242 MEMOIRS OF THE CARNEGIE MUSEUM. 



same general character, but the arrow-shaped spots are more prominent, and often 

 extend over large parts of the sheU in a zig-zag pattern. 



Male and female shells a little more distinct. In the male the posterior part 

 of the lower margin slopes up in a nearly straight line, rendering the posterior 

 end of the shell more sharply pointed, while in the female the lower margm is 

 more expanded and curved, making the posterior end more bluntly pointed. But 

 there are cases in which the difference is hardly noticeable. Sometimes in the 

 female the posterior part of the lower margin behind the expansion is slightly 

 concave. 



L. H. D. 



Size: 1. (No locality) Cat. No. 61.787 (probably cf ) 49 mm. 34 mm. 20 mm. 



2. do. " " do. (probably?) 41 " 26 " 18 " 



Soft parts described, but rather unsatisfactorily, by Simpson in (Baker, 1898, 

 p. 91). I find that the soft parts agree completely with those of A. truncata. Glo- 

 chidia (See Surber, 1912, PL 2, fig. 29; and Ortmann, 1914, p. 67). They also are 

 remarkable for their small size. Surber's dimensions are: 0.060 X 0.063 mm.; 

 mine are: 0.05 X 0.06 nun. 



Breeding season: Surber found eggs and glochidia in July. I received specie 

 mens with glochidia collected on Aug. 18, 1912. 



Remarks: This species is closely allied to A. truncata, and possibly may be 

 only a variety of it. But, as far as I can see, there are no intergrades between them. 



This is the only species previously reported from western Pennsylvania, which 

 I have not collected myself. I have seen the specimens collected by Rhoads (now 

 in the Philadelphia Academy), and thus it is beyond doubt that it once existed in 

 our state. It may yet be found on the Pennsylvanian shores of Lake Erie. 



Localities represented in the Carnegie Museum: 



Lake-drainage: 

 Lake Erie, Vermilion (C. Goodrich) and Cedar Point (0. E. Jennings), Erie Co., Ohio; La Plaisance Bay, 



Monroe Co., Michigan (C. Goodrich). 

 Miami and Erie Canal, Lucas Co., Ohio (C. Goodrich). 



Ohio-drainage: 

 Ohio River, Parkersburg, Wood Co., West Virginia. 



West Fork Wliite River, Riverside, Greene Co., Indiana (J. D. Haseman). 

 Wabash River, New Harmony, Posey Co., Indiana (A. A. Hinkley). 



Tennessee-drainage : 

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



Mississippi River and westward: 

 Mississippi River, Moline, Rock Island Co., Illinois (P. E. Nordgren). 



