ON MOLLUSCA. 167 
Bonnet, that the shell grows by intus-susception. In fact, 
the experiments of Reaumur, in which he has shown that a 
hole made in the shell, or in a part of its spire, or even at the 
edge, is not filled by the circumference, but at once and inde- 
pendently of the shell itself, have placed the matter beyond 
the reach of doubt. 
The form of this shell, and even the predominance of animal 
over mineral matter, must then be in relation with the form of 
the skin, or mantle, and with the age of the animal. Accord- 
ingly the tubular, spiny, lamellary elongations which we often 
remark at the surface of a shell, are only elongated productions, 
lobes, strips as it were of the mantle; in the same manner as 
the sinuses, the emarginations are produced by the habitual 
but intermittent production of some organ, such as the respir- 
atory tube, the head itself, the oviduct, &c.; but properly to 
comprehend their formation, we should follow the developments 
of a conchyliferous molluscum, from the moment of its appear- 
ance in the egg from which it has issued up to the completion 
of its growth, and from that point to its death. 
Every molluscous animal, however large and dispropor- 
tionate to its body the shell may become in the course of 
time, has once presented an inverse disproportion, that is, the 
shell which is perceptible very early in the egg, has been at 
first much smaller than the body, and was consequently very 
far from being able to contain it, as may be seen in the helico- 
limax. It has equally commenced by being almost entirely 
membranaceous ; originally then its free edges were really in 
the skin itself, since they did not yet attain the limits of the 
mantle. By the addition of new interior strata, and the in- 
crease of the quantity of calcareous molecules, the shell is 
thickened and solidified, but at the same time it has grown in 
such a manner that the edges of its aperture have attained to 
the limits of the mantle, at first only in the state of repose or 
retraction. However, the animal has issued from the egg 
