58 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 24 



wide, anterior margin with two low, wide-set teeth; merus of walking 

 legs with one or two spines on anterior margin, that of legs 1 and 2 

 spined at posterodistal angle. 



Description: Carapace naked or lightly pubescent, finely and evenly 

 rugose. Front sinuously triangular, with a shallow median sulcus; no 

 supraocular spine. Outer orbital angle not produced. A single small 

 epibranchial spine. 



First movable segment of antenna with a spine-tipped lamellar 

 lobe on anterior margin; second granular, with a proximal nodular pro- 

 jection on anterior margin ; third nearly smooth ; flagellum naked. 

 Outer maillipeds rugose. 



Chelipeds rugose. Merus armed on anterior margin with a small lobe, 

 blunt or spine-tipped. Carpus a little over twice as long as wide, armed 

 on anterior margin with two low, wide-set teeth, one submedian and the 

 other at proximal end ; surface with a longitudinal crest, strong in some 

 specimens; posterior margin with a crest formed of oblique rugae, one 

 to three of them developed into spines. Manus long and slender, outer 

 margin serrate or with a row of spinules, and with a fringe of long 

 hairs ; outer half of lower surface often, but not always, densely covered 

 with hairs to base of pollex. Gape of fingers with pubescence confined 

 to proximal half of fingers, often produced into a long tuft. 



Walking legs rugose; merus and carpus covered with plumose hairs 

 and a few long, non-plumose setae, other segments with tufts of setae; 

 anterior margin of merus of leg 1 spineless or with a single spine, of 

 leg 2 with one or two spines, of leg 3 spineless or with one; merus of 

 legs 1 and 2 with a posterodistal spine; carpus of legs 1, 2, and 3 with 

 an anterodistal spine. 



Material examined: See Table 13. 



Measurements: Males, 5.4 to 8.0 mm; non-ovigerous females, 5.3 to 

 7.5 mm; ovigerous females, 4.7 to 7.3 mm. The largest specimen on 

 record is the holotype male, which measures 8.6 mm in length, 8.0 mm 

 in breadth. 



Color: The color in life has not been recorded. Glassell ( 1945) noted 

 that the carapace in specimens from the type series had a distinct 

 pinkish tone in alcohol, with the pubescence on the carapace dark 

 brown. 



Ecology: Petrolisthes robsonae is remarkable in that it is able to with- 

 stand great changes in salinity, a phenomenon almost unique among 

 porcellanids (see also Ecology under account of Petrolisthes laeviga- 



