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. 
cuneatus , Barnes,) U. refiexus, Raf. ( U- cornutus , 
Barnes,) and U. triangularis , Raf. Different species 
are found occasionally to have remarkable and 
strongly contrasted peculiarities in the choice of 
habitats; thus Anculotus pictus , nob. adheres only to 
pebbles on the bars, in the Alabama river, whilst 
Anculotus tceniatus, 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 hi 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. 
