Triodopsis - Threetooth Snails (Family Polygyridae)

a Triodopsis snail

Threetooth snails are common woodland snails. The shells are mostly depressed with a wide umbilicus and three teeth on its white reflected lip – two on the lip margin and one curving parietal tooth. Once seen, it is easily identified – to genus. Identification to species, as with many land snails, is a different story. 

There may be up to 9 species of Triodopsis in Indiana. According to the databases, the three most widely distributed species are: Triodopsis tridentata, the Northern Threetooth, Triodopsis vulgata, the Dished Threetooth Snail, and Triodopsis discoidea, the Rivercliff Threetooth.

Three other species are more sparsely distributed and, based on Nature Serve data, are not found in Indiana although they do appear as Indiana specimens in museum collection databases. These three are: Triodopsis fallax, the Mimic Threetooth Snail, Triodopsis fradulenta, the Baffled Threetooth Snail, and Triodopsis juxtidens, Atlantic Threetooth Snail. 

Two further species have been found in one county each: Triodopsis complanata, the Glossy Threetooth Snail, in Jefferson County and Triodopsis vannostrandi, the Alabama Threetooth Snail (Coiled Threetooth?) in Marion County. A third species, Triodopsis tennesseensis, the Budded Threetooth, is not in the museum records but is given for Indiana by NatureServe.

Threetooth snails are mostly woodland species, preferring leafy or woody debris, but some have been collected in open areas near roadsides, railroads or abandoned structures. Oesch (2013) noted that the Rivercliff Threetooth “is found along the limestone bluffs of major rivers” - and that seems to be its distribution in Indiana (see below).

The major characters used to differentiate species include:

1. the direction the paretial tooth points (below - or - at/above the palatal tooth)

2. the position of the palatal tooth relative to the edge of the aperture (recessed inside the margin of the outer lip but not along its margin - or - at the margin of the outer lip)

3. the relative size of the umbilicus relative to the diameter of the shell (narrower or wider would be used in other genera, but here it is in terms of fractional sizes)

4. some variation in the height of the aperture (more depressed to slightly elevated)

The key below is based on information from Burch (1962), Oesch et al. (2013), Hotopp et al. (2013), and Dourson (2010) and (2015). This is a very speculative key and should be used with great caution while reviewing these other sources. Dourson (2010 and 2015) really gives you a sense of the challenges with this genus. Not all species are represented in the key.

​For the three most likely species, Triodopsis tridentata, the most commonly collected species, will be the easiest to identify. In the second most common species, Triodopsis vulgata, the parietal tooth is large and Dourson (2010)  notes that it is directed to a point well above the palatal tooth while the palatal tooth is larger, wider, and placed deeper into the aperture. The third species, Triodopsis discoidea, is geographically limited as noted above, and the parietal tooth directs to a point above the palatal tooth but perhaps not as far as in T. vulgata (based on Dourson 2015 images) and the palatal tooth seems smaller. In addition, the umbilicus of T. discoidea seems to spiral while that of the other two more common species seems more tubular.

​For some of the other species, this following key may suffice, but it is very speculative combining information from the sources given above. As with many groups, there may be more species in the state or fewer. There may be regional differences in characters as well. Absent more thorough new collections and examination of museum specimens, who knows? Three species, Triodopsis complanata, T, vannostrandi, and T. tennesseensis are not included.

For more information on these species, see Hotopp et al. (2013), Oesch (2013), and Dourson (2010, 2015).

The Key

​1a. distal end of parietal tooth directed to a point below the palatal tooth – Triodopsis tridentata [note from Dourson (2010): T. complanata points the same way but T. tridentata is smaller and has stronger transverse striae in KY]
1b. distal end of parietal tooth points at or above its palatal tooth, when viewed from the bottom – go to 2

2a. palatal tooth smaller, pointed or squarish, not recessed, but marginal to outer lip – go to 3
2b. palatal tooth larger and broader, recessed, not marginal to outer lip – go to 4

3a. basal tooth larger - Triodopsis juxtidens
3b. basal tooth smaller - Triodopsis discoidea

4a. Umbilicus narrower, less than 1/6 the shell diameter - Triodopsis fallax
4b. Umbilicus relatively wide, 1/6 – ¼ shell diameter – go to 5

5a. Parietal tooth relatively larger - Triodopsis fradulenta
5b. Parietal tooth relatively smaller - Triodopsis vulgata

Triodopsis tridentata (Say, 1816)

Triodopsis tridentata (Say, 1816)

Northern Threetooth
Triodopsis tridentata (Say, 1816)

12-20 mm diameter

Triodopsis tridentata side view

Dished Threetooth
Triodopsis vulgata Pilsbry, 1940

13-20 mm diameter

Triodopsis discoidea (Pilsbry, 1904)

Triodopsis discoidea (Pilsbry, 1904)

Rivercliff Threetooth
Triodopsis discoidea (Pilsbry, 1904)

14-17 mm diameter

Triodopsis discoidea side view

  image of Triodopsis fallax

Mimic Threetooth
Triodopsis fallax (Say, 1825)

10-14 mm diameter

Baffled Threetooth
Triodopsis fraudulenta (Pilsbry, 1894)

14-17 mm diameter

Atlantic Threetooth
Triodopsis juxtidens (Pilsbry, 1894)

12-19 mm diameter

Triodopsis complanata: image

Triodopsis vannostrandi: image

Triodopsis tennesseensis: image

Glossy Threetooth Snail

Triodopsis complanata (Pilsbry,1898)


Alabama Threetooth Snail

Triodopsis vannostrandi (Bland, 1875)

Budded Threetooth

Triodopsis tennesseensis (Walker & Pilsbry, 1902)

revised: 1/23