In Septifer the anterior adductor muscle is carried on a sort of shelf or myophore, and in Cucullaea the posterior adductor is partly raised on a similar and very prominent formation.
Length and breadth of bivalve shells is variously measured. Most authorities measure length, or ‘antero-posterior diameter,’ by a straight line drawn from the extreme anterior to the extreme posterior margin, and breadth by a similar line, drawn from the umbones to a point, not very clearly marked, on the opposite ventral margin (see Figs. [184] and [185]). Others, less correctly, reverse these terms. Thickness is measured by the extreme distance of the opposite faces of the closed valves. As a rule, the length exceeds, and often greatly exceeds, the thickness, but in a few cases—e.g. the Cardissa section of Cardium—this is reversed.
The periostracum.—Nearly all shells are covered, at some period of their growth, by a periostracum,[351] or surface skin, which serves the purpose of protecting the shell against the destructive effects of the chemical action set up by water or air. It also, in some cases (see p. [258]), acts as a kind of base upon which the shell is deposited. In old shells it is commonly worn away, especially at those parts which are likely to become abraded.
The form and composition of the periostracum varies greatly. Sometimes (e.g. Oliva) it is a mere transparent film, at others (Zonites) it is transparent, but stout and solid. It is corneous in Solenomya, covered with fine hairs in many Helicidae, in Conus, Velutina, and Cantharus it is thick, fibrous, and persistent; in Trichotropis and some Triton it is furnished with long bristles on a thick ground (Fig. [191]). In fresh-water shells it is usually rather thick, in order to protect the shell from the erosive powers of certain kinds of water. In some cases (Mya, Anatina) the periostracum is continued over the siphons, so as to form a protection throughout their whole length.
Fig. 191.—Triton olearium L., Mediterranean, an example of a shell with a stout and hairy periostracum. × ½.
Erosion.—The fresh-water Mollusca generally, and marine mollusca in a few rare cases (Purpura, Littorina) are subject to erosion, or decay in the shell substance. In univalves erosion usually sets in near the apex (Fig. [192]), where the life of the shell may be regarded as weakest, and in bivalves near the umbones. It is commonest in old shells, and rarely occurs in the very young. So long as the periostracum is present to protect the shell, erosion cannot set in, but when once it has been removed the shell is liable to the chemical changes set up in its substance by water. There is abundant evidence to show that erosion is caused by pollution of water. Out of many instances one must suffice. In a certain stream near Boston, U.S., numbers of Mollusca occurred, the shells of which were very perfect and free from disease. Some little way down stream an alkaline manufactory drained its refuse into the water. At and below this point for some distance every shell was more or less eroded, most of them seriously. Farther down, when the alkali refuse became diluted away, the shells retained their normal condition.[352]
Fig. 192.—Example of an eroded fresh-water shell (Melania confusa Dohrn, Ceylon).
A small percentage of lime in the water appears to produce erosion. The result of some experiments by G. W. Shrubsole, in the investigation of this point, may be tabulated as follows:[353]—