The shell of the Patella, a section of which. is represented infig. 178, is a simple cone placed upon the back of the creature, which it com pletely covers, and upon which it is evidently moulded. On making a section of the animal, as in the figure, the shell is found to be entirely lined by the mantle a, 6, by which it is secreted.
That the whole surface of the mantle is capable of secreting the calcifying fluid from which the shell is formed, is distinctly proved by the manner in which a fracture or perforation in any part is speedily repaired by the deposition of a patch of calcareous matter beneath it, but in the ordinary growth of the animal the differ ent portions of the mantle execute different functions. It is obvious that the enlargement of the body of the patella, as its age increases, must necessitate a corresponding enlargement of its habitation, and this is principally effected by additions of calcareous matter in succes sively larger rings around the mouth of the shell only ; the great agent therefore in forming the shell is the margin of the mantle, 6,G. This hangs loosely as a fringe near the mouth of the shell, and being moveable at the will of the animal, the calcareous matter which it pre eminently furnishes may be laid on in succes sive layers to extend the mouth of its abode ; and these consecutive additions are indicated externally by concentric lines running parallel with the circumference of the shell, the num ber of which necessarily increases with age. Whilst the abode of the creature is thus en laraed by the deposition of shell from the vas cular and spongy margins of the mantle, the office of the rest of that membrane is reduced to the increase of its thickness, depositing succes sive coatings of calcareous particles, which are laid on to its inner surface, and when a section of the shell is made (f), these last-formed strata are readily distinguishable by their whiteness and different arrangement. So far the produc tion of an external shell is entirely similar to what we have met with in the formation of the internal defences of the naked Gasteropoda, yet in other respects the former are much more ela borately organised. In the first place many of them are adorned externally with colours, not unfrequently arranged with great regularity and beauty ; these tints belong exclusively to the outer layers of the shell, that is, to those formed by the margins of the mantle, and are produced by a glandular structure appropriated to the secretion of the colouring matter, which only exists in the vascular circumference of the cal ciferous membrane. The colouring matter
becomes thus incorporated at definite points, with the cement by which the shell is extended, and is arranged in various manners according to the position of the secreting organs which furnish it. Another peculiarity which distin guishes external shells is that their outer sur face is often invested with a membranous layer, called the epidermis, which having been re garded by some authors as a part of the true integument of the body, has given rise to the supposition that all shells being placed between two layers of the skin were in fact internal, the difference between the one and the other con sisting merely in the extent of development. In support of this opinion reference has been made to the great thickness of this epidermic coat, which not unfrequently is such as to give to the surface of the shell a felted or pilous ap pearance ; but if such an idea were correct, it is evident that the epidermis must be formed prior to the deposit of calcareous matter be neath it, which observation has disproved, in asmuch as those shells in which the epidermic covering is most dense and shaggy are found whilst in ovo to be without such an investment. The so-called epidermis, therefore, whatever may be the aspect which it presents, whether it be, as is usually the case, a brittle lamella en crusting the shell, or a flocculent and pilous covering, is evidently inorganic, being merely a crust of inspissated mucus, originally secreted with the calcareous particles, and forming when dry a layer encrusting the surface of the shell.
There is yet another structure common to shells of this class, of which it remains to speak, namely, the enamel or pearl, wh ich lines such por tions of them as are immediately in contact with the body of the animal ; this polished material may be likened to the glazing of an earthen ware vessel, and is a varnish produced from the general surface of the mantle, by some mo dification of its secretion the nature of which is unknown, and spread in successive coatings over the more coarse calcareous matter, where ever such a polish becomes needful.