P. euphraticus has a big, good-looking shell, but we don’t know very much about the species. It’s distribution in the Fertile Crescent means shells turn up occasionally in the archaeological literature (e.g., Ridout-Sharpe, 2015). But, the biological classification of the genus Pseudodontopsis is a house of cards.
The species was originally described in the 19th century in the genus Unio. Because of its reduced hinge teeth, P. euphraticus was classified for a while as Pseudodon, Leguminaia, etc., until it got its own genus Pseudodontopsis. Kobelt (1913) gave a long bit of hand-waving to make the case that P. euphraticus (and the various synonyms that he described) was distinct from the other genera in the region, and the name Pseudodontopsis has stuck since then.
However, as we explained last month, monotypic genera often seem like a cop-out — a place-holder for species of unknown affinities. Pseudodontopsis is one of those genera.
Today, we classify Pseudodontopsis in the subfamily Gonideinae based on argumentation rather than data. Both Modell (1964) and Starobogatov (1970) classified Pseudodontopsis in a subfamily called Pseudodontinae,* along with other genera like Pseudodon, Leguminaia, and Microcondylaea. Lopes-Lima et al. (2017) recovered those three genera in a clade with Gonidea, and ipso facto we conclude the Pseudodontopsis also belongs in a clade with the Gonideinae. Unfortunately, the transitive property is not a good basis for phylogeny. What we know about the glochidia of P. euphraticus is consistent with this classification (Al-Mahdawi & Al-Dulaimi, 2009).**
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* We’ll ignore for this post that Modell and Starobogatov classified the Pseudodontinae in the family Margaritiferidae!
** There was a brief, pre-cladistic period in the late 20th century when some Palearctic mussels (including Pseudodontopsis) in the Nearctic genus Pleurobema, but we digress. | 
