12 



rocky bed of the Holston river. On the contrary, the 

 Paludina ponder osa, Say, seems a common inhabi- 

 tant of all the rivers of the West, from the northern 

 districts of Indiana and Illinois to the waters of the 

 Tennessee Valley. The same remark will apply to 

 several species of Unio, such as U. niger, Raf. ( U. 

 ciuieatus, Barnes,) U. rejiexus, Raf. ( U. cornutus, 

 Barnes,) and U.triangidaris, Raf. Different species 

 are found occasionally to have remarkable and 

 strongly contrasted pecuharities in the choice of 

 habitats; thus Anculotus pictus, nob. adheres only to 

 pebbles on the bars, in the Alabama river, whilst 

 A7iciUotus tcEiiiatus, nob. and a pretty species of 

 Melania, are exclusively devoted to the soft calca- 

 reous banks of the same river, which they perforate 

 like the lithophagous Testacea, giving it a honey- 

 comb appearance, for they are extremely numerous. 

 Fresh water shells, in general, are very partial to 

 waters flowing over a bed of limestone or calcare- 

 ous earth, and many a limpid stream have I seen, 

 strongly impregnated with lime, whose rocky bed 

 was paved, as it were, with myriads of univalves. 

 The bivalves are also peculiarly abundant in those 

 rivers of North Alabama and Tennessee, which have 

 cut their channels in the carboniferous limestone, 



doubtless the same as that of Melania, with which genus 

 it has a close affinity. 



