CORRELATION OF SHAPE AND STATION IN FRESH- 

 WATER MUSSELS (NAIADES). 



By a. E. ORTAIANN, Ph.D. 

 (Read April 24, igi20.) 



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; Dronius dromas and 

 caperaius) and Utterback- has found this law to hold good in 

 Missouri (chiefly in the Osage River) for several species (Fiis- 

 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 subrotnnda,^ in the case of Pleurobcma coccineum 



1 Bur. Fisher. Doc. no. 781, 1914, pp. 21 and 63. 



2 Amer. Midi. Natural., 4, 1916, p. 2. 

 ^ Ann. Cam. Mus., 5, 1909, p. 192. 



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