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 general. But as the age of the animal advances, this surface is covered by an other ; the primitive colours disappear, others are disposed over them, and the substance itself of the shell becomes con thickened. This process is per formed by a simple, but most curious or. ganization. Two soft, membraneous flaps, or winged processes, pass out of the opening of the shell, turn back on the external convex surface, and cover it so completely as not to leave the least por tion to be seen at the line where they meet each other, on the back of the shell. From the superior surface of these mem. braneous bodies, that surface which clings to the convex part of the shell exudes that secretion, by which the shell is in creased in bulk ; and a new arrangement of the beautiful colours of its drapery is effected.
Brugulere ascertained this formation of a second surface by actual observation at Madagascar ; 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 positive proof is rendered by rub bing down the second coat, when the markings of the first coat will make their appearance.
Although the colours are thus dispos. ed by the animal, the action of light ap pears to have a considerable effect in aug menting their brilliancy : climate also oc casions considerable 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 temperate zones. But the circumstance just no ticed, there is great reason for attributing the high degree of colouring in shells more to the effects of light than to the heat of the climate.
It is not the colouring only of the se cond coat of the porcellaneous shells which is given by this second operation ; the several asperities observable in vari ous shells, as in the Cypraa tuberculosa, and in the Cyprus pediculus, are super added at the same period.
The form of the shell will necessarily depend on that with which the worm is first invested ; but it is a curious subject of investigation, on what peculiar modifi cation 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 organiza tion ; it being sufficient, he supposes, to consider the fibres 011 one surface of the body to be shorter than those on the op posite surface. From this inherent pecu liarity of formation, the prolongation of the animal must necessarily be in a spiral form ; and from the various obliquities of disposition of these shorter and longer fibres will proceed that variety of direc tion, in which the spiral turns will be dis posed.
The 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 $nimal has extended the shell. Thus, if the turns of the spire have been carried round an axis of a co nical 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 terms depart more or less from the centre. But if the terms 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.
To enable us to account for the longi tudinal, raised and thickened ribs, which cross, in a longitudinal direction, the turns of the spire, and which are termed varices by Linn us, it is necessary to re fer 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 re turn of the animal through the opening, has an additional portion of the testaceous matter repeatedly deposited upon it, by which it acquires a bordered edge, which differs in size and thickness in the shells of different animals. But in sea-shells, the growth of the shell is not completed on the formation of this bordered lip ; the continuation of the spire still proceed ing after its formation. To explain the formation of these ribbed longitudinal pro jections, it is therefore only necessary to suppose the animal, after having formed the bordered edge to the mouth of the shell, to have proceeded uninterruptedly for a certain period in the formation of the common smooth surface of the shell,which as it would then extend beyond the lately formed lie. would now render it a rib nro jecting from the general surface ; and by a repetition of this process, a succes sion of these ribs, with intermediate spa ces of the common surface, such as fre quently exists in turbinated shells, would consequently be formed.