Excerpt from A Monograph of the Crag Mollusca, or Descriptions of Shells From the Middle and Upper Tertiaries of the East of England, Vol. 2: Bivalves

The muscular fibres by which the edges of the mantle are withdrawn adhere to, and leave a linear impression somewhat within the margin of the shell; and, in some of the Bivalvia, at the posterior side of the animal, are two siphonal tubes, formed by the prolonged portions of the mantle, the lower one is called the inhalent, the upper one the exhalent siphon, these tubes are capable of being protruded by the animal with the assistance of muscles for that purpose, and again withdrawn under the protection of the shell. In animals possessed of these tubes, the withdrawal of them is indicated in an impression on the body of the shell by the retractor muscle, leaving what is called a siphonal scar, or palleal sinus, which generally denotes, by its depth, a cor responding proportion in the length of the tubes; and where the muscular fibres of the mantle adhere to the interior, leaving the impression without an inflection, the animal either has no prolongation of the mantle, or that the tubes are so short as scarcely to be capable of extension beyond the margin of the valves, and the im pression in that case formed by the mantle is parallel, or nearly so, to the outer edge of the shell.

These marks, therefore, are of essential service to the Palaeontologist, as they afford the only indications of the form possessed by the animal inhabitant, thus im pressed upon the interior of the valves. It is however to be feared, that a perfectly strict reliance cannot always be placed upon the peculiar magnitude of this siphonal scar, even in specific determination, as a marked deviation from what might otherwise be considered its typical form may occasionally be detected, but it is in those species which are most subject to variation in the outward forms of the shell; as a general rule, this line, when visible, is of the greatest assistance, and at all times a good auxiliary character in the determination of a species. The length of the siphonal tubes, or the consequent indenture or sinuation of the mantle mark in the shell, points out a difference in the animal from those in which the sinus is wanting, or at least nearly so, where it indicates a mantle either without or with very short siphons, giving fair grounds for generic separation; but occasionally, species are met with that are otherwise very closely allied, having - a similar dentition, and bear the same general relationship in regard to the shell, although very unlike in the form of the mantle mark, such as Leda and Nucula, Cardium and Adama, lucina and Lucifzopsis, and cannot, without violence to a natural arrangement, be removed to any distant position.

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