Pennsylvania Land Snails

Photo(s): Bill Frank

Helicodiscus parallelus (Say, 1817)

Common name: Compound coil
Family Helicodiscidae
Order Stylommatophora

The shell of this small snail can be recognized in the field as a little greenish tire. It is flattened and has spiral lirae that look like tire treads at first glance, though upon close inspection they are seen to be raised rather than indented.

The compound coil’s shell is approximately 3.5mm wide and 1.3mm tall, and has a wide umbilicus (Pilsbry, 1948). The four whorls are tubular, expanding slowly, the aperture is relatively small. Inside the final whorl of the shell are two or three series of denticles that may be seen from the outside as opaque spots. The shell is often tinted green and not at all reflective, possibly helping to camouflage this animal.

The compound coil is sometimes not easily distinguished from close relatives, as the diagnostic characters are relatively subtle variations in shell shape, lirae and denticles. In Pennsylvania, it can be separated from the temperate coil (Helicodiscus shimeki) by its smaller, steeper-sided umbilicus (Hubricht, 1962).

Like many others of its family, the compound coil is similar to cave dwelling animals, being blind and having a mostly white body. It lives in damp leaf litter or decaying wood. This coil is widespread in Pennsylvania and the Eastern United States and Canada.

Ken Hotopp, 4/29/06

Development of this site was supported by the generous contributions of Pennsylvanians to the Wild Resource Conservation Fund.