160 Cincinnati Society of Natural History. 
conditions of variation enumerated in another part of this paper reach 
their maximum, are so intimately united by varieties as to render 
their separation into distinct species, in most cases, utterly impossi- 
ble, as the shells from different localities are so completely blended, 
that it is no exaggeration to say that fifty per vent. of the described 
species are the merestsynonyms. At the north, even, the difficulty be- 
gins; and it vastly increases in the mountainous region further south. 
This fauna differs essentially from A and B, in that it is not, nor- 
mally, lacustrine, but fluviatile. A very few species are found in 
lakes, occasionally; but there is in these shells, an inherent aversion to . 
still water, which characterizes all the genera, leading them to seek 
rather the rapid parts of rocky streams; and herve it is that we meet 
their greatest diversity of types, and the greatest variety of coloration 
and ornamentation. ‘This peculiarity of station is so persistent, that 
no skilled collector ever searches for them in level reaches of deep 
water, unless in the case of a few species of Plewrocera, which affect 
such localities; but Zo, Angitrema, Lithasia, Anculosa, Schizostoma, — 
Goniobasis, and Strephobasis, all genera represented by an infinity of 
varietal forms, seek always clean, rapidly flowing water, in rocky or 
gravelly river beds; and these groups are only represented by the — 
genus Melanopsis, over the same range in Europe and Asia, and by 
Goniobasis and Pleurocera at the north, in America, their grand 
metropolis; in foreign landstheir representatives, also, are confined to 
a range mostly south of that occupied by A and B. This fauna 
has a very limited distribution of genera and species west of the 
Mississippi, a fact easily traced, I think, to true geological causes, some 
of which are past, and others now in operation. 
The shells designated as Ohio River Types in my previous article, I 
shall call Fauna D. Of its geographical distribution, varieties, and 
persistent forms, enough was said in that paper; and since it was 
written, I have received, from the very southwestern borders of Texas, 
a collection of Uniones gathered at random, which contains nothing 
but absolutely typical Ohio river species. South of the Ohio, in par- 
allel streams, beginning with Kentucky river and Green river, and con- 
tinuing to the eastern and southeastern tributaries of the Tennessee, 
we find, as has already been stated, a group of shells of a distinct facies, 
requiring nu expert knowledge of chonchology to enable one to see 
that it differs, as a whole, from the Fauna D, with which it is associ- 
ated. Its southern distribution is co-extensive with that of Fauna C, in 
all the larger and many of the smaller streams. Here occurs the greater 
