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).
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