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. 
“Tn 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 Chame 
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.”—Gray, Phil. 
Trans. 1833, 776. ' 
“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 im 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. 
“Tn 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 Jsocardia 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 bemg 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. 1838, 
775. 
It thus appears that the valve of a bivalve shell resembles the 
univalve shell of a Gasteropodous mollusk— 
1. In shape, one valve being like a dextral, and the other like 
a sinistral univalve. 
