2298 SUPPLEMENT 
lusca. Be this as it may, we know that this caries aug- 
ments in breadth and depth with age, and that the uniones of 
all countries exhibit this singularity. 
The anomalies or deformities of shells are of two sorts: the 
one is sufficiently explicable, the other not so. 
We may rank in the first kind the relative thickness 
which individuals of one and the same species may attain to 
in their growth; and, in fact, we find some individuals in 
certain genera, which, although complete, are much smaller 
than others. This, without doubt, is owing to a difference in 
the quantity of nutriment, either in the same, or in a different 
locality, as may be seen among the hexapod insects. Accord- 
ingly, we cannot admit the notion of Bruguiéres, that this 
difference necessitates the change of skin in the animal, some- 
thing like that of the epidermis in insects. 
We must also consider in the same light the double swel- 
lings which are formed in certain univalve individuals, after 
that, having come to their full growth, the normal swelling has 
been produced. This, no doubt, is referrible to an over 
excitation of the vital powers, determined by some local cir- 
cumstance. ‘ 
We must equally place in the same class the artificial form 
which certain slender bivalve shells may assume, and the in- 
ferior valve of which adheres in its full extent. Not only does 
this assume the form of the body on which it is applied, but 
the upper valve follows the form of the lower. This observa- 
tion, made upon the anomie, is owing to M. Defrance, and is 
explained by the circumstance, that the upper valve must 
follow the form of the body, which has itself been modified by 
that of the lower valve moulded on the foreign body. 
A phenomenon pretty nearly inexplicable is the degree of 
elevation in the spire of the univalves. In fact, we know that 
the same species presents, under this relation, some differences 
