18 THE NAUTILUS. 



the Marston collection was revised. In spite of its small size, it 

 ranges under the group Rivulina, and has resemblance, in shape, 

 with forms of P. compressum Pr. and with handwerkii St.; from the 

 latter it is easily distinguished: it is smaller, more oblique, its beaks 

 are comparatively broader, more posterior, and less elevated, its strife 

 much finer and slighter. 



Pis. fragillimum, n. sp. Mussel rather small, inequipartite, ob- 

 lique, well inflated ; superior margin short, nearly straight, bounded 

 by angles from which the supero-anterior and posterior slopes fall 

 down nearly straight at first, then passing over the rounded anterior 

 and posterior ends into the moderately curved inferior margin ; an- 

 terior part longer than posterior and anterior end narrower; beaks 

 somewhat broad and full, well projecting over the superior margin, 

 flattened on top ; surface with very fine striae and a few slightly 

 marked lines of growth, dullish, pale horn-colored ; shell subtranslu- 

 cent, very thin and fragile ; hinge very slight, short, but well formed, 

 plate narrow ; cardinal teeth thin, lamellar, nearly straight and longi- 

 tudinal, the right long, its posterior end not or slightly thicker, 

 the left ones rather long, nearly parallel ; " laterals " slight, thin, 

 the anterior short, their cusps close to the cardinals, especially so the 

 left which is high, sharp pointed, abrupt, the right comparatively 

 large ; the outer ones of the right valve small but distinct and rather 

 long ; ligament slight. 



Long. 3, alt. 2.6, diam. 1.8 mill. 



Habitat: Lake Geneva, Wisconsin, sent for examination by Mr. 

 F. C. Baker. Types are in the collection of the Chicago Acad. Sc, 

 and that of the Carnegie Museum. 



Although only ten specimens were seen, this Pisidium is quite 

 distinct by its shape and the formation of the hinge ; the shell is the 

 thinnest and most fragile yet seen in Plsidia. The mussel has some 

 resemblance in shape with P. walkeri St., but is much smaller, 

 shorter, especially its anterior part, the surface sculpture is finer 

 and slighter, and the hinge is quite different. Its closer affinities 

 are yet unknown. 



Pis. levissimum. n. sp. Mussel small, somewhat elongate, slightly 

 oblique, moderately inflated; superior margin moderately curved, 

 posterior rounded, sometimes with a slightly projecting, rounded 

 angle at the scutum; inferior slightly curved, supero-anterior slope, 

 at some distance from the beaks, in some specimens straight or 



