Hauffenia is a genus of freshwater snails of the family Hydrobiidae found in south-eastern Europe (Slovenia, Croatia, adjoining parts of Italy and Austria, etc.) Hydrobiids are a very diverse but very minute group of gastropods: Hauffenia species, for instance, are less than three millimetres in diameter and less than 1.5 millimetres in height. Hauffenia species differ from many other hydrobiid genera in having much flatter shells with only the slightest of turrets. This shape, which commenter 'tf' described as "almost but not quite planiform", is known as 'valvatiform', after Valvata, another freshwater snail with a similar shell. Though the shells of Hauffenia and Valvata are similar enough that the two genera have been confused in the past, the internal anatomy of Valvata shows that it is not a hydrobiid or even closely related. Hydrobiids belong to the major gastropod clade known as caenogastropods but Valvata is a heterobranch, more closely related to garden snails or sea slugs than to hydrobiids (Dayrat & Tillier, 2002). Valvata species are also hermaphroditic while hydrobiids such as Hauffenia have separate males and females.
Distinguishing Hauffenia from other valvatiform hydrobiids is difficult and requires examination of the internal anatomy. Bodon et al. (2001) characterised Hauffenia as possessing a penis with a stylet in the male, while females possessed proximal seminal receptacles only (no distal receptacles) and a reduced bursa copulatrix. Identifying these characters can be difficult because Hauffenia species are subterranean, mostly living in caves and springs in limestone karsts though the Hungarian Hauffenia kissdalmae was recently described from a spring in andesite (Erőss & Petró, 2008), making collecting fresh material difficult. Most early studies on Hauffenia were based on shell morphology only, and species previously assigned to the genus from western Europe or North America were regarded by Bodon et al. as belonging to other genera. Even more doubtful is the assignation of Miocene marine fossils to this genus, refuted by Iljina (2010) on the basis that the Hauffenia-like opercula attributed to the fossils were probably not validly associated, and possibly not even gastropod opercula. Some Hauffenia species, such as H. tellinii in the figure at the top of this post, possess a distinctive knob on the inside of the operculum that distinguishes them from other valvatiform hydrobiids. Earlier authors distinguished separate subgenera in Hauffenia based on whether or not a species possessed such a knob, but Bodon et al. (2010) did not use such a formal distinction.
And if you've just been reading this post to see who won the ID challenge, three points go to 'tf' who recognised the animal as a hydrobiid and provided the diagnostic features; two points go to 'intercostal' who mistook it for a valvatid but pointed out that the presence of an operculum meant that it couldn't be a pulmonate.
Bodon, M., G. Manganelli & F. Giusti. 2001. A survey of the European valvatiform hydrobiid genera, with special reference to Hauffenia Pollonera, 1898 (Gastropoda: Hydrobiidae). Malacologia 43 (1-2): 103-215.
Dayrat, B., & S. Tillier. 2002. Evolutionary relationships of euthyneuran gastropods (Mollusca): a cladistic re-evaluation of morphological characters. Zoological Journal of the Linnean Society 135 (4): 403-470.
Erőss, Z. P., & E. Petró. 2008. A new species of the valvatiform hydrobiid genus Hauffenia from Hungary (Mollusca: Caenogastropoda: Hydrobiidae). Acta Zoologica Academiae Scientiarum Hugaricae 54 (2): 159-167.
Iljina, L. B. 2010. On the taxonomic position of Miocene valvatiform gastropods and their ecological features. Paleontological Journal 44 (4): 391-394.