No.416.] MOLLUSCAN FAUNA OF THE GENESEE. 663 
A study of the table will show a few interesting facts. The 
first will be the abundance of Unios above the falls and their 
total absence below, showing that the upper falls afford an 
insurmountable barrier to their further distribution in this 
direction. Another fact of equal note is the abundance of 
certain gastropods in all four sections of the river (Planorbis, 
Physa). The general absence of ctenobranchiates below the 
falls (Bythinia excepted) is noteworthy.  Bythinia is a recent 
addition to the fauna of the lower river, appearing in immense 
numbers in 1898. It was very common at the mouth of the 
river, in Lake Ontario, in 1895, and in these three years it 
became one of the most abundant mollusks in the lower river. 
The distribution of Physa is also peculiar. Physa integra 
is the dominant and (so far as I know) the only form above 
the falls, and Physa gyrina below. Physa sayii is the only 
species between falls 1 and 2 and is very abundant only between 
the upper and lower falls, a very peculiar distribution, for which 
I cannot account. P. integra does not occur between the upper 
river and falls 2 and 3, and I was not able to find it between 
falls 1 and 2, which, if borne out by future observations, offers 
an interesting phase of distribution. Why P. integra should 
be so interrupted in its distribution may be accounted for by 
two hypotheses : (1) that specimens may have been carried over 
the first and second falls and found lodgment above the lower 
falls ; or (2) specimens might have been carried by birds or 
other animals. Why Physa sayii is found everywhere except 
In the upper river is also an interesting question. 
Limnza seems to be evenly distributed, and yet in this 
senus we find the same peculiar distribution as in Physa. 
L. falustris has been collected in small numbers in the upper 
Diver, but Z. catascopium and L. caperata take its place in the 
other three regions, the former being very abundant. Ancylus 
has been found only in the upper river, in the dead valves of 
Unios, but it may be that it inhabits all parts of the river. As 
in the case of Physa integra, we find the distribution of Lim- 
"à palustris to be peculiar, jumping, as it does, the first fall and 
occurring sparingly between falls 2 and 3. P/anorbis trivolvis 
's the only evenly distributed species. Among the bivalves, 
