A Monograph of the Crag Mollusca, Vol. 2

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Excerpt from A Monograph of the Crag Mollusca, Vol. 2: Or Descriptions of Shells From the Middle and Upper Tertiaries of the East O EnglandThese 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 Addend, Lacz'na and Luciuopsz's, and cannot, without violence to a natural arrangement, be removed to any distant position.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.