The pictured shell was 1.8 mm in diameter.
Hawaiia minuscula is a widespread land snail in the U.S. My specimens came from Monocacy Natural Resources Area in Frederick County, Maryland. Its small, more or less flat and mostly smooth shell with a wide umbilicus make H. minuscula easy to recognize until one realizes that there is another species, Lucilla inermis1, whose shell is almost identical to that of H. minuscula. According to Pilsbry2 the shells of the species in the subgenus Hebetodiscus "resemble Hawaiia minuscula somewhat, but are smoother and more depressed". I think the resemblance is a bit stronger than "somewhat", for after I examined the drawings in Pilsbry, I had no idea how tell if the shells I had were minuscula or inermis.
Then, I noticed in Pilsbry that there was a tiny difference between the anatomies of Hawaiia and Hebetodiscus: the latter has one retractor on its penis and another one on its epiphallus, while the former has a retractor only on its penis. Luckily, I had 2 alcohol specimens. So I dissected them. Now, if these were large snails, it would be quite easy to find their genitalia, but the snails I dissected were barely 1.6 mm across and what was worse was that they were apparently not adults; their genitalia were nowhere to be found.
As I was about to give up, Tim Pearce came to the rescue and pointed out that the teeth on the radulae of H. minuscula and L. inermis have different shapes, the most significant being the shapes of the marginal teeth, blade-like in minuscula but serrated in inermis (see the drawings below).
The teeth of Lucilla inermis (left) and Hawaiia minuscula (right). Notice the different shapes of the marginal teeth (#7 for inermis and #6 for minuscula). Drawings from Pilsbry.
No sooner did I read Tim's e-mail, than I set to work. I removed the radula of one of my specimens, transferred to a slide and put it under the microscope. The marginal teeth were blade-like (see photo below). I had H. minuscula.
Flattened radula of Hawaiia minuscula from Monocacy Natural Resources Area. The capital M marks the column of median teeth; the red arrows point at the blade-like marginal teeth.
But, now there is another puzzle. Frank C. Baker3 gave the "most common" habitat of H. minuscula in Illinois as "woodlands of oak, hickory and sycamore". All of my specimens also came from leaf litter samples from woods. In contrast, this is what Hubricht4 said about H. minuscula: "A species of bare ground. I have never found it in leaf litter." Interestingly, he lists similar habitats for inermis: "open, grassy situations..." Go figure.
1I've been told that Helicodiscus (Hebetodiscus) singleyanus inermis (H.B. Baker, 1929) is now Lucilla inermis.
2Pilsbry, H.A. 1948. Land Mollusca of North America . Volume 2, parts 1 & 2.
3Baker, F.C. 1939. Fieldbook of Illinois Land Snails. p. 72.
4Hubricht, L. 1985. The distributions of the native land mollusks of eastern United States. Fieldiana #24.