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 
preéminently, 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 headwaters, 
makes up, so to speak, for the loss by a gain in sige, that 1s 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. subr. kirtlandiana: 133 mm. 
Fusconaia pilaris: 70 mm.; F. pil, lesweuriana: 84 mm.; F. pil. bursa- 
pastoris: 100 mm. (Simpson gives even 110 mm. for the last 
one). 
Fusconaia flava trigona: 67 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. 
Fusconaa barnesiana tumescens: 67 mm.; F. barnesiana: 77 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, 1920. 
