Lucilla - Coil Snails (Family Helicodiscidae)
The coil snails are easily distinguished from other land snails by their small size (< 4 mm) and coiled cinnamon roll shape. Coil snails of the genus Lucilla lack spiral threads as found in the genus Helicodiscus. Two species are found in the databases: the Smooth Coil Snail, Lucilla singleyana, and the Oldfield Coil Snail, Lucilla scintilla. The latter is listed as presumed extirpated by Nature Serve in Indiana and Illinois. Its Indiana collections were in the late 1920s early 1930s.
Lucilla singleyana (Pilsbry, 1889) with mm scale
Smooth Coil Snail
Lucilla singleyana (Pilsbry, 1889)
Oldfield Coil Snail
Lucilla scintilla H. B. Baker, 1929
Characters: Heliciform, simple lip, depressed, umbilicate, ~ 2-2.5 mm. They lack spiral threads as found in the genus Helicodiscus. For more information and images of both species, see Hotopp et al. 2013 and Dourson (2015).
Comparison: The shell of the Smooth Coil (pictured) is translucent with a glossy surface. There are growth wrinkles and radial grooves. The whorls of the shell of the Oldfield Coil gradually increase in size as they grow outward more so than the Smooth Coil. The aperture of the Smooth Coil tends to more oval than the more round on the Oldfield Coil. Both sources above have more complete descriptions. Lucilla belongs to the group of small (<3.5 mm), heliciform, smooth-lipped snails that are a bother to separate.
Habitat: These are not forest-dwellers. They both prefer grassy areas as in fields, meadows, and verge habitats. The Smooth Coil may be found among bare rock and talus.
Status: Both species are uncommon and their true status is not known. More collections are needed.