Unio tumidus Philipsson in Retzius, 1788

Synonyms (in the study area) Unio conus Spengler, 1793.

Unio muelleri Rossmässler, 1838.

Description. Shells are thick up to 11.3 mm in the anterior shell region, wedge-shaped, moderately to strongly inflated. The ratio of shell width to its height is 0.62–0.88, ratio of shell height to its length is 0.34–0.58. Umbo is wide, and its sculpture consists of several rows of double-looped folds. Pseudocardinal teeth are thick (0.35–0.42 mm), lateral teeth with circa 0.25 mm thickness and slightly curved. Pseudocardinal teeth of the left valve are generally separated, with the inner tooth projecting more than the outer tooth, but they can also occur merged as a single tooth. Periostracum is yellow-brown. Nacre color is light blue, yellow or pink tone. A dark strip on the foot may be visible or not. Papillae of the incurrent aperture have a swelling in the base and are arranged in two–three rows. Pigmentation of excurrent aperture is homogenous, weakly expressed or quite unnoticeable.

Comparisons. This species differs from U. pictorum by its wedge-shaped and thicker shell, thickened pseudocardinal and lateral teeth, double-looped umbo sculpture, distinct papillae on the incurrent aperture and virtual absence of pigmentation of the excurrent aperture.

Distribution. In the Ukraine and European Russia restricted to the Black, Caspian and Baltic Sea drainage basins, also found in a restricted range in Transbaikalia, Asian Russia (Fig. 1).