In both cases, the nucleus is generally of a more horny and transparent composition than the parts subsequently produced. As soon as the animal is hatched, or, in other words, leaves the egg or body of the parent, of course it begins to increase in size, and to require a corresponding enlargement in the shell. To effect this, a small quantity of mucus substance, secreted by the mantle of the animal, is deposited on the edge of the aperture. When this is dry and become sufficiently hard, it is lined by a more calcareous secretion; and these together form a new layer, which is followed by others in succession; each new layer being larger than the one that preceded it until the whole being complete, the full-grown animal is invested with a shell commensurate with its own proportions. Thus from the apex or nucleus the formation proceeds, as it were, downwards, taking the shape of the part which secretes it, on which it is in a manner moulded. The nucleus, or first formed portion, may for technical purposes be considered, mathematically, as the apex of a spiral cone. And here it must be observed, that whether the shell consist of one or several pieces, each piece has a separate nucleus, and the process of formation is separately repeated with each. The word cone is used for convenience, and its meaning extended so as to include all those structures which commencing at a point enlarge downwards. [Illustration: 4. Imaginary cone. _a._ Apex. _b._ Base. _l._ Lines of growth.] From the apex, the next layer is deposited on its edge, and advancing beyond it necessarily adds to its extent. Thus, suppose for the sake of illustration, the part marked _a_ in the diagram, fig. 4, to represent a nucleus, the cross lines (_l_) will shew the consecutive layers, which enlarge their circle as they add to their numbers. This disposition of shelly matter into layers is marked externally by concentric striæ, or _lines of growth_, while on the inside the edges of the laminæ are consolidated into a kind of enamel. If a perpendicular section of a solid portion of a shell were magnified, it would present, in many instances, an appearance resembling the diagram, fig. 5; _a_ may be taken to represent the horny part of the layers which form the outer coating, named "_Periostaca_," or "_Epidermis_;" the undulating line _b_, is formed by the edges of the calcareous layers, and causes the striæ, or lines of growth, which are often distinguishable on the surface of the shell; the space _c_ is the middle part of those layers, and at _d_ they are consolidated into the enamel which lines the interior. [Illustration: 5. Supposed section of a part of a solid shell.]