ORTMANN: monograph op the naiades of PENNSYLVANIA. 323 



The female of L. orbiculata is entirely different from that of A. ligamentina 

 in shape. The posterior dilatation is not found in A. ligamentina, and the latter 

 never approaches the globular shape. 



Both male and female of L. orbiculata often present freakish shapes, and the 

 proportion of length and height, convexity of the valves, and curvature of the 

 lower margin is hardly alike in any two specimens. 



A very good character, excepting in the case of very young shells, is always the 

 thickness of the shell. In fact for its size this species is possibly the heaviest shell 

 of the Ohio-drainage. 



m 



Localities in Pennsylvania represented in the Carnegie Museum: 



Ohio Eiver, Shippingport, Cooks Ferry, and Industry, Beaver Co.; Neville Island, Allegheny Co. 

 Allegheny River, Godfrey and Kelly, Armstrong Co. 

 Monongahela River, Charleroi, Wasliington Co. (G. A. Ehrmann). 



Other localities represented in the Carnegie Museum: 



Ohio River, Toronto, Jefferson Co., Ohio; St. Marys, Pleasants Co., West Virginia; Portland, Meigs 



Co., Ohio; Portsmouth, Scioto Co., Ohio. 

 Tennessee River, Florence, Lauderdale Co., Alabama (H. H. Smith). 

 Clinch River, Solway, Knox Co., and Offutt, Anderson Co., Tennessee. 

 Mississippi River, Muscatine, Muscatine Co., Iowa (Hartman collection); Andalusia, Rock Island Co., 



Illinois (A. D. Howard). 

 Black River, Black Rock, Lai^Tence Co., Arkansas (H. E. Wheeler); Pocahontas, Randolph Co., Arkansas 



(H. E. Wheeler). 

 Ouachita River, Arkadelphia, Clark Co., Arkansas (H. E. Wheeler) .-"*'' 



Distribution and Ecology in Pennsylvania (See fig. 33) : This is a rare shell in 

 Pennsylvania, and is only found in the larger rivers. The largest number of speci- 

 mens has been secured in the Ohio in Beaver County, but here also it is by no 

 means abundant. It vised to be in the Ohio in Allegheny County, and in the 

 Monongahela as far up as Charleroi, Washington Co. (only one individual is at 

 hand from this locality). It also ascends the Allegheny to Armstrong County, 

 but here it is extremely rare, and only a few specimens were taken. 



So far as I am able to judge, it is a shell of strong currents in large rivers. I 

 found all my specimens on riffles; 'but farther down in the Ohio it is present upon 



'"'*'" See Wheeler (1918, p. 117). Specimens from the Ouachita River agree very well in shape with 

 specimens from the Ohio River and the color of the epidermis is of the same brown hue in some of them, 

 but in others it is more brownish-olive (with a suggestion of green). They are surely not L. higginsi, 

 because the beaks are not at all inflated. The form from Black River is remarkable on account of its 

 rays, which are broad and distinct; and also by the brownish-olive epidermis. It belongs to the forms 

 intergrading with L. higginsi (Lea). 



