Ruthenica, 2019, vol. 29, No. 1: 55-70. Published online January 14, 2019 © Ruthenica, 2019 http: www.ruthenica.com ABSTRACT. Unio mongolicus Middendorff, 1851 (Bi- valvia: Unionidae) was described from Gorbitza moun- tain stream (in the east of Trans-Baikal Territory, Rus- sia) and the taxonomy has changed over the past cen- tury and a half. A specimen of Middendorffinaia mon- golica from the Gorbitza stream collected 160 years later is a match with the type illustration of Unio mon- golicus from the type locality. Comparison of the shell morphology of the type specimen of U. mongolicus with the modern specimen collected from this location, combined with our analyses of molecular, anatomical and conchological data of M. cf. mongolica species from rivers of the Upper Amur River Basin and the far eastern Russia, revealed that these taxa all belong to a single polymorphic species. The molecular analysis of the COI gene fragment of mtDNA of examined Compar- atory Species assigned to Middendorffinaia Mosk- vicheva et Starobogatov, 1973 confirmed they are only intraspecific forms of M. mongolica. Morphological and genetic distances between M. mongolica and Nod- ularia douglasiae (Griffith et Pidgeon, 1833) confirmed their assignment to two independent genera. Based on conchological characteristics and anatomical features, the differences between morphologically similar spe- cies M. mongolica, N. douglasiae and Unio crassus Retzius, 1788 were determined. History of the taxono- mic changes for Unio mongolicus is discussed. Introduction Unio mongolicus Middendorff, 1851 (Bivalvia: Unionidae) was originally described from a Gorbit- za mountain stream near Gorbitza village in Dauria (in the eastern area of Trans-Baikal Territory of Russia) [Middendorff, 1851: 277-278, pl. XXVII, figs 7–8], and has been an enigma for taxonomists. Middendorff [1851] noted that he only found a single specimen of Unio mongolicus, thus a holo- type by monotypy. The holotype has been pre- sumed lost, and this species has not been found or collected until recently. With only a rough descrip- tion and the shell figure provided by Middendorff (showing an elongate oval shell with an extensively eroded umbo, pronounced pseudocardinal and lat- eral teeth) it was difficult for taxonomists to recog- nize U. mongolicus as a true species and to establish its generic placement. For that reason, its taxonom- ic placement has changed considerably over the last 150 years. Zhadin [1938] suggested that U. mon- golicus represented a very rare species of freshwa- ter pearl mussel, and in his monographic summary of large bivalves of the USSR, placed U. mongoli- cus within Margaritana (=Margaritifera) Schuma- cher, 1817 as Margaritana mongolica (Middendor- ff, 1851). A few years later, the same author after considering the well-developed lateral teeth which are characteristic of Unio, changed his mind mov- ing the species back into Unio Retzius, 1788, as a subspecies of Unio douglasiae Griffith et Pidgeon, 1833, i.e. Unio douglasiae var. mongolicus [Zhad- in, 1952]. Haas [1969] considered Unio mongolicus a subspecies of the European Unio crassus (Philips- son in Retzius, 1788), i.e. Unio crassus mongolicus. Solution of taxonomic status of Unio mongolicus Middendorff, 1851 (Bivalvia: Unionidae) from the type locality in Transbaikalia and history of its taxonomy O.K. KLISHKO 1 , M. LOPES-LIMA 2, 3 , E. FROUFE 3 , A.E. BOGAN 4 1 Institute of Natural Resources, Ecology and Cryology Siberian Branch, Russian Academy of Sciences, Nedorezova Str. 16a, 672014, Chita, RUSSIA. E-mail: olga_klishko@mail.ru (corresponding author) 2 Research Center in Biodiversity and Genetic Resources (CIBIO/InBIO), University of Porto, Campus Agrário de Vairão, Rua Padre Armando Quintas, 4485-661 Vairão, PORTUGAL. E- mail: manuelpmlopeslima@gmail.com 3 CIIMAR Interdisciplinary Centre of Marine and Environmental Research of the University of Porto, Terminal de Cruzeiros do Porto de Leixões; Av General Norton de Martos, S/N; 4450- 208 Matosinhos, PORTUGAL. E-mail: elsafroufe@gmail.com 4 Research Laboratory, North Carolina Museum of Natural Sciences, 11 West Jones St, Raleigh, NC 27601, United States of America. E-mail: arthur.bogan@naturalsciences.org