132 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL.24 



Basal segment of antenna not produced forward to meet anterior 

 margin of carapace; usually somewhat produced inward to form a 

 partial suborbital margin. 



Basal segment of antennule broad, anterior margin truncate; an- 

 terior margin sinuate, sometimes armed with one or two spinules on 

 inner side. 



Chelipeds large, thick, and robust, one distinctly larger than the 

 other; carpus short and stout, usually about as broad as long; manus 

 and fingers often different in form in the two chelipeds. 



Walking legs of moderate length, somewhat flattened; propodus 

 with movable spinules on posterior margin; dactylus ending in a simple 

 spine, with small movable accessory spinules on posterior margin (and 

 very rarely with a few fixed spines as well). 



Telson of abdomen composed of either seven or five plates, the 

 number within each species constant in males, sometimes variable in 

 females. Male pleopods frequently reduced in size or completely lack- 

 ing. 



Remarks: Pisosoina was very poorly defined by its author (Stimp- 

 son, 1858), and a heterogeneous assemblage of species has been as- 

 signed to it from time to time by various writers. Ortmann (1897) 

 considered it a subgenus of Petrolisthes, placing in it forms in which 

 the basal antennal article is short, the side walls of the carapace are 

 entire, and the chelipeds are somewhat thickened ; but at the same time 

 he assigned its type species, Porcellana pisinn H. Milne Edwards, 1837, 

 to Pachycheles. Miyake (1943), recognizing that Pisosoma must be con- 

 lined to species related to Pisosoma pisum, gave the first satisfactory 

 definition of the genus on that basis. Among the five Nipponese spe- 

 cies in which the side walls of the carapace are divided, Miyake was 

 able to distinguish between Pachycheles and Pisosoma on the basis of 

 several characters, the most clear-cut distinctions being the presence in 

 Pachycheles of a tuft of hairs on the front and spinules on the anterior 

 margin of the basal antennal article, and the absence of these features 

 in Pisosoma. However, when these criteria were applied to the eighteen 

 eastern Pacific species no clear distinction between the two genera 

 could be drawn. In this report Pisosoma is considered a synonym of 

 Pachycheles. 



