104 THE VOYAGE OF H.M.S, CHALLENGER. 



with two (or three) pairs of slender uniramous appendages borne on the fourth, fifth (and 

 tliird) segments ; males with a single genital pair on the second segment. 



The members of this well-defined group are not confined to any special geographical 

 area, but occur in all seas, and are found under stones between tide-marks, a situation 

 for which their flattened body and chelipedes are peculiarly adapted, or in shallow water 

 living among Corals, Sponges, or stones. Stimpson in his useful Synopsis of the 

 Anomura has arranged the genera in two divisions, which form, however, but a single 

 family ; in the first of these the basal joint of the antennal peduncle is of small size and 

 partially concealed in the orbital cavity, whereas in the second this joint forms an 

 acute and somewhat flattened projection placed externally to the orbit. They may be 

 arranged as follows : — 



I. First joint of the antennal peduncle short, not reaching the superior margin of the 

 carapace — 



Petrolisthes, Stimpson. j Pisosoma, Stimpson. 



PetrocheJes, Miers. 



II. First joint of the antennal peduncle more or less produced, and joined to the 

 margin of the carapace, the second joint placed at a distance from the orbit — 



Porcellana, Lamarck [restrictum). 

 Porcellanella, White. 

 Rcq^liidopus, Stimpson. 



Pachycheles, Stimpson. 

 Megalohrachmm, Stimpson. 

 Minyocerus, Stimpson. 



Polyonyx, Stimpson. 



Family Porcellanid/E. 



Genus Petrolisthes, Stimpson. 



Petrolkilies, Stimpson, Proc. Acad. Nat. Sci. Philad., p. 65. 1858. 

 „ Miers, Catal. New Zealand Crust., p. 59, 1876. 



Haswell, Catal. Austral. Crast, p. 145, 1882. 



Carapace subovate, depressed, the length usually slightly greater than the breadth. 

 Frontal region triangular, usually depressed, with the margin more or less undulated. 

 Eyes of rather large size. First joint of the antennal peduncle remarkably short. 

 Chelipedes broad and flattened, the carpus of moderate length and often provided with 

 teeth on the inner margin. Ambulatory limbs with the dactyli short and robust, 

 terminating in a single claw. 



The species, many of which live between tide-marks, are distinguished from those of 

 the genus Porcellana, in addition to the important diff"erence in the antennal peduncle, 

 by the form of the chelipedes and front. They appear to be scarcely represented in the 

 temperate and colder seas of the northern hemisphere. 



