BRE NEIN NAT 
No. 416.] MOLLUSCAN FAUNA OF THE GENESEE. 661 
At Rochester, below the bridge of the New York Central 
Railroad, the river drops to the valley below in a fall 96 feet 
in height, cutting through Niagara limestone and Clinton 
shale. Below this fall the current is rapid, the river flowing in 
a sinuous course, leaving on either side at intervals a low, flat 
piece of ground with a shallow, shelving shore, the opposite 
side, however, being very deep. The upper part of the river 
is so contaminated with oil and gas products from the gas 
works and a number of manufactories that mollusks cannot 
live here. A short distance down the river, where the water 
is not so badly contaminated, a few Physz, Planorbis, and Lim- 
nea live. These grow more abundant as the purer waters are 
reached. About two miles farther north a second fall occurs, 
26 feet in height, cutting through the Clinton shale. Below 
this fall the river runs swiftly, is very deep, and forms a circu- 
lar pool just beneath the fall. The walls are very steep and 
stony, the river having cut away the soft shale, leaving great 
projecting ledges. Physa, Planorbis, and Limnza are here 
notably abundant, living on ledges of rock where the water is 
a few inches deep. 
A few hundred feet north of this fall a third one occurs, 
cutting through Medina sandstone 83 feet in height, and the 
river drops to the valley below, flowing at first rather rapidly, 
but after a mile or so rather quietly, between well-wooded 
banks 210 feet in height. The riveris here bordered by many 
Swampy flats covered with rank vegetation, and the shores are 
muddy. -Near the falls the river is somewhat shallow, flow- 
ing over fallen rocks, but it soon becomes quite deep. The 
senera present are Limnzea, Physa, Planorbis, Bythinia, and Caly- 
culina ; Limnzea and Planorbis predominating. Inthe mouth of 
the Genesee there are some eight or ten species of Unios, with 
Some gastropods, which have come in from Lake Ontario, but 
they have not as yet reached far enough up the river to form 
a factor in this discussion. 
The following table shows the comparative distribution of 
the species in each section of the river. An asterisk (*) indi- 
cates that the species is very abundant, while the dagger (T) 
Shows that the species is more rarely found. 
