44 Remarks on the Unios of the United States. 



specimen of the Fhiviatilis 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 

 specie6 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. peruvianns (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. 



