SHELL. 
To enable us to account for the longitu- 
^Sina], raised, and thickened ribs which 
«ross in a longitudinal direction, the turns 
©f the spire, and which are termed varices 
by Linnicns, it is necessary to refer to 
the formation of the opening of land shells, 
when they have obtained their full growth. 
At this period the lip of the shell, from the 
frequent egress and return of the animal 
through the opening, has an additional por- 
tion of the testaceous matter repeatedly 
deposited upon it, by which it acquires a 
bordered edge, which diflfers in size and 
thickness in the shells of different animals. 
But in sea shells, the growth of the shell is 
pot completed on the fonnation of this 
bordered lip ; the continuation of the spire 
still preceding after its formation, lo ex- 
plain the formation of these ribbed longitu- 
dinal projections, it is therefore only neces- 
sary to suppose the animal, after having^ 
formed the bordered edge to the mouth of 
the shelf to have proceeded uninterrupt- 
edly for a certain period in the formation 
of the common smooth surface of the shell, 
which, as it wonld then extend beyond the 
lately formed lip, 'would now render it a rib 
projecting from the general surface; and 
by a repetition of this process, a succession 
of these rib.=, with intermediate spaces of 
the common surface, such as frequently 
exists in turbinated shells, would conse- 
quently be formed. 
From a process exceedingly similar we 
may suppose the formation of the warted 
protuberances and tubercles to have pro- 
ceeded; corresponding fleshy projections, 
existing on that part of the surface of the 
animal to which this part of the shell has 
been applied, having been the moulds, as 
it were, on which these have been formed. 
Whilst these fleshy protuberances have con- 
tinued, these processes have remained hol- 
low,; but where the fleshy protuberances 
have been diminished, these processes have 
become partly filled up ; and where they 
have been removed, these have become en- 
tirely solid, from the deposition of additional 
shelly matter from the surface of the animal, 
the existence of the cavities being necessary 
no longer than the continuance of the fleshy 
processes themselves. 
The production of the spinous processes 
so frequent on many shells approaches still 
nearer to that of the ribbed protuberances, 
since they have been formed by little, long, 
tongue-like processes, with which the neck 
of the animal is beset, and which have 
»erved iirthe same manner to produce these 
spines as the neck of the aniibal has served 
to form the bordered edge of the shell, by 
an addition of the testaceous matter at 
each time the neck of the animal, at certain 
periods, passed out or returned into the 
shell. The confirmation of this being the 
mode of their formation is yielded by the 
circumstance of these spines being ranged 
in lines, at equal distances, on the ribbed 
protuberances, or varices of Linnasus, the 
formation of which has not only been simi- 
lar, but simultaneous. 
The production of the channel, or gutter, 
the caiida of Linnaeus) depends on circum- 
stances extremely similar to those on which 
has depended the formation of the vai ions 
parts already noticed. All those animals 
whose shells have this termination are en- 
dued with an, organ of a cylindrical form, 
capable of being contracted and extended, 
so as to allow the animal to explore its path, 
or to attach itself to neighbouring bodies. 
By the frequent employment of this organ, 
and by its ^necessarily accompanying, fre- 
quent, alternate contractions and exten- 
sions, its surface, which possesses the pro- 
perty of exuding a testaceous substance, 
must at each passage and return contribute 
to the formation of this canal, which would 
serve the purpose of a sheath to it. 
By an ingenious application of these prin- 
ciples of Reaumur, Bruguiere and Brog- 
niart (“ Bulletin de la Societe Philomathe- 
que,” No. 25) have very satisfactorily ex- 
plained the production of the several fissures, 
stride, grooves, and other modifications of 
the forms and surfaces of different shells. 
The author fast mentioned has shown, that, 
in some instances, the organ by which some 
changes have been produced in the later 
formed part of the shell was not .acquired 
by the animal until it had obtained its full 
size. By an application of these principles 
to bivalves, and even to multivalves, their 
peculiarities of form have also been very 
ingeniously accounted for. It has been 
found, that, in bivalves, the part of the 
animal which is termed the mantle is the 
organ on which the peculiarity of the form 
of” each chiefly depends; it answering in 
this respect to the collar, or neck of the 
animal, belonging to univalve shells. 
It does not seem necessary here to dwell 
on the labours of those who, previous to the 
time of Linnmus, had endeavbured to dis- 
pose of shells under the most comprehen- 
sive and distinctive arrangement. What- 
ever had been done before with this view 
gave way to the more lucid and correct 
