Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/291

Rh spoken only of as lateral ridges passing through the chambers and septa, they will be found in most specimens to be formed of a solid spathose substance. But in the specimen No. VI, in which is a transverse section of one of these shells near to its upper extremity, these ridges will be seen, in consequence of the cavities not having been filled by the infiltration of calcareous matter, to have originally possessed a singular organization: a substance of extremely loose and light texture is disposed in plates, which radiate towards the sides of the containing tubes, leaving interstices between them, nearly equal in size to that of the radiating substance itself.

It is indeed impossible to speak decidedly as to the manner in which this peculiar organization could influence the buoyancy of the animal and its dwelling. It is however not difficult to conceive that the animal might possess the power of contracting and of expanding this radiating substance; and might, by thus admitting or expelling the water, produce that change in the specific gravity of the whole mass as might occasion it to sink or rise according as the necessities of the animal might direct. The great substance of this shell, it being generally about half an inch in thickness, does not, it must be acknowledged, appear at first sight to be favourable to the opinion of its having been capable of being rendered buoyant. But the appearances offered to our observation by the specimen No. VII. give reason for supposing that the texture of the shell was such, that its size might rather promote than prevent its buoyancy. In this specimen the looseness of the texture of the shell is very remarkable; it must however be admitted, that it is difficult to ascertain how much of this depends on the original lightness of structure, the interstices not having been filled up by calcareous matter, and how much on subsequent decomposition.

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