one and the same plane, more or less 
visible on the two faces, and constantly so on the internal one. 
But the summit of the spire varies much in its degree of 
eccentricity; sometimes it is even altogether central, as in the 
corneous opercle of the trochi, which is formed of nine or ten 
spiral turbinations; it is much less so in that of turbo; and 
finally, in nerites it is completely lateral. ‘The presence or 
absence of colour on the external face of this kind of opercle, 
the disposition of the striae which adorn it, the furrows with 
which the internal or adherent face is often marked, may also 
furnish excellent characters to confirm the distinction of genera 
and species. Unhappily, this part of the organization of the 
mollusca has been too much neglected. 
This same property of entering or not with the animal into 
the shell, appears also to have some influence on the chemical, 
corneous, or calcareous nature, and on the thickness of the 
operculum. In fact, in the first case, it is constantly corne- 
ous, and most frequently slender and flexible, especially on the 
edges, while in the second it is often calcareous and very 
thick. It may, however, be simply corneous; accordingly, 
we sometimes find in the same natural genus of the anento- ° 
mostomata some species which have a corneous operculum, 
and others a calcareous operculum, which is never observed 
among the siphonostomata and entomostomata. 
Finally, the last relation under which the operculum can 
vary, is that of adherence. All the calcareous opercula, and 
VOL. XII. N 
