212 THE CHICAGO ACADEMY OF SCIENCES. 
Geological distribution: Pleistocene; Loess. 
Habitat: In moist localities, under fallen logs and sticks, 
or buried in the earth beneath them. Associated with Vitrea 
electrina, Zonitoides arboreus, etc. : 
Remarks: A very distinct little species which is at once 
distinguished by its strongly ribbed surface and very wide um- 
bilicus. The animal is not rapid in movement but is slow and 
hesitating, seeming to calculate every motion. It isa widely | 
distributed species, and is fully as common as the Vitreas. It 
is frequently mistaken for P. perspectiva Say, a species not found 
in this territory, which is a much larger shell. 
Genus HELICODISCUS Morse, 1864. 
Mantle posterior, thin, simple; shell discoidal, widely um- 
bilicated; aperture with several pairs of tubercles at intervals 
within, on the inner surface of the outer whorl; peristome 
simple.* 
a 
QWo— 
Fig. 50. 
Animal of HELICODISCUS LINEATUS Say. (After Binney.) 
ee 
82. Helicodiscus lineatus Say, pl. xxviii, fig. 25. 
Helix lineata Say, Journ. Phil. Acad., Vol. I, p. 18, 1817. 
Planorbis parallelus Say (?) Proc. Phil. Acad., Vol. II, p. 164, 1821. 
Shell; Small, flat, discoidal, widely umbilicated; surface 
roughened by numerous equidistant, parallel, raised lines re- 
volving about the whorls, the spaces between the lines show- 
ing fine, wavy lines of growth; color greenish horn; periph- 
ery broadly rounded; sutures deeply impressed or channeled; 
apex large, without revolving lines; whorls four and one-half, 
rounded, discoidal, the last not at all expanded; spire flat, 
showing all the whorls distinctly; aperture in the same plane 
as the whorls, narrow, semilunate, the outer lip bearing several 
(one to three) pairs of very small, conical teeth, and situated 
from the region of the peristome to the inner part of the last 
whorl; peristome simple, thin, acute, the terminations con- 
nected by a thin callus; umbilicus forming a concave depres- 
*W. G. Binney, Man. Amer. Land Shells, p. 74. 
