THE NAUTILUS. 65 



acute or acutish, not pinched ; superior and inferior margins mod- 

 erately curved, posterior well rounded or slightly truncated, joining 

 the inferior without any marked angle; antero-superior margin 

 sloping, oblique, slightly curved, meeting the inferior at an angle 

 situated rather inferior, more distant in the adult than in younger 

 examples ; surface very finely striated, polished ; color pale or yel- 

 lowish to greenish-horn, sometimes whitish or straw in old speci- 

 mens ; shell thin, translucent ; hinge moderately strong ; cardinal 

 teeth of the right valve moderately curved, its posterior end thick- 

 ened, those of the left valve lamellar, almost equal, the superior 

 rather short, slightly oblique and little curved ; lateral teeth rather 

 strong ; ligament short, thin. 



Long. 3'2, alt. 2'7 ; diam. 1'9 mill., in the average. 



It has a wide geographical distribution, and is one of the most 

 common Pisidia, having been seen from Massachusetts : Winchester 

 (E. W. Roper) ; New York : Mohawk, Herkimer County, Erie 

 Canal (E. W. Roper, A. Bailey, Dr. Jas. Lewis) ; Hudson River 

 (R. E. C. Stearns) ; Pennsylvania : Philadelphia, in different waters 

 (M. Schick) ; New Jersey : White Pond, dredged (Pilsbry and 

 Rhoads) ; Michigan : Ann Arbor, High Island Harbor in Lake 

 Michigan ; East Sagiuaw, Pine Lake, dredged (Br. Walker) ; Grand 

 Rapids (L. H. Streng) ; Wisconsin : Fox River (Geo. T. Marston) ; 

 Minnesota: Clearwater and Mississippi Rivers, Heath Lake (H. E. 

 Sargent) ; fossil, at White Pond, N. J. (Pilsbry and Rhoads). 



Our species is one of modest appearance, and yet somewhat 

 unique. Being so common, it has evidently been overlooked, or 

 taken for younger specimens of some others, owing to its want of 

 striking features ; hence the name given to it. Almost always the 

 mussel is more or less coated with a blackish or rusty substance in 

 a rather characteristic way, especially over the beaks and upper 

 part, even when found in company with other Pisidia not thus 

 coated, so that this is a feature of the species, usually independent of 

 the habitat. Yet sometimes all specimens in a place are found clean, 

 e. g., those (dredged) from White Pond, New Jersey. Dead shells 

 are of a rather characteristic plumbeous-gray color. 



The species is variable, though being more constant in each place. 

 There are marked differences in size and shape, prominence of the 

 beaks and color. Especially notable is a form from Michigan, with 

 less curved superior and inferior margins, the posterior end more 

 abrupt, obliquely, so that the outline of the mussel resembles an ob- 



