The small Mahakamby Island (= Island of the Shrimp) is located in the Mozambique Channel (Indian Ocean) along the northwestern coast of the Mahajanga Province, (Soalala District, NW Madagascar). Miocene outcrops of Mahakamby were examined during the expeditions of two naturalists from the Muséum national d'Histoire naturelle, Paris: the first by Perrier de la Bâthie in 1915 and the second by Waterlot in 1922. Among the fossil invertebrates collected during the Waterlot's Mission, some decapod crustaceans were partially reported by Collignon & Cottreau (1927) but their specimens are nowadays almost completely lost. However, additional materiel collected by Waterlot was rediscovered in the palaeontological collections of the Paris Museum. It allows a review and a discussion of the crustacean samples previously studied on the basis of iconographic material. Moreover, new specimens of brachyurans, collected during the Waterlot mission, are here assigned for the first time to Portuninae Rafinesque, 1815.