STATION IN FRESH-WATER MUSSELS. 293 



large knobs, but also the nodules.^' It hardly, however, forms a 

 distinct race here, being rare, and passing insensibly into the normal 

 type. 



In the upper Tennessee; there is an analogous form, also greatly 

 compressed, and without the large knobs. It is, however, not 

 smooth, but thickly covered with small nodules, even more so than 

 the normal form. It has been called var. strigillata (Wright). 

 Simpson (1. c.) does not accept this, because there is an "absolute 

 graduation " to this form. Yet in the upper Clinch, from the Ten- 

 nessee-Virginia state line upward, this variety becomes a pure race, 

 which is very striking. It is also in Powell River and the North 

 Fork Holston, but not so well developed. 



In this instance, it is clearly seen that analogous, compressed 

 forms are being developed independently in the upper Ohio and the 

 upper Tennessee systems, and their independence is shown by the 

 difference in the character of the nodules : the one is practically 

 smooth, the other strongly nodulous. But in the disappearance of 

 the large knobs they agree again. 



I shall give here the measurements of the form in the Clinch 

 River. In the last column, in this case, tl^e name given refers to the 

 development of the large knobs. 



Loc. No. Max. Min. Av. 



Edgemoor 3 35 34 35 O'^- 



Clinton 2 41 31 39 cyl. 



Clinch River Station 6 39 34 36 o'^- 



Speers Ferry 2 38 34 36 cyl. 



Clinchport 5 35 32 33 cyl. (strig.) 



Fink I 31 31 31 cyl.-strig. 



Cleveland 6 35 28 31 strig. 



Raven 2 30 30 30 strig. 



Richland 4 3i 28 30 strig. 



Cedar Bluff i 27 27 27 strig, 



RoTUNDARiA TUBERCULATA (Rafinesque) , and R. granifera (Lea). 

 Quadrula tub. and gran. Simpson, '14, pp. 903, 905. 

 According to Wilson and Clark, ^^ these two forms show, in the 

 Cumberland River, the phenomenon that the swollen form (grani- 



1' Sterki, Pr. Ohio Ac, 4, '07, p. 390, mentions this form first. 

 IS L. c, '14, p. 63. 



PROC. AMER. PHIL. SOC, VOL. LIX, S, AUGUST 20, I92O. 



