STATION IN FRESH-WATER MUSSELS. 277 
of West Virginia (below Clarksville) is polluted, and its fauna is 
destroyed. I have older material, however, from one locality in 
the Monongahela proper; farther up, I collected some near the 
mouth of Cheat River, just at the state line; and at one locality in 
the headwaters (West Fork River). Vhe measurements are as 
follows. 
Loc. No. Max. Min. Av. 
Monongahela, Charleroi ........ 13 61 48 53 subrot. (kirtl.) 
@heateCheatmlaviery a scs)s's1eiet ts) - 2 52 40 46 kirtl. (subrot.) 
West Fork, Lynch Mines ....... 2 44 41 42.5 kirtl. 
This material is rather scanty; nevertheless the decrease in 
obesity in the upstream direction is clearly seen. The locality 
Charleroi has a higher percentage (63) than the next localities below 
in the Ohio (47 and 48 at Industry and Coraopolis), but agrees with 
that of Cooks Ferry. This, probably, is again due to the age of the 
shells, In the Ohio below Pittsburgh, giant specimens, with the 
length over 100 mm., are not uncommon, while among the shells 
from Charleroi there is not a single one over 100 mm. (largest 
QI mm.). 
Very probably, this law holds good also in other tributaries of 
the Ohio. I have not enough material to demonstrate this. In one 
case, that of the Tuscarawas River in Ohio, I have rather abundant 
material from the Holland collection, but, unfortunately, no exact 
localities are given; both forms, however, are represented in this 
river. From the Levisa Fork Big Sandy River, Prestonsburg, 
Floyd Co., Ky., I have a single specimen, the diameter of which is 
50 per cent., that is to say, it is a subrotunda standing just at the 
lower limit, and close to kirtlandiana, and this would correspond 
to the station well up the river. From Little Kanawha River, 
Grantsville, Calhoun Co., W. Va., I have one specimen, the diameter 
of which is 43 per cent. this is the typical kirtlandiana, correspond- 
ing to the small size of this river, 
All this serves to show that our contention is upheld, that F. 
subrotunda and kirtlandiana are forms of the same species, differ- 
ing only in obesity. The former is the swollen form of the large 
rivers ; the latter is the compressed form of the small streams. Both 
PROC. AMER. PHIL. SOC., VOL. I.IX, R, AUGUST 20, 1920. 
