When last we left our fearless jpeg naturalist, he had just received several images of hydrobiid snails from Ms. Rachel Vinsel, the curatorial assistant at the Illinois Natural History Survey. One image depicted cotypic Marstonia ozarkensis, collected by A. A. Hinkley in 1914 from the White River drainage in northern Arkansas , declared extinct by the US Fish & Wildlife Service in late 2018 . That’s the first image in the photomontage below, reprinted from last month’s essay.
Ah, but there were “several images” attached to Rachel’s late February email. She went on to add:
“While I have you, I was wondering if you'd mind taking a look at image 0097 as well? This one was collected in the Kishwaukee River (Rock River Dr.) Winnebago Co., IL. It's not quite 3mm tall.”
Rachel’s image 0097 is labelled “Kishwaukee unknown,” second from left below:
|All four approx 3 mm standard length|
In that charmingly-befuddled essay, you may remember my fumbling with a mysterious population of 3 mm hydrobiid snails collected from the muck of Lake St. Clair, 25 miles east of Detroit. Check out the photos in that essay, one of which has been clipped and inserted third in the montage above. And now could I ask you to refer to my follow-up essay of 5Feb16:
The matches between cotypic M. ozarkensis, Rachel’s Kishwaukee unknown and the Lake St Clair Marstonia letsoni is pretty darn near perfect, am I right? And let me add yet another observation.
In January of 2017 I was combing through a remarkable set of collections made by our good friend Ryan Evans up in Kentucky, when my eyes fell on a single, tiny shell from Elkhorn Creek, about 10 km north of Frankfort. That shell is depicted at the far right of the montage above. That’s pretty good match as well, am I right?
So we have now established two things. In 2016, we demonstrated that populations of Marstonia letsoni are quite literally obscure – tiny snails, inhabiting dark recesses, sometimes in deep water. To find one, you’d need to be more persistent or more lucky than I, your humble correspondent, who has never seen one in his entire 45-year career combing the lakes and rivers of America, overtly and deliberately committing premeditated acts of freshwater malacology in the first degree.
And we have now documented that Marstonia letsoni has a strikingly broad range. Lake St Clair is 500 km north of Frankfort, KY, and 500 km east of the Kishwaukee River at Rockford, IL. Could Marstonia letsoni range another 700 km to the Ozarks? Might Marstonia ozarkensis (Hinkley 1915) be a junior synonym of Marstonia letsoni (Walker 1901)?
Let me make one final point in closing. All authors who have any first-hand experience in this arcane little corner of malacology – Hinkley, Bob Hershler , and Shi-Kuei Wu  – have been unanimous that in overall morphology and life habit, both Marstonia ozarkensis and M. letsoni are very similar to a third species, Marstonia scalariformis. The primary distinction is a carina or keel on the shell of scalariformis, which Wu and colleagues  observe “may be absent or only vaguely apparent” in some natural populations. See Wu’s Figures 26 – 29 below.
|M. ozarkensis (26, 27) and M. scalariformis (28, 29) from Wu .|
Marstonia scalariformis rivals M. letsoni in both obscurity of life habit and vastness of range. Wolf  described “Pyrgula” scalariformis in 1869 from a single shell found by the banks of the Illinois River. Hinkley  described wabashensis from the Wabash River at the Illinois/Indiana border in 1908, which Hershler  synonymized under scalariformis in 1994, noting “variable carina development.” Hershler’s figure of the penial morphology of scalariformis sampled from the Meramec River in Missouri shows bifurcation reminiscent of the letsoni penis figured by Berry .
And the “very incompletely known” range of scalariformis ranges all the way from central Illinois south down to tributaries of the Tennessee River in north Alabama .
Hinkley thought that the previously-described species most similar to his ozarkensis was wabashensis. Hershler agreed, suggesting that wabashensis was a junior synonym of scalariformis and adding further that letsoni was also most similar to scalariformis, without directly comparing ozarkensis to letsoni. I am not sure here today whether Marstonia ozarkensis (Hinkley 1915) is actually extinct, or if it was simply a local population of what has been called elsewhere letsoni (Walker 1901), or wabashensis (Hinkley 1908) or possibly even scalariformis (Wolf 1869), now here in the 21st century misunderstood into oblivion.
 Hinkley, A.A. (1915) New Fresh-water Shells from the Ozark Mountains. Proceedings of the United States National Museum, 49:587-589. This is actually the 1916 volume of the PUSNM, but Hinkley’s date of publication is given as “December 23, 1915” in the index.
 USFWS 2018. Ozark snail species presumed extinct following science-based surveys.
 Hershler, R. (1994) A review of the North American freshwater snail genus Pyrgulopsis (Hydrobiidae). Smithsonian Contributions to Zoology 554: 1-115.
 Wu, S-K, R. D. Oesch & M. E. Gordon (1997) Missouri Aquatic Snails. Missouri Department of Conservation, Jefferson City. 97 pp.
 Wolf, J. (1869) Descriptions of three new species of shells. American Journal of Conchology 5: 198.
 Hinkley, A. A. 1908. A new species of Pyrgulopsis. Nautilus 21: 117-118.
 Berry, E. G. (1943) The Amnicolidae of Michigan: Distribution, ecology, and taxonomy. Misc. Publ. Mus. Zool. Univ. Mich. 57: 1 – 68.
 Walker, B. 1906. New and Little Known Species of Amnicolidae. Nautilus, 19:114-117. Walker identified the population collected by Mr. Hinkley near Florence, Alabama, as “Pyrgulopsis mississippiensis (Pilsbry),” which is a junior synonym of M. scalariformis, according to Hershler . There are also museum records of M. scalariformis from the Flint River near Huntsville that need confirmation.