470 Annals of the Carnegie Museum. 



are over the beaks and extending o\-er about the central one-third of 

 the valves, gradually evanescent; color pale corneous to whitish or 

 straw, shell thin, translucent to opaque; hinge short, slight, plate 

 narrow; cardinal teeth small and slight, the right somewhat curved, 

 its posterior end not or slightly thicker, both in the left valve short, 

 nearly straight, the anterior close to the edge of the plate; laminae: 

 the right anterior, inner with a cusp not abrupt, in about its middle, 

 the posterior quite small with a slightly marked cusp; outer ones both 

 small, about half as long as the inner; left: both small, short, with 

 cusps somewhat distal, rather abrupt, obtusely pointed; ligament and 

 resilium short and slight. 



Long. 2.4, alt. 1.8, diam. 1.2 mm. fioo : 75 : 50). 



This Pisidiuni is rather inconspicuous and has hardly any striking 

 features. The mussel appears to be the young of some other species, 

 but hundreds of specimens from nepionic to manifestly adult prove its 

 being distinct. It shows little variation, except in the form noted 

 below. In size and shape it is somewhat like a form of splendididum, 

 but in other respects is markedly different; in color and appearance it 

 somewhat resembles imbeciUe, but the latter is somewhat more elon- 

 gate and more inflated, the superior margin is more curved, the beaks 

 are narrower and more prominent. 



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

 collected by Mr. L. E. Daniels, in 1912 and 1915. The types are 

 from Lost Horse Creek, near Charlos (No. 8103, Carnegie Museum). 



129. Pisidium abortivum exiguum var. nov. 



Smaller than abortivum, more inflated, outlines somewhat more 

 rounded, surface more glossy and striae finer, colorless-glossy to whit- 

 ish. Long. 1.6, alt. 1.2, diam. i mm. A number of specimens, in the 

 type lot of abortivum, were thus noticeably different, from young to 

 adult, and plainly represent a variety, or subspecies, not merely 

 individual \ariation (No. 8105, Carnegie Museum). 



130. Pisidium hannai Sterki. 



Pisidium hannai Sterki, Proc. U. S. National Miis., 1916, p. — . 

 Habitat. — St. Paul Island, Pribilof Islands, Bering Sea. 



131. Pisidium singleyi Sterki. 



Pisidium singleyi Sterki, Nautilus, XI, 1898, p. 112. 



Ai)parently belongs to a group of neotropical Pisidia, found in the 

 West Indian Islands, and Central and South America. 



Habitat. — Alabama to Texas, Mexico, Guatemala. 



