178 University of California Publications in Zoology [Vol. 23 



The telson composed of seven plates: a central (triangular), two terminal, and 

 two lateral, each of which has a smaller plate separated off at its proximal end. 

 These smaller plates together constitute a second pair of lateral plates which are 

 characteristic of this species as compared with the other two listed above. 



Dimensions. — Type : length of carapace 15 mm., width 15 mm. ; of the largest 

 specimen I have examined : length of carapace 18 mm., width 19 mm. 



Type Localities. — Drake's Bay, Farallon Islands and Humboldt County, Cali- 

 fornia. 



Distribution. — From Port Orchard, Puget Sound, Washington, to Monterey 

 Bay, California. I have seen specimens from both of these localities and Oakland, 

 California, as well, where a single specimen was taken by Henry Hemphill. 

 Misaki, Japan (Balss). 



Genus Petrolisthes Stimpson 



Carapace subovate; front triangular, with a more or less undulated margin, 

 which may be either smooth or dentated; epimera (pleural, subbranchial, or lateral 

 portions of the carapace) entire. First or basal joint of antennal peduncle very 

 short, not reaching the upper margin of the carapace; the second joint is flattened 

 and more or less cristate. Chelipeds equal or subequal, broad and flattened; 

 carpus more or less elongated. 



Key to the California Species of Petrolisthes 



I. Carpus of chelipeds about twice as long as wide, or less; minutely granulated 

 or finely tuberculated, never very rough. 



A. Carpus with parallel anterior and posterior margins, about twice as long 



as wide; upper surface with irregularly scattered small and tiny 

 tubercles; lobe at inner margin if present never very prominent and 

 not interfering with parallelism of margins. Carapace finely striated 

 over entire surface, except in cardiac region; anterior branchial and 

 gastric regions with small, irregularly placed tubercles, which scat- 

 teringly follow the areolations. Merus of ambulatory legs hairy. 



eriomerus, p. 180. 



B. Carpus of chelipeds with sides not parallel, with a prominent lobe at 



inner angle, and a distinct distal convergence of anterior and pos- 

 terior margins, almost invariably one and one-half times as long 

 as wide; surface of carpus granulated rather than tuberculated, an- 

 terior margin usually smooth, lobe at inner angle well granulated. 

 Carapace similar to preceding, but with a longitudinal convexity 

 greater and more pronounced than any of the species here listed. 

 Merus of ambulatory legs not hairy. 



cinctipes, p. 179. 



II. Carpus of chelipeds about two and one-third times as long as wide, or longer: 



anterior and posterior margins subparallel; carpus either very smooth 



or very rough. 



A. Carpus very smooth, punctate, or minutely and almost imperceptibly 



scaled or tuberculated, more than two and one-half times as long as 



wide. Carapace smooth and flattened, though distinctly areolated, 



minutely striated on branchial regions, tending to become punctate 



elsewhere, never tuberculate. Merus of ambulatory legs not hairy. 



(Not known north of Monterey.) 



gracilis, p. 181. 



