STATION IN FRESH-WATER MUSSELS. 287 



The sudden jump between Edgemoor and Needham Ford is ex- 

 plained by the long distance between these two points. 



In the Powell River, this shell is also found, and material is at 

 hand from two places, Combs and Rose Hill. At the former place, 

 the average diameter is 42 per cent., at the latter, 43 per cent. 

 Here the lowest percentage has been found to be 38. 



I did not collect shells of this type in the Holston below the 

 forks, which is quite a remarkable fact. But the fiat type turns up 

 again in the North Fork, from Rotherwood to Holston P. O. Here 

 the diameter lies between 41 and 48 per cent., and there is no ap- 

 parent change in this comparatively short stretch of river. 



Three shells from the Tennessee at Florence, Ala., have the dia. 

 55' 70» 70 per cent., with the average of 65, thus being more swollen 

 than any shells from the upper Tennessee region. From a tribu- 

 tary some distance farther up. Paint Rock River, I have the fol- 

 lowing table. 



Loc. No. Max. JMin. Av. 



Paint Rock 10 54 47 51 cdgar. {anal.) 



Trenton 6 54 47 52 cdgar. {anal.) 



Princeton i 49 49 49 ayial. 



This series may appear as unreliable, since the uppermost sta- 

 tion contains only one shell ; yet the fact is evident that the Paint- 

 Rock-River-shells are less obese than those from the Tennessee 

 below. 



Although, on the whole, my material of the edgariana-gron^ i.- 

 somewhat fragmentary, the law is distinctly seen, chiefly in a por- 

 tion of the Clinch River, from Needham Ford upwards, where con- 

 ditions are typical. These are perfectly analogous to those observed 

 in the cuneoliis-gron'^, and thus we are justified to distinguish also 

 here two varieties, a swollen river form, and a fiat headwaters-form, 

 although the latter has not been named previously. 



Group of Fusconaia barnesiana in the Upper Tennessee 



System. 



I have treated of this group previously,^^ and have given an ac- 

 count of its taxonomy. Thus it suffices here to repeat only the 



^^ Nautil., 31, 1917, p. 58 ff. 



