STATION IN FRESH-WATER MUSSELS. 303 
Ohio system, and on account of its close affinity to Pl. oviforme, 
we might expect to find our law expressed. But the fact is that in 
this region no large-river-type is present, P/. clava avoiding larger 
streams. Measurements of the material at hand have furnished no 
evidence for a change of obesity correlated with station. The 
measurements correspond roughly to those of the typical ovwiforme 
(between 40 and 50 per cent.), and there is also no form parallel to 
argenteum. 
I also have investigated material from the Coosa River in 
Georgia and Alabama belonging to Pleuwrobema decisum (Lea) and 
Pl. chattanoogaénse (Lea). These two species undoubtedly repre- 
sent the clava-group and the typical oviforme. Very slight evidence 
was found of a change of obesity according to station, yet some 
indications of it were present. The lowest figures for the average 
diameter were obtained in specimens from Conasauga River in 
Whitfield Co., Ga.; the highest figures in the lower reaches of 
Coosa River in Chilton and Elmore Cos., Ala. Yet the differences 
are slight at the best (max. 55, min. 38 per cent.), and the irregu- 
larities are many. Also the material is very unequal as regards the 
number of specimens measured from one and the same locality. 
Thus I pass over these forms, only calling attention to the posibility 
that the existence of the same law might be demonstrated also in 
these cases. 
Dromus promas (Lea) and DR. DROMAS CAPERATUS (Lea). 
Simpson, 14, pp. 341, 343. 
Wilson and. Clark (14, pp. 54, 63) have first pointed out that, 
in the Cumberland River, these two forms run together, and that 
the flattened caperatus is in the headwaters, and the swollen dromas 
in the main river (see also Ortmann, ’18, p. 566). 
In this species, conditions are somewhat complicated by the fact 
that the swollen form has also a peculiar “ hump,” or large tubercle, 
upon each valve, which becomes more or less obliterated in the flat 
form, being often entirely missing. Thus it is rather hard to draw 
a line. We may assume, that D. dromas is the swollen type, with 
the diameter of 50 per cent. or over, and a well-developed hump ; 
