Opinions and declarations Rendered by the International Commission on Zoological Nomenclature

Increasing attention to the systematics of Southeast Asian pinnotherid crabs has required the re-evaluation of many species described in older works in order to correctly recognise and characterise new species. The pinnotherids from Thailand are known on the basis of only four studies, the most important being that by Mary Rathbun in 1909, who described seven new species from the Gulf of Thailand: Pinnotheres buergeri, P. kamensis, P. kutensis, P. lanensis, P. nigrans, P. quadratus, and P. siamensis. The seven species are reviewed and figured based on type material. Pinnotheres buergeri, currently assigned to Viridotheres, is a senior synonym of P. kutensis and P. siamensis, both of which are earlier juvenile stages of the same species. Pinnotheres kamensis, also based on a juvenile holotype, is a junior synonym of Arcotheres similis (Bürger, 1895). Pinnotheres lanensis and P. quadratus are each valid species of Arcotheres, and mature at a much smaller size than known congeners. Pinnotheres nigrans is a junior synonym of A. purpureus Alcock, 1900. The re-assessment of P. nigrans required a study of A. purpureus and its close congener, A. boninensis (Stimpson, 1858), both of which are redescribed, the latter based on a newly designated neotype

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