118 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 24 



Movable segments of antenna smooth or nearly so, without anterior 

 projections; flagellum with long hairs. Outer maxillipeds lightly ru- 

 gose. 



Chelipeds covered dorsally with small granules, lightly pubescent; 

 smooth on ventral surface. Merus armed on anterior margin with a 

 small conical lobe. Carpus about one and a half times as long as wide, 

 or a little less; armed on anterior margin with a strong, rounded lobe, 

 anterior and posterior margins markedly converging distally from its 

 highest point, lobe sometimes minutely denticulate along its edge; sur- 

 face with a strong longitudinal crest; posterior margin with a strong 

 crest covered with slightly elongate granules a little larger than those 

 on rest of chelipeds, this crest usually produced distally into a small 

 tooth ; dorsal and posterior crests defined by two deep, broad grooves 

 on dorsal surface; another, faint crest sometimes present on dorsal sur- 

 face near base of lobe. Manus with a distinct longitudinal crest about 

 center of dorsal surface, on its inner side another, shorter crest joined 

 to first at proximal end and extending to base of dactylus; outer mar- 

 gin with a smooth, distinct crest, defined by a deep, broad groove ex- 

 tending well onto pollex. Dactylus with a deep longitudinal groove ; gape 

 of fingers with a short pubescence. 



Merus of walking legs unarmed, anterior margin with a short fringe 

 of plumose hairs, a few long non-plumose setae near posterior margin; 

 that of leg 3 slightly inflated. Other segments covered with long setae. 



Measurements: Holotype male: length 5.0 mm, width 4.8 mm. Para- 

 types: males, 2.8 to 4.6 mm; ovigerous females, 3.3 to 5.2 mm. 



Color: No color or markings remain on any of the material ex- 

 amined. 



Ecology: The type series was taken in the intertidal zone under 

 stones. Twelve females out of twelve were ovigerous ; they were collected 

 in January and March. 



Relationships: Petrolisthes cocoensis is very closely related to P. 

 lewisi (Glassell), from which it is distinguished only by a slight dif- 

 ference in the shape of carapace and front, and by the presence of a 

 lobe instead of teeth on the anterior margin of the carpus. It was prob- 

 ably derived originally from an isolated population of P. lewisi', in- 

 deed, it might be considered a third subspecies of the latter in which 

 complete coalescence of the carpal lobes has taken place. However, it 

 differs considerably more from the two subspecies of P. lewisi than 

 they do from each other, and it is here considered a full species. 



