2 MOLLUSCA FROM THE CRAG. 
like the two covers of a book, this is furthermore often strengthened by prominences 
and depressions in a part of the shell kept thickened for the purpose, interlocking each 
other, preventing, in most instances, the possibility of any material inconvenience 
arising from lateral motion without a fracture. The common action of the valves in 
their separation or opening is from the relaxation of the adductor muscles, when from 
the natural elasticity of the ligament the valves are drawn apart, and again closed by 
the contraction of the muscle or muscles that pass from one valve to the other, strongly 
adhering to the inner surface of the shell on which, in most cases, a distinct, and often 
a deep indentation is left. 
The muscular fibres by which the edges of the mantle are withdrawn adhere to, 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the assistance of muscles for that purpose, and again withdrawn under the protection 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 Palzeontologist, 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 Adacna, Lucina and Lucinopsis, and 
cannot, without violence to a natural arrangement, be removed to any distant position, 
