664 THE AMERICAN NATURALIST. 
Calyculina transversa is interrupted in its distribution, but, like 
Physa gyrina, P. heterostropha, and Bythinia tentaculata, may 
have come up the river from Lake Ontario. This, however, 
will hardly account for Calyculina partumeta, which is found in 
the lower river and between falls 2 and 3. 
As one glances over the table the fact presents itself that 
there is a marked division in the faunas between the upper 
river and the series of falls, and the lower river. This may be 
accounted for by the fact that for a distance of about two miles 
above the first fall the river is shallow and very rocky, in: fact 
flowing over great ledges of Niagara limestone, and the Unios 
all seem to prefer the deeper, more quiet waters above this 
section of the river, only a few stragglers, like Lampsilis luteo- 
lus, L. iris, and Alasmodonta rugosa, being found in this rocky 
region. The writer was unable to find Unios within a half mile 
of the upper falls, showing conclusively that this environment 
is unsuitable for them. Another reason why Unios are not 
found below the falls is probably that their heavy shells and 
also their habit of burying themselves in the mud prevent 
them from rising to the surface and being swept over the falls, 
as might be the case with Planorbis, Limnzea, or Physa, which 
come periodically to the surface for air. 
The foregoing discussion indicates that a series of falls like 
those at Rochester will prove an effective barrier to the dis- 
tribution of some mollusks (as the pelecypods with mud- 
burrowing habits and the ctenobranchs, which cling to the 
rocks and do not come to the surface), while to others (like the 
fresh-water pulmonates, which come to the surface frequently 
and hence could be swept over the falls) it is not a barrier. 
Future studies and collections, however, may modify the 
above conclusions. 
