THE NAUTILUS. 81 



This large and beautiful Pisidium cannot be mistaken for any other 

 species. It seems to be related to abditum Hald., and some of the 

 old-world Pisidia. Young specimens are very little inflated, and 

 of a light, almost whitish color. 



Pis. cuneiforme n. sp. Mussel inequipartite, oblique, moderately 

 to rather well inflated, mostly so near the beaks; hinge margin 

 slightly curved, the angles at the scutum and scutellum projecting; 

 anterior part considerably longer, attenuated, somewhat cuneiform, 

 and directed downward, the end rounded, supero-anterior slope well 

 marked, straight or slightly curved ; posterior part short, subtruncate ; 

 beaks moderately large and slightly elevated over the hinge margin ; 

 surface finely and irregularly striate, pale to yellowish horn-colored, 

 or whitish, dull to shining ; shell opaque to subtranslucent, thin ; 

 hinge slight, short, plate narrow ; cardinal teeth placed far towards 

 the anterior, well formed ; the right curved, its posterior part slightly 

 thicker and grooved ; the left anterior well curved, the posterior 

 slightly so, almost longitudinal and above the anterior, long ; right 

 lateral teeth slight, cusps low and rounded, the outer ones well 

 formed ; in the left valve the cusps are short, high, abrupt, pointed ; 

 ligament slight. 



Long. 2.8, alt. 2.4, diam. 1.8 mill. 



Habitat: Michigan and Minnesota. In Michigan: Byer's trout 

 pond, Kent Co. ; Blue Lake and Green Creek, Muskegon Co.; Hess 

 Lake, Newaygo Co.; Lake Michigan, at High Id. Harbor. Clear- 

 water River, Stearns Co., Minn. Collected by Messrs. Bryant 

 Walker, H. E. Sargent and R. G. Kirkland. 



Specimens were received in 1895, and again in '98 and '99, and 

 then regarded as representing a distinct species. Yet the number of 

 specimens from each place being limited, it seemed advisable to wait 

 for more materials. 



Pis. cuneiforme has some resemblance with P. subtruncatum Malm, 

 and (var.) cuneatnm Biz., of Europe ; but the beaks are broader, less 

 elevated ; the surface striation and appearance are different, and the 

 young of both show more differences between each other than the 

 adult. Of our North American species, it has resemblance only with 

 some forms of P. compressum, but our species is much smaller and 

 its anterior part is longer, comparatively. Young and half-grown 

 specimens are comparatively shorter, less inequipartite and less ob- 

 lique. 



