PROC. BIOL. SOC. WASH. 93(2). 1980, pp. 443-472 A NEW CRAB FAMILY FROM THE VICINITY OF SUBMARINE THERMAL VENTS ON THE GALAPAGOS RIFT (CRUSTACEA: DECAPODA: BRACHYURA)i Austin B. Williams Abstract. — Bythograea thermydron, a new genus and new species of brachyuran crab, is described from the vicinity of submarine thermal vents on the Galapagos Rift. About 160 of the crabs in various states of maturity were collected by means of baited traps, suction, and mechanical arm with the aid of the deep submersible, Alvin. Bythograea represents a new family that exhibits some characters of the families Portunidae and Xanthidae, with superficial resemblance to the family Potamidae s. 1. It is placed in an in-dependent superfamily. Introduction The Galapagos Rift zone supports a community of organisms living on hard bottom within and somewhat beyond the influence of thermal vents in the ocean floor. Investigators aboard the deep submersible Alvin viewed and photographed these organisms, taking a few samples in February and March 1977 (Corliss and Ballard, 1977; Corliss et al., 1979), and on a second expedition in February 1979, secured far more abundant samples of the biotic community among which were numerous adult brachyuran crabs tak-en in baited fish traps and a few megalopae associated with mussel clumps (Grassle et al., 1979; Ballard and Grassle, 1979). Further observations were made and a few specimens collected during a third expedition in November-December 1979. The crabs are here described. I am greatly indebted to J. F. Grassle, Woods Hole Oceanographic In-stitution (WHOI), chief scientist on the second and third expeditions, for transmitting to me the material on which the description is based as well as observations from the submersible, and to his associates L. Morse-Porteous and I. Williams for assistance in laboratory study of the crabs; to D. M. Cohen, National Marine Fisheries Service, Systematics Laboratory, Na-tional Museum of Natural History, whose traps baited for capture of fishes were the adult crab collectors, not only for that good fortune but also for his observations on crab sizes, distribution and behavior; and to J. R. Chil-' This article is contribution number 5 of the Galapagos Rift Biology Expedition supported by the National Science Foundation.