250 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



portion of the body, it is brought more and more into line with the 

 hinge and posterior adductor muscle, and consequently ceases to be 

 needful.' 



It is possible that in the case of Mytilus the predisposing cause may 

 be due to the long- continued action of gravity operating on successive 

 generations of suspended animals, aided perhaps by some other 

 morphological influence. Whether a similar tendency to mono- 

 myarianism observable in forms that, like Pecten, Ostrea, etc., rest 

 on their sides, may be attributable to a like cause is not clear, but it 

 is at least remarkable that so many of the Monomyaria should be 

 forms that assume a position out of the normal vertical. 



Tridacna, so long a puzzle, and concerning which it was even held 

 that the animal must have rotated in its shell, has been successfully 

 shown by Mons. R. Anthony - to be simply a case of a monomyarian 

 that has taken to live with its umbo downwards. All its anatomical 

 features correspond closely in arrangement and position with those of 

 Mytilus, only it occupies a relatively reverse position, and its huge 

 plastic body tends by its own weight to spread out and consequently 

 to form a shell that has its longer axis at right angles to that of the 

 Mytilus shell. 



To return, however, to the pelecypod shell. Most of the bivalves, 

 as a matter of fact, do not live in exposed positions, but burrow more 

 or less deeply into soft sand or silt. Here those that do not penetrate 

 to any depth below the surface, and do not live in deep water beyond 

 the reach of ground swells, are liable to considerable pressure from the 

 shifting of the loose material that surrounds them. Hence these 

 generally have acquired stout, more or less globular, shells, as in 

 Isocardia, Cardiiim,^ the Veneridse, etc. 



The disadvantage of this form of shell, of course, is the amount of 

 muscular power required to force a passage with it down into the 

 sand. A gauge of this may be seen in the huge scar of the retractor 

 pedis muscle in the Veneridse, that has generally been overlooked 

 because it is situated at the back of the broad hinge-plate. 



In proportion, however, as the bivalve seeks shelter from the strains 

 of the shifting sand, either in quieter waters or by burrowing deeper, 

 so the shell in response tends to become less heavy and solid, and to 

 assume a flatter shape, permitting of more rapid passage down into 

 the silt. This is seen in the later date forms, such as Tellina, Psammohia, 

 and Scrohicularia. The habit of deeper burrowing is of necessity 

 accompanied by an increase in the length of the siphons to ensure 

 proper respiration, and this in turn results in the prolongation of the 



1 TMs was first pointed out by Mr. B. Sharp, Proc. Acad. Philad., 1888, 



pp. 121-124, and first illustrated by specimens in the Index Hall of the Natural 

 History Museum shortly afterwards. 



2 Comptes-rendus Acad. Sci. Paris, torn, cxxxviii (1904), pp. 296-298. 



^ It is interesting to note that the spines on the shell of the prickly species of 

 Cardium are moi'e pronounced on those dwelling in sand than on those 

 individuals inhabiting muddy or silty sea- floors, the more shifting material 

 exacting a better means of anchorage. 



