Description de Segonzacia gen. nov. et remarques sur Segonzacia mesatlantica (Williams) : campagne HYDROSNAKE 1988 sur la dorsale médio-Atlantique (Crustacea Decapoda Brachyura)

— The HYDROSNAKE Cruise 1988 with the submersible "Nautile" was the exploration of the neovolcanic ridge of the Snake Pit in the mid-Atlantic Ridge, 23"'22'N. During 10 dives on this site a carcinological material was coUected. The Brachyuran crab reported as Bythograea mesatlantica Williams, 1988, from the Mid-Atlantic Rift valley on the Mark vent, very near of the Snake Pit, is hère attributed to a new genus, Segonzacia gen. nov. It differs in several respects from the gênera Bythograea Williams and Cyanagraea de Saint Laurent, discovered on the submarine thermal vents of the East Pacific Rise. The curions suborbital area is the most obvions character of Segonzacia : it is studied, with the différent forms of that structure in adult maie, female, and juvénile spécimens. The location of the crabs in particular hydrothermal areas, recorded in situ on videotape during "Nautile" dives, is explained in détail. Mots-clefs. — Hydrothermalisme, sites hydrothermaux, dorsale est-Pacifique, dorsale médio-Atlantique, Crabes Bythograeidae, Cyanagraea, communautés hydrothermales associées. D. GuiNOT, Muséum national d'Histoire naturelle, 61, rue Buffon, 75231 Paris cedex 05, France.