of Gasteropodous Mollusca. 479 



spread out on a table, with the umbones above and the front end 

 towards the observer, the valve to the right (the left when on 

 the animal and in its usual walking position) resembles a dextral, 

 and that on the left a sinistral, very depressed, spiral shell. This 

 is well illustrated by comparing the left valve of an Isocardia 

 with a Concholepas. 



" In some very rare instances these shells also are reversed, but 

 the fact is not easily observed except in the unequal- valved kinds. 

 There were formerly in the Tankerville collection (they are now 

 in the Museum) two specimens of Lucina Childreni, in one of 

 which the right valve was a dextral shell in opposition to the 

 general structure. A much more remarkable variation is to be 

 observed in some of those bivalve shells whose under valve is at- 

 tached to foreign bodies ; thus for example, most of the Chamce 

 are attached by their left valve, but some species, such as Chama 

 Lazarus, are frequently attached by the right valve, under which 

 circumstance the teeth proper to the left and usually attached 

 valve are transferred to the right, and vice versd.'' — Grayy Phil. 

 Trans. 1833, 77Q. 



" In bivalve shells the apex of each valve is always placed on 

 or near the dorsal or upper margin, varying its position on this 

 part in the different groups. Thus in the Pectines and other 

 suborbicular shells, which having a very large subcentral poste- 

 rior adductor muscle, were called by Lamarck Monomyaires, 

 the apex is generally in or near the centre ; while in most of the 

 other genera it is placed more or less towards the anterior extre- 

 mity of this margin, and is sometimes incurved. 



" In some of these shells the apex is spirally twisted, and the 

 spire becomes more developed as they increase in size. 



" Now this could not take place if the valves remained insepa- 

 rably united together at the same part of the dorsal margin, but 

 it is provided for by the hinge of the shell being gradually moved 

 backwards on the edge of the valves, the ligaments separating 

 in front of the hinge into two parts, one of which diverges along 

 each of the umbones and forms a spiral groove down the suture 

 of the whorls. In Isocardia the umbones seldom make more 

 than half a turn, but in one specimen of Chama in my collection 

 they have made an entire revolution, and in another a revolution 

 and a half. The valves of these shells being unequal, the spiral 

 part of the lower or attached valve is produced into an elongated 

 cone, while in the other it is depressed and simply marked with 

 a spiral groove like that of an operculum." — Phil. Trans. 1833, 

 775. 



It thus appears that the valve of a bivalve shell resembles the 

 univalve shell of a Gasteropodous moUusk — 



1 . In shape, one valve being like a dextral, and the other like 

 a sinistral univalve. 



