434 Annals of the Carnegie Museum. 



to strongly inflated ; upper margin short, curved, bounded by an obtuse 

 angle to the supero-anterior slope, which is straight or nearly so, 

 anterior end rounded angular, posterior margin well rounded, form- 

 ing one continuous curve from the beaks to the anterior end. the 

 inferior margin somewhat less strongly curved; beaks somewhat be- 

 hind the middle, narrow, almost pointed, projecting well over the 

 hinge margin; surface polished, with very slight, somewhat irregular 

 striae, and one or two rest-lines; color whitish to straw, with a few 

 narrow alternating narrow zones which are glassy transparent ; hinge 

 short, little over one-half the length of the mussel, curved, moderately 

 stout; plate somewhat broad; cardinals small, slight, high up on the 

 plate, the right moderately curved or nearly straight, its posterior part 

 a little thicker, grooved to somewhat bifid; the left both slightly 

 curved, of nearly equal lengths, or the posterior somewhat larger; 

 lamina short, in the right valve the inner both with cusps occupying 

 their entire length, pointed, bent upward, the anterior especially so, 

 and markedly concave on its upper face, somewhat spoon-shaped, the 

 outer ones both small and short; in the left valve both with cusps 

 very abrupt, almost spine-like ; ligament rather short and moderately 

 thick. Long. 4.2; alt. 3.7; diam. 3.4 mm.; long. 4.2; alt. 3.6; diam. 

 2.8 mm. 



Soft parts colorless or light yellowish (in preserving fluid). Al- 

 most every specimen, even barely half-grown, contains a number of 

 nepionic young. 



Habitat: Upsilon Lake, Turtle Mountains, North Dakota, in mud 

 and sand, water about one foot deep. Collected by Mina L. Win slow, 

 July 15, 1919. Type C. M., No. 9160 and seq. ; paratypes in the 

 Museum of Zoology, University of Michigan. 



This Pisidiiim is so characteristic and so greatly different from 

 others that there can be no question about its representing a distinct 

 species, apparently not seen or noticed before. Probably it is of 

 northern distribution. The specimens, several score, ranging from 

 quite young to full-grown, are remarkal)ly uniform, in regard to size, 

 shape, and color. However, a few have a small, but well-marked, 

 sharp scutellar angle at the junction of the upper margin and the 

 supero-anterior slope, as frequently seen in some other forms — e. g., 

 scutellatnm, sargcnti, ovum, and typical clcz'atiim. 



8. Pisidiiim notopJitJiahni n. sp. Mussel minute, elongate, subequi- 



