STATION IN FRESH-WATER MUSSELS. 303 



Ohio system, and on account of its close affinity to PI. oviforme, 

 we might expect to find our law expressed. But the fact is that in 

 this region no large-river-type is present, PL 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 oviforme 

 (between 40 and 50 per cent.), and there is also no form parallel to 

 argentenm. 



I also have investigated material from the Coosa River in 

 Georgia and Alabama belonging to Plcurohcnia dccisnin (Lea) and 

 PL chattanoogaense (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 Conasaitga 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 dromas (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 ; 



