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Unio rectangularis Tapparone-Canefri, 1889 is a little-known nominal species of freshwater mussels described from a tributary of the Ayeyarwady River in Myanmar. This taxon was considered a synonym of Gibbosula laosensis (Lea, 1863), a margaritiferid species. However, the range of Gibbosula laosensis does not encompass the Ayeyarwady River watershed. Here we re-examine the holotype of Unio rectangularis and provide a conchological re-description of this species. Based on conchological features such as the shell shape, elevated umbo, and the structure of lateral and pseudocardinal teeth, we transfer this taxon to the genus Indonaia Prashad, 1918 and propose I. rectangularis (Tapparone-Canefri, 1889), comb. nov. It appears to be a rare freshwater mussel species with a restricted range, because it has not been found since the original description. Two additional species in this genus are known from Myanmar, i.e. Indonaia andersoniana (Nevill, 1877) and I. subclathrata (Martens, 1899). [ABSTRACT FROM AUTHOR]

The Indochinese freshwater pearl mussel Gibbosula laosensis is a little-known but endangered species with a disjunctive range in the Mekong and Sittaung drainage basins. Here we provide an overview of two taxonomic names, i.e., Unio sella and U. sula that were linked to Gibbosula laosensis. We show that the name Unio sella has been introduced by Cesare Maria Tapparone-Canefri on the labels of at least two shell lots, which were collected by Leonardo Fea during his travel to the Karen Hills in British Burma in the years 1887-1888. However, Tapparone- Canefri did not publish this name for unknown reasons. Fritz Haas mentioned this name in the year 1912 as an unpublished name and a synonym of Gibbosula laosensis based on a study of two specimens of the latter species from the Natural History Museum Vienna (NHMW). Ten years later, Baini Prashad also provided a description of what he accepted as Unio sella, but he used a series of four shells from the Museo Civico di Storia Naturale di Genova (MSNG) that contained two specimens of Yaukthwa nesemanni and two specimens of Gibbosula laosensis. Prashad followed the handwritten labels of Tapparone-Canefri and considered the sample of the first species as young specimens of Unio sella, while Gibbosula laosensis specimens have been treated by him as adult shells of this taxon. To retain the concept of Unio sella published by Prashad, we designate a specimen of Yaukthwa nesemanni that was among Tapparone-Canefri's series in MSNG to be the lectotype of U. sella Prashad, 1922. Hence, there are two different taxa under the name Unio sella, i.e., U. sella Haas, 1912, a synonym of Gibbosula laosensis (Margaritiferidae: Gibbosulinae), and U. sella Prashad, 1922, a primary junior homonym of Haas's taxon and a senior synonym of Yaukthwa nesemanni (Unionidae: Rectidentinae). However, in both cases this name is initially unavailable, because it was introduced as a synonym and was never used as a valid name (Art. 11.6 of the International Code of Zoological Nomenclature). Finally, the name Unio sula Simpson, 1900 nom. nudum seems to have originated through an incorrect spelling of U. sella Prashad, 1922, because "sella" is written like "sula" on the original label of Yaukthwa nesemanni sample in MSNG. [ABSTRACT FROM AUTHOR]