THE NATURAL HISTORY SURVEY. 401 
Pisidium fallax ae 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, Ill. (Ferriss); Lilycash Creek, Ill. (Hand- 
werk); Wisconsin, Michigan, Maine (Sterki). 
Habitat: Found in company with P. compressum and cru- 
ciatum. 
Remarks: This isa distinct littlespecies. 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 Pvs. 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 walkeri Sterki, pl. xxxi, fig. 15. 
Pisidium walkert STERKI, The Nautilus, Vol. IX, p. 75, 1895. 
Pisidium walkeri var. mainese STERKI, |. 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 
