Genus Rhombuniopsis Haas, 1920

Rhombuniopsis Haas (1920): 146 –151 (type species: Unio (Cuneopsis) tauriformis Fulton, 1906).

Diagnosis. Shell small, ovate to triangulate ovate, thick, periostracum dark brown to black, rough, with uneven-distributed sparser growth lines, the umbo occupies central position, teeth well developed, umbonal depression deep, lateral teeth strong, ligament developed, anterior adductor muscle scar deep.

Comparison. Rhombuniopsis (Fig. 2 F–K) differs from most Unionidae genera by its small and thick shell with developed pseudocardinal teeth and strong lateral teeth, a deep groove near the anterior adductor muscle scar, and the umbo occupies a central position. It differs from Middendorfnaia (Moskvicheva & Starobogatov, 1973) (Fig. 2 C) by having larger pseudocardinal teeth, which are thicker and closer to the umbo, and a deeper and more obvious anterior adductor muscle scar. Rhombuniopsis also differs from Cuneopsis (Simpson, 1900) (Fig. 2 E) by having more rounded posterior margin region, shorter lateral teeth, and less smooth periostracum with uneven-distributed sparser growth lines. Rhombuniopsis differs from Pseudocuneopsis (Fig. 2 D) in having shorter lateral teeth, pseudocardinal teeth with blunt edges and less smooth periostracum with uneven-distributed sparser growth lines. Rhombuniopsis differs from Inversidens (Haas, 1911) (Fig. 2 A) by having developed pseudocardinal teeth, the indentations between the teeth not reaching the umbo, and the dorsal margin with fewer sculpture. It differs from Leoparreysia (Vikhrev, Bolotov & Aksenova, 2017) (Fig. 2 B) by having a dorsal margin with fewer sculpture, a narrower anterior adductor muscle scar, and brighter periostracum with silky luster, and a certain metallic luster (Fig. 3).

Taxonomic remarks. The genus Rhombuniopsis was proposed for Unio (Cuneopsis) tauriformi s (Fulton, 1906) from Lake Dianchi, Yunnan, China based on the dry shells (Haas 1920). Previously, Simpson (1914) had assigned Unio tauriformis to the genus Cuneopsi s. Subsequently, several species from lakes and surrounding areas of the Yunnan Plateau, as well as from other regions, were also attributed to this genus based on dry shell specimens. Martens (1874) described one fossil species from West Siberia, Rhombuniopsis prona (Martens, 1874). Neumayr (1899) described two other species from Tali-fu (Lake Erhai) in Yunnan: Rhombuniopsis superstes (Neumayr, 1899) and R. heres (Neumayr, 1899). Bogachev (1924) described a fossil species from Kazakhstan, Rhombuniopsis kutschum (Bogachev, 1924). Modell (1931) described one fossil species from Germany, Rhombuniopsis weithoferi (Modell, 1931). Moskvicheva & Starobogatov (1973) described Rhombuniopsis fultoni (Moskvicheva & Starobogatov, 1973) from Yunnan. Zykin (1980) described two fossil species from West Siberia, namely Rhombuniopsis madernyi (Zykin, 1980) and Rhombuniopsis superstoides (Zykin, 1979), as well as another fossil species from Kazakhstan, Rhombuniopsis divulgata (Zykin, 1980). Species of Rhombuniopsis can best be differentiated from each other based on the teeth structure. Here, we recognize six species of this genus as valid, including two new species.