FORMS OF SHELL IN BIVALVES. 
39 
The much greater difficulty found in di.ssolving the calcareous 
shell of Anodonta, than was experienced in dissolving those of Helix 
pomatia and Limna’a stagitalis, suggests the probability that the 
calcareous basis of the shell in Anodonta maybe more especially com- 
posed of that form of carbonate of lime termed Aragonite. 
Bivalves are mostly Equivalve, the right and left valves corres- 
ponding with each other in form, size, etc., as in Anodonta cygnea 
and other species. All our fresh-water Bivalves are, however, strictly 
Equivalve shells, with the exception of Dreissensia polymorpJia which 
is undoubtedly Inequivalve, the left 
valve being somewhat larger than the 
right, and to some extent overlapping 
it near the acuminate umbones, and 
in addition to this di.sparity in size 
and shape, the byssal aperture or sinus 
is usually placed exclusively upon the 
right valve, though in some specimens 
the byssal opening modifies more or less unequally the shape of both 
valves and, in rare instances, that of the left valve only. Although 
our .species are thus with one exception Equivalve, they are almost 
invariably and universally Inequilateral or more or less unequal- 
Fig. 93. — An Inequivalve Inequi- 
lateral Bivalve. 
Dreissensia polymorpha (Pallas), 
Baker's Dock, Stourport, 
Collected by Mr. J. W. Williams. 
Fig. 94. — A Posteriori}* produced Inequilateral Bivalve. 
Unio pictoriun (L.), right valve. 
Hethersett Lake, near Norwich, collected by Mr. A. G. Stubbs. 
sided, the umbo being placed towards one end, and the anterior side 
being usually the .shortest, as in Unio pictorum, etc. ; but this is 
reversed in Pisidium, as in many .species of that genus, the posterior 
side is distinctly shorter than the anterior ; this interesting struc- 
tural difference can very easily be verified by observing the position 
of the ligament which in Unio will be found to be always at the 
