Sterki: North American SPH.ERiiD.4t. 469 



somewhat shining, with fine, somewhat sharp, crowded, irregular to 

 subregular concentric striae and a few slightly marked lines of growth; 

 color pale corneous to straw or light yellowish, generally with narrow, 

 irregular, alternating zones of these shades; shell thin, fragile, some- 

 what translucent; inner surface microscopically wavy rugulose, muscle 

 insertions and pallial line distinct; hinge slight, rather short; cardinal 

 teeth: the right curved, thin, its posterior end more or less bifid, 

 left; anterior short, thin, strongly curved, posterior short, oblique, 

 slightly curved to nearly straight, its anterior end extending to 

 barely above the middle of the anterior; laminae rather short, slight, 

 little projecting inward; of the right valve; the anterior inner with 

 its cusp near the middle, little elevated and barely pointed, the outer 

 small and barely one-fourth the length of the inner, posterior similar; 

 left both short with the cusps distal, pointed, somewhat abrupt; 

 ligament rather long, resilium not thick, strongly folded, with the 

 insertions extending rather far downward on the hinge plates. 



Long. 8, alt. 6.2, diam. 4.2 mm. (100 : 81 : 52). 



Soft parts not examined. 



P. fabale appears not to be closely related to any of the other N. 

 Am. species, so far as known. It is of about the same size as P. 

 roivelli (western) and alleni (northeastern), but dift"ers from both in 

 shape and appearance, and the formation of the hinge. It is less 

 inflated, less inequipartite, the beaks being less posterior, and less 

 elevated. 



Habitat. — Bitter Root Mountains, Montana, various places. Mr. 

 L. E. Daniels collected several hundred specimens in 1915, and had 

 secured some in 191 2. They are of all stages of growth, and fairly 

 constant, except some which appear to be hybrids. The types are 

 from Lost Horse Creek, near Charlos (Carnegie Museum No. Sioo), 



128. Pisidium abortivum sp. nov. 



Mussel small, subequipartite, slightly oblique, sub-quadrangular, 

 moderately inflated; superior and inferior margins little curved, pos- 

 terior subtruncate to rounded, supero-anterior slope well marked, 

 slightly curved, anterior end rounded-angular, situated well below 

 the longitudinal median line, beaks barely posterior, little elevated, 

 somewhat flattened on top; surface somewhat glossy, with very fine, 

 slight, crowded concentric strige and generally a few slightly marked 

 lines of growth; radial lines, very fine, crowded, irregularly broken, 



