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Freshwater bivalves (Bivalvia, Unionida, Unionidae) are among the most threatened groups of animals in the world. Taxonomic uncertainties pose a particular challenge due to significant intraspecific variation and apparent plasticity in shell morphology among these species, impeding efficient conservation planning and management. The species Unio szechenyii that is endemic to China has long been regarded as a synonym for Cuneopsis heudei due to the high convergence in shell morphology. The recently validated species Middendorffinaia mongolica is currently restricted to the Amur Basin and Primorye in eastern Russia. This species is classified under the tribe Middendorffinaiini of the subfamily Unioninae within the family Unionidae based on multilocus phylogeny. However, the lack of comprehensive taxonomic information and sufficient molecular phylogenetic data casts uncertainty on the validity and taxonomic status of these two species. We aim to reassess the validity of Unio szechenyii and confirm the taxonomic status of Middendorffinaia mongolica , a new record for China, by integrating taxonomy with robust mitochondrial phylogenomics. We support the validity of Unio szechenyii and propose a recombination as Cuneopsis szechenyii comb. nov. by integrating shell morphometry, soft-body anatomy and molecular evidence. By determination of the mitochondrial genome of Cuneopsis szechenyii , Cuneopsis kiangsiensis , Middendorffinaia mongolica and Pseudocuneopsis wuana , we elucidate intrageneric relationships within the genera Cuneopsis and Pseudocuneopsis. In addition, based on the updated consensus taxonomic framework including tribes and subtribes in Unioninae, mitochondrial phylogenomics strongly support the transfer of Middendorffinaia mongolica from the tribe Middendorffinaiini to the tribe Unionini and classification under the subtribe Middendorffinaiina stat. rev. ZooBank: The maternal mitochondrial genomes of Unio szechenyii , Cuneopsis kiangsiensis , Middendorffinaia mongolica and Pseudocuneopsis wuana are sequenced. The validity of Unio szechenyii , previously considered a synonym of Cuneopsis heudei , is supported by the integration of conchometry, soft-body anatomy and molecular evidence, and a recombination as Cuneopsis szechenyii comb. nov. is proposed. The intrageneric relationships within the genera Cuneopsis and Pseudocuneopsis are elucidated based on the mitochondrial phylogenomics. Middendorffinaia mongolica , a new record for China, is proposed for transfer from the tribe Middendorffinaiini to the tribe Unionini and classification under the subtribe Middendorffinaiina stat. rev. (Image credit: Ruiwen Wu.) [ABSTRACT FROM AUTHOR]