88 



ENDODONTIDAE 



Genus HELICODISCUS Morse 



The small snails of this genus may be distinguished by their 

 flattened and discoidal shells. The umbilicus is very wide and 

 shallow and shows all of the whorls. The genus is North 



American in distribution. The figure above pictures a specimen 

 of a member of this genus, Helicodiscus parallelus. 



Key to Species 



Whorls of shell encircled by parallel spiral ridges; aperture with 

 1 to 3 minute denticles parallelus, p. 88 



Whorls of shell without ridges; aperture without denticles. . . . 

 singleyanus inermis, p. 89 



HELICODISCUS PARALLELUS (Say) 



The flat, greenish shell of Helicodiscus parallelus measures 

 about one-eighth inch (3.5 mm.) in diameter. It differs from 

 all other Illinois shells in having a number of parallel, spiral 

 ridges encircling the whorls. The 4 

 whorls are equally coiled above and 

 below, so widely coiled below that 

 no true umbilicus is present. Inside 

 the outer wall of the aperture, par- 

 allelus has from 1 to 3 minute con- 

 ical white denticles. 



The animal is almost white ex- 

 cept for small dark blotches. The 

 fore part of the body is long, the 

 posterior part short. The eye pe- 

 duncles are rather short. When the 

 animal is crawling, the shell lies flat 

 on the short posterior part of the 



