A conchological manual / By G.B. Sowerby.

  • George Brettingham Sowerby II
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: A conchological manual / By G.B. Sowerby. Source: Wellcome Collection.

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    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 w:th 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 pur¬ poses 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. From the apex, the next layer is deposited on its edge, and advancing beyond it necessarily adds to its extent, 1 hus, suppose for the sake of illustration, the part marked a in the diagram, fig. 4, to represent a nucleus, the cross lines (Z) 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 striae, or lines of growth, while on the inside the edges of the laminae 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 4. Imaginary cone. a. Apex. b. Base. 1. Lines of growth.
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