174 SUPPLEMENT 
attachment of the tubes, constitute what we name the abdomi- 
nal impression ; the one descending from the anterior ad- 
ductor muscle follows the direction of the shell, in a variable 
breadth, and at a variable distance, and arrives at or passes 
the impression of the attachment of the tubes, which forms 
an excavation, or a sinuosity more or less deep, and open 
behind. 
When a shell is finally come to its greatest degree of 
development in extent, the changes which it undergoes, 
always in relation with those of the animal, which tends to 
contract itself, especially in the lobes of its mantle, consist of 
little more than its augmentation in thickness, not by the aug- 
mentation of the strata which compose it, but by that of the 
vitreous matter, and in its increase of weight, by the diminu- 
tion of the proportion of organic to inorganic matter in its 
composition. The external strata lose more and more the fili- 
form productions, and the little epidermis which they might 
have had. The colours grow pale, are effaced, and disappear ; 
the striz, the tubercles, and even the varices, grow blunt, wear 
out, and become more and more levelled ; the shell is covered 
with earthy cretaceous deposits, and with animals which 
excavate lodges there; the spiny and tuberculous elongations 
are filled and solidified. On the contrary, the ordinary sinu- 
ses grow hollow and become larger. Some cavities of this 
kind are even developed, especially in the female individuals, 
in places where there were none during the greater part of 
life, so as to form plewrotomata in a great number of genera. 
The aperture is narrowed, the posterior extremity of the cavity 
is filled or partitioned by the successive advancement of the 
animal, and the death of the latter, the necessary consequence 
of life, determines that of the shell. This shell then loses by 
little and little the animal matter which it contained, and 
finishes by being nothing but a composition of carbonate of 
lime, and consequently often becomes very friable. The in- 
sensible movement produced by the laws of attraction between 
