SHELL. 
shell itself : the colouring matter appearing 
to be here disposed, ready for its deposi- 
tion, with the other substances of the shell. 
In this manner, and on these principles, has 
Reaumur been able to explain almost every 
circumstance, respecting the varieties of 
form and of colour which are observable in 
these bodies. 
But a circumstance is observable in the 
Olives and the Porcellaneae, to which Reau- 
mur has not attempted to apply his doctrine. 
In these shells the colours are disposed in 
two layers, the outer of which is the pro- 
duction of an organization different from 
that of any of the other inhabitants of shells, 
and from which proceeds an operation, also 
different from what occurs in any other in- 
stance. At the period at w'hich Reaumur 
wrote, several of the olives and of the 
cowries, in consequence of certain differ- 
ences of colour, and of some difference also 
in their forms, were considered as of dif- 
ferent species from others, of which they 
were in fact individuals of the same spe- 
cies ; bat had not yet acquired their perfect 
state. For want of reckoning upon this 
circumstance, the celebrated Linnaeus has 
unnecessarily multiplied the number of spe- 
cies of these shells. 
The highly ornamented surface of these 
shells is formed at two different periods, 
and by two different processes. The first 
appears to be that deposition which takes 
place from the surface of the body of the 
animal ; and in which but little takes place 
different from what occurs in shells in gene- 
ral. But as the age of the animal ad- 
vances, this surface is covered by another ; 
the primitive colours disappear, others are 
disposed over them, and the substance 
itself of the shell becomes considerably 
thickened. This process is performed 
by a simple, but most curious organiza- 
tion. Tw'o soft, membraneous flaps, or 
winged processes, pass out of the open- 
ing of the shell, turn back on the external 
convex surface, and cover it so completely 
as not to leave the least portion to be seen 
at the line where they meet each other, on 
the back of the shell. From the superior 
surface ■ of these membraneous bodies, that 
surface, which clings to the convex part of 
the shell, exudes that secretion by which 
the shell is increased in bulk ; and a new 
arrangement of the beautiful colours of its 
drapery is effected. 
Bruguiere ascertained this formation of a 
second surface by actual observation at 
Madagascar j and the fact derives addi- 
tional proof from the pale line which passes 
frequently along the back of these shells, 
marking the part where the edges of the 
membraneous wings met. Still more posi- 
tive proof is rendered by rubbing down the 
second coat, when the markings of the first 
coat will make their appearance. 
Although the colours are thus disposed 
by the animal, the action of light appears 
to have a considerable effect in augmenting 
their brilliancy ; climate also occasions con- 
siderable differences in this respect ; hence 
the shells obtained from the torrid zones 
are much more rich in their colouring than 
those which are found in the more tempe- 
rate zones. But notwithstanding the cir- 
cumstance just noticed, there is great rea- 
son for attributing the high degree of co- 
louring in shells more to the effects of light 
than to the heat of the climate. 
It is not the colouring only of the second 
coat of the porcellaneous shells which is 
given by this second operation, the several 
asperities observable in various shells, as in 
the Cypraea tuberculosa, and in the Cypraea 
pediculus, are superadded at the same pe- 
riod. 
The form of the shell will necessarily 
depend on that with which the worm is 
first invested j but it is a curious subject of 
investigation, on what peculiar modification 
of the animal depends the spiral form of 
univalve shells. This has been attempted 
to be explained, by Reaumur, by a very 
simple peculiarity of organization : it be- 
ing sufficient, he supposes, to consider the 
fibres on one surface of the body to be 
shorter than those on the opposite surface. 
From this inherent peculiarity of formation, 
the prolongation of the animal must neces- 
sarily be in a spiral form ; and, from the 
various obliquities of disposition of these 
shorter and longer fibres, will proceed that 
variety of direction in which the spiral 
turns will be disposed. 
Tlte umbilicus, the cavity in the centre 
of the spire, which is seen in the lower 
part of the shell, depends on the direction 
in which the animal has extended the shell. 
Thus, if the turns of the spire have been 
carried round an axis of a conical form, 
each turn having departed more and more 
from the centre of the shell, an umbilicus 
will be formed, more or less open, as the 
turns depart more or less from the centre. 
But if the turns are made round an axis so 
straight and fine as to occasion the turns 
to touch each other, then no umbilical cavity 
will be formed at the bottom of the shell. 
