STATION IN FRESH-WATER MUSSELS. 309 



small streams, and varies greatly in obesity. But no correlation 

 between the diameter of the shell and the size of the stream has 

 been discovered. 



In the tables given above, one character has been brought out 

 preeminently, that is obesity, or diameter of the shell as related to 

 length. 



But, in addition, another fact has been discovered, which is the 

 following. A shell which decreases in diameter in the headzvaters, 

 makes up, so to speak, for the loss by a gain in size, that is to say, 

 in circumference, and this is best expressed by the total length of 

 the shell. The latter not having been given in the tables, I supple- 

 ment this here for the several species discussed. 



The following maxima of length have been observed (the large 

 river form is given first; the headwaters form in the second place). 



Fusconaia subrotunda: 107 mm. ; F. sitbr. kirtlandiana: 133 mm. 

 Fusconaia pilaris: 70 mm. ; F. pil, lesneiiriana: 84 mm. ; F. pil. bursa- 



pastoris: 100 mm. (Simpson gives even no mm. for the last 



one). 

 Fusconaia flava trigona: Gy mm. ; F. flava: 90 mm. (This is not 



everywhere so striking, for locally the typical flava appears as a 



dwarfed race, as for instance in Crooked Creek in Pa., where 



the largest specimen is only 74 mm. long, but longer yet than the 



largest trigona.) 

 Fusconaia cuneolus appressa: 63 mm. ; F. cuneolus: 68 mm. 

 Fusconaia edgariana: 63 mm. ; F. edg. analoga: 70 mm. 

 Fusconaia barnesiana tumescens: 67 mm. ; F. barnesiana: yy mm. ; 



F. barn, bigbyensis: 86 mm. 

 Lexingtonia dolabelloides: 56 mm. ; L. dol. conradi: 68 mm. 

 Pleurobema oviforme holstonense: 58 mm.; P. oviforme: 68 mm.; 



P. ovif. argenteum: 96 mm. 

 Obovaria subrotunda: 59 mm. ; O. subr. lens: 67 mm. 



Only in Pleurobema cordatum catillus and P. cord, coccineum 

 this phenomenon is not evident, and it could not be established in the 

 case of Dromus dromas and caperatus. For the others, as may be 

 seen by the above figures, it stands out as a striking fact, and cannot 



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



