404 THE CHICAGO ACADEMY OF SCIENCES. 
above, rounded below; superior and inferior margins moder- 
ately curved, the 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, 1.80 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, Ill. (Mississippi drainage) (Hand- 
werk). 
Geological distribution: Pleistocene: Loess. 
Habitat: In creeks, pools and rivers. 
Remarks: “This species resembles somewhat Fis. 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 lessgrayish. 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 
