Confirmation of Ostracotheres H. Milne Edwards, 1853 from Southeast Asia in a new species from Singapore and Indonesia (Crustacea: Decapoda: Brachyura) Author Ahyong, Shane T. Australian Museum, 1 William St., Sydney, NSW 2010, Australia, and School of Biological, Earth & Environmental Sciences, University of New South Wales, Kensington, NSW 2052, Australia Author Mendoza, Jose Christopher E. Lee Kong Chian Natural History Museum, Faculty of Science, National University of Singapore, 2 Conservatory Drive, 117377, Singapore text Zootaxa 2024 2024-07-04 5476 1 9 16 http://dx.doi.org/10.11646/zootaxa.5476.1.4 journal article 299675 10.11646/zootaxa.5476.1.4 e56a71f3-97d9-4e26-a7bc-d4482adff2d5 1175-5326 12716667 10D2E92D-25C2-4390-83ED-143B861F37FA Key to species of Ostracotheres 1. Pereopods 2–5 dactyli stout, falcate, apices distinctly curved, some turning almost perpendicular to main axis. Pereopod 5 dactylus shorter than propodus. Maxilliped 3 exopod flagellum 2-segmented. In tridacnine clams. [Red Sea and western Gulf of Aden]...................................................................... O. tridacnae ( Rüppell, 1830 ) – Pereopods 2–4 dactyli slender, gently curved, apices weakly curved, to less the 45° to main axis. Pereopod 5 dactylus relatively straight, as long as or longer than propodus. Maxilliped 3 exopod flagellum unsegmented. In ascidians................. 2 2. Buccal margin of epistome with small triangular median projection. Pereopods 2–4 dactylus flexor margin lined with corneous spinules for no more than proximal two-thirds. Longest walking leg (pereopod 3) about 1.1 × cl in females. [Red Sea and western Gulf of Aden].............................................................. O. cynthiae Nobili, 1906 – Buccal margin of epistome straight, without triangular median projection. Pereopods 2–4 dactylus flexor margin lined with corneous spinules for about proximal three-fourths. Longest walking leg (pereopod 3) about 1.5 × cl in females. [Southeast Asia]............................................................................... O. caramica sp. nov.