SHELL. 
ligament is placed, by which the valves are 
closely connected. This chink is formed 
by a slight separation of the valves at this, 
the anterior, part, which having a distinct 
surface from the general disc, and being 
indeed separated from it by an angle, or by 
a raised or sunk line, it may be distin- 
guished as the corslet : this surface is par- 
ticularly distinguished in the shells of the 
Venus kind. On these parts, in some spe- 
cies, are spines, in others various markings, 
and in others, these parts are plain. At 
the lower part of the anterior margin of the 
valves, within the limits of the corslet, and 
above the ligament, is a part generally distin- 
guishable from the rest of the surface by a 
difference in the colour, stri®, &c. This 
part varies in its form, being canaliculated, 
replicated, &c. Its edges are termed lips. 
On the other side of the hinge, on the 
posterior part, and near to the hinge, is ge- 
nerally a lunated depression ;■ the crescent, 
like the part just described, it varies in its 
form and markings in different shells. Both 
these parts derive half their form from each 
of the valves ; and are consequently sepa- 
rated in the middle. To determine the side 
to which a valve belongs, the shell need 
only be placed on the hinge with the ante- 
rior side forward, when it directly shows 
itself. 
In some irregular shells, as of oysters and 
Spondyles, the shells are divided into upper 
and under, the upper shell being flatter 
than, the under. In the Terebratul®, the 
upper shell, the beak of which is pierced, 
is more tumid than the under : but in most 
shells, as in the Pectens, and the oysters, 
the upper valve is almost always the least 
tumid. 
The hinge is, in general, furnished with 
teeth ; but sometimes it is without. When 
placed on the side, they are said to be lateral ; 
and when on the inferior extremity, terminal. 
The central teeth, essential to the genus, 
are termed the cardinal teeth ; and the 
others, accessory or secondary. The teeth 
are in some shells articulated, in a cavity, 
in the opposite valve ; in others, they are 
not. In multivalves also the shell is tiie 
combination of all the valves, whether con- 
nected by articulation, or by ligaments. 
From the observations and experiments 
of Reaumur, which have been since consi- 
derably extended by Bruguiere and others, 
the formation and growth of shells have 
obtained considerable illustration. Leu- 
wenhoeck, Swammerdam, Lister,and others, 
had observed, that, at the first escape of 
the animal from the egg, it was invested by 
a complete spiral turn, at least, of the shell ; 
and, it appears, from the observations of 
Adanson, that viviparous shell fish are like- 
wise brought forth in a similar state. The 
growth of shells was discovered by Reau- 
mur to take place in that which is most dis- 
tant from the first formed part. Here a 
part of the animal exists which is not yet 
covered w'ith shell ; but which is beset with 
a vast number of vessels, which separate 
and deposit, on the existing edge of the 
shell, those glutinous and calcareous mat- 
ters, by which its due increased extent of 
surface is obtained. In proof of this being 
the process by which the augmentation of 
the shell is effected, he broke the shells of 
various living testaceous animals in different 
parts, and was thus enabled to perceive, 
that the newly added matter was not depo- 
sited line after line from the shell, at the 
edges of the fractured part; but was sepa- 
rated, in a pellicle, from the body of the 
animal, and thus applied at once to the 
whole of the vacuity. Those, who denied 
this mode of increase, denied also the remo- 
val of the posterior termination of the 
worm, from the extreme termination of the 
first formed spire. But it is certain that 
the animal, in many shells, have their pos- 
terior termination attached to the point of 
the shell only in their earliest stages ; and 
that, when older, it is found adherent to the 
second or third turn : and it has also been 
ascertained that the tail of the Nerite is at- 
tached beneath the left or columellar lip. 
In proof of these animals possessing this 
power of removing themselves frbm these 
turns of the shell, or indeed from those parts 
in which their residence is no longer neces- 
sary, the circumstance may be adduced of 
the animal belonging to the Porcellanea, 
sometimes abandoning its old shell, and 
forming a new one. 
The various tints of different colours with 
which shells are so beautifully adorned, 
result, according to the remarks of the same- 
ingenious naturalist, from an oeconomy and 
organization somewhat similar to that which 
has been just mentioned. On the neck of 
the animal, that part from which the matter 
of the shell is supposed to be secreted, the 
colours of the shell may be detected. Thus 
if tlie ground of the shell be yellow, and it 
be marked with dark brown or black bands, 
then the neck of the animal will be seen of 
a white inclining to a yellow hue, with 
dark spots, answering in their number and 
direction to the bands observable ip the 
