BIVALVIA. 225 



A peculiarity exists in many of the species, as well as in some of the species of its 

 generic allies, by which a considerable variation is produced in the degree of tumidity 

 at the posterior part of the opposite valves, causing the impression of the mantle upon 

 the interior to display a difference in form as well as in extent ; the compressed or 

 right valve having its sinus shorter and somewhat broader or higher, while in the 

 more tumid one it extends forward so as almost to touch the anterior adductor. This 

 does not appear to depend upon a difference in length of the two siphons, as in some of 

 the members of this, so called, family, which are furnished with tubes of an unequal 

 length, this difference in the sides of the animal does not exist, and the mantle- 

 mark is the same in both valves ; but in others, in which the tubes are precisely alike, 

 this inequality in the valves is very conspicuous ; the difference in the mantle-mark 

 appearing to be coexistent with the difference in the tumidity of the valves. 



If the drawings by malacologists be correct, the inhalent siphon is the longer 

 one in some species, while in others, this lower or indrawing tube is the shorter of the 

 two, and apparently without producing any difference of tumidity at the posterior part 

 of the valves. It is, however, doubtful whether a safe reliance can be placed upon the 

 published figures of the Bivalve Molluscs, as some of the animals are represented 

 with the upper siphon most extended, while in other very proximate species the 

 lower projects beyond the upper one. Mr. Alder informs me these siphons are so 

 elastic that either may be made to appear the longer at the will of the animal, which 

 perhaps is the cause of this apparent diversity. 



The inequality of proportions in the two valves gives to them a degree of obliquity, 

 and, when viewed in a position with the animal upon its ventral margin or standing 

 upon its foot, indicates an inflexion or incipient spirality in a dextral direction. 

 The want of symmetry in the two valves does not pervade the whole group ; for 

 species evidently otherwise very closely allied are not possessed of this deformity. 



Amongst other peculiarities of this genus may be mentioned T. Burneti, rather an 

 aberrant species, brought from the Coast of California, it has its right valve quite flat, 

 while the left is convex or lenticular ; and in the newer Tertiaries of South Carolina 

 there is a fossil species strongly resembling it in general form, but which differs from 

 it in having the left valve the flat one. In both of these species the sinus in the 

 mantle-mark is large and deep, but is unlike that of our other unsymmetrical shells in 

 being of the same form and magnitude in both valves. 



Notwithstanding the great curtailment of this genus since its original establishment 

 by Linnseus, it still contains a very large number of species, particularly in the recent 

 state ; and these have a wide geographical distribution, extending from the Equator to 

 the Polar Regions, and they range vertically from low-water mark to nearly 100 fathoms. 

 The genus appears as early as the Coral Rag, with some doubtful forms in the Palaeozoic 

 Formations. In the older Tertiaries, twenty-three species have been figured and 

 described by Mr. Edwards from the deposits of that period in this country alone, and 

 ten were inhabitants of the seas by which the Crag was deposited. 



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