122 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 24 



Merus of cheliped smooth, armed on anterior margin with a small, 

 strongly projecting lobe. Carpus less than one and a half times as long 

 as wide, with three or four broad teeth on anterior margin, proximal 

 one the largest; covered with rough, oblique granules, especially along 

 posterior margin ; a median longitudinal crest, often indistinct, it and 

 posterior crest defined by a groove; small specimens with a trace of 

 another dorsal crest anterior to median one. Manus covered with rough 

 plications; surface not swollen, outer margin with a crest defined by a 

 distinct groove continuing onto pollex. Fingers punctate ; gape devoid 

 of pubescence. Chelipeds without pubescence. 



Walking legs nearly smooth, all segments with long setae. Merus 

 not armed with spines. 



Telson of abdomen with five plates. 



Material examined: See Table 38. 



Measurements: Males, 2.1 to 4.3 mm; non-ovigerous females, 1.8 

 to 4.4 mm; ovigerous females, 1.9 to 5.6 mm. A male syntype examined 

 by the writer measured 4.0 mm; an ovigerous female syntype, 3.9 mm. 

 Nobili gave measurements for a male specimen of 5.0 by 4.5 mm. The 

 holotype of Pisosoma flagraciliata (an ovigerous female) measures 4.9 

 by 5.1 mm; a male paratype, 4.2 by 4.0 mm. 



Color: In alcohol, carapace with a ground color of red; plications 

 red and white. Chelipeds with maroon blotches, the plications banded 

 with red and white ; fingers red with white tips. Ambulatoiy legs 

 banded with red and white. (Glassell) 



Ecology: Petrolisthes hians is commonly found in coral heads from 

 the littoral to a depth of about four fathoms. It occurs less frequently 

 under stones in the intertidal zone. A Hancock expedition recovered 

 it on one occasion from a sponge dredged in 1-4 fathoms at Acapulco, 

 Mexico. It was once dredged by the Velero III in 14-18 fathoms 

 from a sand and nullipore bottom off Isla Socorro; this depth must 

 be considered exceptional. 



Aside from the common coral and occasional sponge association, 

 the only reference to commensal ism for the species was by Steinbeck 

 and Ricketts (1941), who found it attached to a sea urchin, probably 

 Centrostephanus, in the Gulf of California. 



Ovigerous females have been collected in every month from De- 

 cember through June. 



Relationships: This species is most closely related to Petrolisthes 

 jugosus Streets, 1872, a western Atlantic form. P. jugosus differs in 



