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°77 ; 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 Saginaw, 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- 
