Carapace squarish to transversely oval and smooth. Propodus of third maxilliped not projecting distally beyond extremity of dactylus. Chelipeds equal in size and smooth, upper margin of merus, inner of carpus and lower of dactylus sparsely setose at the most, propodus slightly swollen, dactylus sometimes with a tooth on inner proximal margin; second to fifth pereiopods with setose margins, dactylus long, usually as long as propodus.
Female carapace soft and translucent (?hard in swimming stage), postero-lateral margins sometimes divergent. Male carapace hard and opaque (in swimming stage), frontal region of carapace slightly projecting; second to fifth pereiopods stouter than in female and, in swimming stage, with long setae.
Carapace length up to female 12 (12.8) mm; male 7 mm.
Depth range from intertidal region to about 55 metres, and in the following bivalves: Atrina fragilis (Pennant), Modiolus modiolus (Linnaeus), Ostrea edulis (Linnaeus); and in the ascidians Ascidia mentula O. F. Muller and Ascidiella aspera (O. F. Muller) (see Schmitt et al., 1973). Ovigerous crabs July, larvae August-October (Lebour, 1928).
Carapace brown, sometimes reddish (suffused colour of gonads), chelipeds and pereiopods lighter brown.
Yorkshire. Thames. Portland. Plymouth. Channel Isles. Scilly Isles, N. Cornwall, Bristol Channel, Isle of Man, Cork, Nymphe Bank.