ON MOLLUSCA. 227 
ed nd 
The diseases of the mollusca are doubtless not very numerous, 
but assuredly they are not very well known, at least as far as 
the animal itself is concerned. Should we regard as such the 
peculiar alteration exhibited by oysters when they pass to a 
state of greenness? Nothing can be less certain than this. 
Nevertheless, observing that the oysters which pass into 
this kind of state live in stagnant water, and that they are 
in general smaller and less fleshy than the others, we may 
be allowed to think that the particular vibrio, to which they 
owe their green colour, does not sufficiently nourish them, and 
that the water, half fresh, and seldom renewed, in which they 
are, does not sufficiently excite their organic activity. 
The diseases of the shell are probably more numerous, and 
better known. The first is the separation or breaking of the point 
of the spire. This is observed in many species of univalves, and 
among others in some of the genus bulima. Although this 
takes place only with shells of the turriculated form, neverthe- 
less it cannot be this circumstance alone which determines 
this fracture, since most shells of this form exhibit no such 
appearance. It is more probably owing to the circumstance 
of the animal growing very fast, and quickly abandoning the 
commencement of the spire, while the vitreous matter, de- 
posited to fill the deserted cavity, is more fragile and less 
lamellated. 
A kind of change, which is remarked at the summits or 
hooks of a great number of fresh-water bivalve shells, which 
compose the genera unio and anodon, has perhaps some analogy 
with what we have just mentioned concerning the univalves. 
This, however, is not certain ; and in fact many authors have 
supposed that this kind of caries, which appears to eat in an 
irregular manner, not only into the summit, but even into other 
parts of the unio, and that often pretty deeply, was owing to 
the destructive action of the animals which feed on the mol- 
Q2 
