THE NATURAL HISTORY SURVEY. 401 



Pisidium fallax var. boreale STERKI, The Nautilus, Vol. XIII, p. 12, 

 1899. 



Shell: "Mussel rather small; it is of the same type with 

 P, compressum Prime, but smaller, more rounded in outline, the 

 upper margin is less strongly curved, not angular, the ridges 

 on the beaks are comparatively larger and situated less high 

 up; the striation is finer, crowded, somewhat irregular and 

 sharp; the color commonly greenish or yellowish-horn in the 

 younger, more yellow in older specimens; the hinge is strong, 

 more regularly curved than in compressum, the hinge plate 

 broad, the cardinal tooth of the right valve more oblique, the 

 lateral teeth strongly projecting inward; nacre more glassy- 

 whitish; ligaments strong." (Sterki.) 



Length, 3.20; height, 2.90; breadth, 2.10 mill. 



Animal: Unknown. 



Distribution: Tuscarawas River and Sugar Creek, Ohio 

 (Sterki); Joliet, 111. (Ferriss); Lilycash Creek, 111. (Hand- 

 werk); Wisconsin, Michigan, Maine (Sterki). 



Habitat: Found in company with P. compressum and cru- 

 ciatum. 



Remarks: This is a distinct little species. Dr. Sterki says 

 it has been found in the stomach of the fish known as "Buffalo 

 Sucker," with P. cruciatum and other shells. "Old specimens 

 of P. fallax are almost always badly eroded, and covered with 

 a thick, blackish coat, while Pis. compressum from the same 

 places were intact and clear." (Sterki.) 



This species is common about Joliet, but has not been 

 found in any other part of the area. Specimens of compressum 

 should be carefully compared with the above description to 

 separate the two species. 



156. Pisidium walker! Sterki, pi. xxxi, fig. 15. 



Pisidium walkeri STERKI, The Nautilus, Vol. IX, p. 75, 1895. 



Pisidium ivalkeri var. mainese STERKI, 1. c., Vol. XII, p. 79, 1898. 

 Shell: "Mussel of medium size, elongated, rather inflated, 

 resembling P. abditum, but is smaller than good-sized -exam- 

 ples of that species; it is also more elongated, and the smaller 

 and moderately prominent beaks are more posterior; thus the 

 anterior part is comparatively long, with the end rounded, the 

 posterior quite short, truncated at the end; scutum and scutel- 

 lum well marked. The shell is very thin, translucent, the sur- 

 face very finely, and sharply striated, often somewhat scaly, 

 dullish, or with a silky gloss; color grayish, along the margin 



