CORRELFATION OF SHAPE AND STATION IN FRESH- 
WATER MUSSELS (NATADES): 
By A. E. ORTMANN, Pu.D. 
(Read April 24, 1920.) 
While studying the Naiad-shells of the upper Ohio-drainage, the 
fact was forced upon my mind, that certain species which inhabit 
the headwaters and smaller streams are represented, in the larger 
streams, by different, but very similar forms, which are distin- 
guished from them chiefly by one character, namely obesity. The 
headwaters-forms are rather compressed or flat, the large-river- 
forms more convex and swollen. I also found that in the rivers of 
medium size integrades betwen the extremes were actually present. 
Subsequently, similar conditions appeared to prevail elsewhere, 
and the existence of this rule was brought home to me very forcibly 
during my study of the Naiad-fauna of the upper Tennessee 
drainage. 
Also other authors have observed this fact, and have referred to 
it in their publications. Thus Wilson and Clark’ indicated it for 
certain species of the Cumberland River (Amblema peruviana and 
costata; Rotundaria tuberculata and granifera; Dromus dromas and 
caperatus) and Utterback? has found this law to hold good in 
Missouri (chiefly in the Osage River) for several species (Fus- 
conaia undata and trigona; Amblema peruviana and costata; Pleuro- 
bema obliquum and allied forms). 
I myself have alluded to this relation between a compressed 
headwaters-form and a swollen large-river-form in the case of 
Obovaria lens and subrotunda,? in the case of Pleurobema coccineum 
1 Bur. Fisher. Doc. no. 781, 1914, pp. 21 and 63. 
2 Amer. Midl. Natural., 4, 1916, p. 2. 
3 Ann. Carn. Mus., 5, 1900, p. 192. 
Reprinted from Proceedings American Philosophical Society, Vol. lix., 1920. 
