Discus - Disc Snails (Family Discidae)
Discus is a genus of small round flattened, shells. Their umbilicus is wide open, the lip is thin but not reflected, and the shell’s surface presents with radial ridges. Of the small land snail species, some Discus can be found along trails at the bases of tree trunks or stumps with a little bit of looking.
Three species may be found in Indiana. First, most widely is Discus patalus, the Domed Disc Snail followed by Discus whitneyi, the Forest Disc Snail. The other species, less often in the collections, is Discus catskillensis, the Angular Disc Snail.
Key (modified from Burch 1962)
1a. periphery of last whorl rounded – go to 2
1b. periphery of last whorl angular or flattened – Discus catskillensis
2a. Shell with relatively large coarse ribs (3-4 ribs/mm) on body whorl – Discus patulus
2b. Shell with relatively small, fine ribs (6-8 ribs/mm) on body whorl – Discus whitney
Discus patulus (Deshayes, 1830)
Domed Disc Snail
Discus patulus (Deshayes, 1830)
This is the commonest species and about 8-9 mm diameter. It has about 5.5 whorls and, in some specimens, a small tooth is present just inside the aperture. The radial ribs are very well-developed, the periphery rounded, and the aperture considered to be more egg-shaped. The Domed Disc Snail can be found associated with woody and leafy debris and, also, climbing on stumps and the lower reaches of tree trunks.
Forest Disc Snail
Discus whitneyi (Newcomb, 1864)
This second-most common species is about 5-7 mm in diameter with about 3.5-4.5 whorls. It has no teeth, the ribs are well-developed, and the periphery usually well-rounded. The Forest Disc Snail is reported to prefer low-lying wet areas such as near springs, wetlands, low-damp meadows, or marshes.
Angular Disc Snail
Discus catskillensis (Pilsbry, 1896)
Found in only four counties, this species is about 5 mm in diameter with 4 whorls. Its radial ribs are also well-developed, but its periphery is more bluntly angular. It has no teeth and is smaller than the other two species.
It can be found in the same kind of habitat as Discus patalus. Presumed extirpated in Indiana by Nature Serve, but there are credible recent collections from 1990 (Brown County) and 1992 (Orange County). Dourson (2010) noted it as presumed extinct in Kentucky.
Notes:
Two other species appear in the databases. The first, Discus perspectivus (Megerle von Mühlfeld, 1816) (common name not known) is a European species. But it has from 17-47 Indiana observations in the databases, most from a few separate collectors, but it is not listed in NatureServe. It is omitted here.
The second is Discus shimekii (Pilsbry, 1890), the Striate Disc with only one observation in the GBIF database. In Nature Serve, this species is not given in the Midwest east of the Mississippi and is likely extirpated in MO, IA, KS, and NB on the western side. Extant in western states. It is also omitted here.