340 Natural History of Molluscous Animals : — 
reous operculum previously to hybernation, is unable to make 
any thing more than a membranous substitute “ when deprived 
of nourishment ;” and that the snail itself thrives better, and 
is found in greater numbers, in chalky districts than elsewhere. 
This may all be tr ue, and yet it proves little; for we are far 
from denying that the food is the source of the lime; and 
that lime is not necessary to be eaten in order to the perfect 
formation of the operculum is proved, as Mr. Bell remarks, 
by many snails in his possession having formed that part, 
though during the whole summer they had no access to any 
prepar ation of lime. * And, further, sea-water and sea-weed 
contain a mere fraction of calcareous matter ; yet what and 
how many large and compact shells are produced by creatures 
which have no other sustenance! I repeat, then, that though 
the food unquestionably is the source of the material of shells, 
yet has the architect as certainly the power of changing its 
constituent principles, and of alteri ing their qualities. 
The mode in which the shell is built i is less a mystery; so 
that the second question admits a more satisfactory answer. 
If you will examine the snail of any common Helix, you will 
perceive that where the body rises into the shell there is a 
fold or membrane of a semicircular shape. ‘This part is 
denominated the collar, from the manner in which it surrounds 
the body, and it is the organ which secretes the shell. ‘The 
animal is born with the rudiments of its future covering, and 
by its gradual increase of growth is enabled to push the col- 
lar for a space, and from time to time, beyond the original 
margin. In these operations a thin layer of membranous and 
éalcar eous matter is excreted and deposited, which is gradually 
thickened by successive lay ers being laid on within the first 
by the repeated protrusions and retractions of the collar. 
This portion being formed, the animal commences another, 
and finishes it in “the same manner; and the extent of each 
portion is marked in some shells by an elevated rib, in others 
by aslight depression. There is not, as the language of some 
authors - would seem to imply, a regular and alrenaate deposi- 
tion of a layer of membrane and a layer of lime; but in all 
shells the animal and earthy matters are obviously secreted 
and deposited at the same moment and in commixture. And 
although it be true that the exterior or smallest layers are 
formed first, and that others are successively deposited sub- 
jacent to ‘hen yet it does not follow, as is commonly stated, 
that each new layer is complete, and extends beyond the mar- 
gin of the former one, so that the shell, by every addition, 
* Zoological Journal, 1. 96-7. 
