A taxonomic review of Trapezidens (Bivalvia: Unionidae: Lamellidentini), a freshwater mussel genus endemic to Myanmar, with a description of a new species Author Konopleva, Ekaterina S. Federal Center for Integrated Arctic Research, Russian Academy of Sciences, 163000, Arkhangelsk, Russia Author Bolotov, Ivan N. Federal Center for Integrated Arctic Research, Russian Academy of Sciences, 163000, Arkhangelsk, Russia Author Kondakov, Alexander V. Federal Center for Integrated Arctic Research, Russian Academy of Sciences, 163000, Arkhangelsk, Russia Author Kononov, Oleg D. Federal Center for Integrated Arctic Research, Russian Academy of Sciences, 163000, Arkhangelsk, Russia Author Gofarov, Mikhail Yu. Federal Center for Integrated Arctic Research, Russian Academy of Sciences, 163000, Arkhangelsk, Russia Author Tomilova, Alena A. Federal Center for Integrated Arctic Research, Russian Academy of Sciences, 163000, Arkhangelsk, Russia Author Lunn, Zau Fauna & Flora International - Myanmar Program, Yangon, Myanmar Author Chan, Nyein Fauna & Flora International - Myanmar Program, Yangon, Myanmar Author Win, Than Department of Zoology, Dawei University, Dawei, Tanintharyi Region, Myanmar * Corresponding author: es. konopleva @ gmail. com Author Vikhrev, Ilya V. Federal Center for Integrated Arctic Research, Russian Academy of Sciences, 163000, Arkhangelsk, Russia text Ecologica Montenegrina 2020 2020-01-12 27 45 57 http://dx.doi.org/10.37828/em.2020.27.6 journal article 10.37828/em.2020.27.6 2336-9744 13307088 urn:lsid:zoobank.org:pub:DCB77D14-C674-469C-B4EF-6020AF6FA8CA Trapezidens scutum ( Sowerby, 1868 ) Fig. 3d = Unio scutum Sowerby (1868) : Pl. 94, sp. 510. Type locality: Tenasserim. Type: Holotype BMNH_ 1907-10-28 -239 [by monotypy]; Natural History Museum , London , UK (examined by us). = Lamellidens scutum ( Sowerby, 1868 ) . – Simpson (1900) : 857 ; Konopleva et al. (2017) : 210 . = Trapezidens scutum ( Sowerby, 1868 ) . – Bolotov et al. (2017b) : 10 . Topotypes examined. MYANMAR : Nga Wun River , 11.8567°N , 99.1181°E , Tanintharyi River basin, 25.xi.2018 , 4 specimens [ RMBH biv 630_1, biv 632_1, biv 630_2 (ethanol-preserved tissue snip only), and biv 632_2 (ethanol-preserved tissue snip only); all sequenced], Bolotov , Nyein Chan and Vikhrev leg. ; Nga Wun Stream , 11.8233°N , 99.14934°E , Tanintharyi River basin, 2 specimens [ RMBH biv 644 and biv 645_1; all sequenced], Nyein Chan leg. Differential diagnosis. Shell rhomboid, winged posteriorly, inequilateral, moderately thick and inflated. Periostracum dark-brown to black. Pseudocardinal teeth strong, anterior tooth smaller, posterior tooth bigger, somewhat leaf-like, ribbed. Umbo cavity deeper than that in other Trapezidens species , with small granules on its surface. Molecular diagnosis. Trapezidens scutum is closely related to T. exolescens (mean uncorrected COI p-distance: 3.56%), but it can be distinguished from the sister species by nine fixed substitutions in the COI gene fragment as follows: 68 A, 134 C, 179 C, 308 A, 335 G, 443 C, 638 A, 647 C and 653 T; three fixed substitutions in 16S rRNA gene fragment as follows: 14 G, 285 C, and 335 G. Distribution. Endemic to the Tanintharyi (Great Tenasserim) River, Myanmar . Comments. Previously, we assumed that Trapezidens scutum may be conspecific with T. exolescens based on the proximity of their ranges and conchological similarity ( Konopleva et al. 2017 ). However, this hypothesis was not supported by the novel molecular data, because these taxa represent two divergent phylogenetic lineages.