404 THE CHICAGO ACADEMY OF SCIENCES. 



above, rounded below; superior and inferior margins moder- 

 ately curved, th*e former rather short, the latter long; scutum 

 little, scutellum moderately marked, both forming slight an- 

 gles; vertical section heart-shaped, horizontal, short, lanceo- 

 late-rhombic; margins very slightly acute; beaks moderately 

 full and prominent, with a longitudinal, slightly oblique ridge 

 (sometimes obsolete) below the culmination; nave moderately 

 thick, whitish, with crowded, small pits, from which it appears 

 as if dotted; hinge moderately strong; cardinal teeth fine, in 

 the left valve two, lamellar, longitudinal, about equally long, 

 a little curved, almost parallel, the upper little anterior; in the 

 right valve one, longitudinal, little curved, lamellar, the poste- 

 rior end slightly thickened; lateral teeth rather small and thin, 

 in the left valve one, pointed, in the right valve two, the outer 

 quite small; hinge-list fine, rather regularly formed; ligament 

 rather long and fine." (Sterki.) 



Length, 1.80; height, 1.60; breadth, 1.30 mill. (Sterki.) 



Animal: "Soft parts colorless, rest whitish." (Sterki.) 



Distribution: "Ohio; Tuscarawas River, Bear Run, tribu- 

 tary to the Mahonig River, Portage County, a spring brook 

 at Rootstown Station, Portage County, emptying into Cuya- 

 hoga River (Lake Erie and St. Lawrence drainage) (Sterki); 

 Lilycash Creek, near Joliet, 111. (Mississippi drainage) (Hand- 

 werk). 



Geological distribution : Pleistocene; Loess. 



Habitat: In creeks, pools and rivers. 



Remarks: "This species resembles somewhat Pis. com- 

 pressum Prime, in having a ridge or appendage, but not in the 

 same place, as it stands on the outside, below the culmination 

 of the beaks, while in the mature P. compressum it has its place 

 rather on top. The shape of the shell is different and the size 

 is very much smaller, its bulk being only about one-tenth of 

 that shell. And while the upper part of the posterior margin 

 in P. compressum. is rounded or flattened, it is sharp, somewhat 

 "pinched" in P. punctatum. In this the shell is purely whitish, 

 or light straw-colored, the latter more so when dry, while the 

 mature P. compressum, is always more or less grayish. A marked 

 feature is the finely and densely pitted interior surface of the 

 shell, the dots being distinctly perceptible through the shell 

 from the outside. Yet this is not unique, as I have also seen 

 it in other Pisidia. Some specimens show not a trace of the 



