44 B.emarks on the Unios of the United States. 
specimen of the Fluviatilis so much twisted as not to be 
very unlike the Arca tortuosa. These malconformations 
are produced when the young shell becomes fastened be- 
tween hard substances, and as it increases in size it assumes 
in some degree the figure of the substance. 
No genus of bivalve shells is more liable to have the pe- 
riostracha eroded or carious, than the Unio. This decorti- 
cation always commences round the beaks, and seems to 
arise from two causes. In those cases where the carious 
part appears channelled or grooved, some parasitic animal 
has no doubt occasioned the defect, for these animals have 
been found making their depredations under the periostra- 
cha. They always penetrate into the solid calcarious por* 
tion of the shell, but perhaps never perforate it. Those per- 
forations which we often see both in the recent and fossil 
species of some marine bivalves, are produced by a very 
different parasite. But the principal cause of the decortica- 
tion of the Unios seems to be the following — These shells 
are commonly about half imbedded in the mud of the streams 
which they inhabit — the foot and basal margin of the shell 
are of course downwards, and the beaks above the surface 
of the mud, where they are more exposed to the influence 
of light and to the changes in the temperature of the water, 
which causes are sufficient to produce a slow decomposition 
in the exposed part — the motion of the water may also tend 
to hasten the decay. In confirmation of this opinion, we find 
that those shells which inhabit very shallow water, where 
the first mentioned causes operate most powerfully, are much 
more carious than those found in deeper streams. The full 
grown specimens of the U. peruvianus (called sometimes 
U. plicatus or undulatus) seem, peculiarly liable to this de- 
fect — These shells are commonly found in the shallowest 
parts of the Ohio, 
