122 PROCEEDINGS OF THE; NATIONATj MUSEUM vol. loi 



Pleon. — Composed of a single somite. Each lateral border with 

 two large, posterolaterally directed, angulate, projections; posterior 

 border produced, apex blunt. Two large dorsally directed spines 

 present on dorsal surface near anterior end. 



First antenna. — Distal joint with few olfactory setae. 



Second antenna. — All segments lacking spines or tubercles. Flag- 

 ellum with three segments, last segment with a large apical claw. 



Maxilliped. — With two coupling hooks, palp with five articles. 



First maxilla. — Outer lobe with 11 apical setae. Inner lobe with 

 three apical setae, two of which bifurcate at tip. 



Second maxilla. — Each of the two outer lobe lappets with three 

 ciliated setae at apex. Inner lobe apex with about nine small simple 

 setae and two large ciliated setae, margin with three large ciliated 

 setae. 



Mandible. — Palp lacking. Left mandibular molar process with 

 smooth, truncate cutting edge, incisor process with three teeth, lacina 

 with two teeth, setal row with two setae. Eight mandible incisor with 

 three teeth, setal row with one large bidentate lacinoid seta and a 

 smaller seta, molar process with inferior cutting edge finely serrate, 

 lacina lacking. 



First peraeo'pod. — Contained in buccal area, probably functions as 

 a maxilliped. Distal segment lacking terminal claw, propodus with 

 many serrate setae on medial surface, carpus with serrate setae on 

 inferior margain. 



Second to fourth peraeopods. — All similar, fourth largest ; segments 

 furnished with long ciliated setae, distal joint a simple claw. 



Fifth to seventh peraeopods. — ^All ambulatory in structure; most 

 segments with a fine pubescentlike covering of pectinate scales. 



Uropod. — Concealed ramus with two long ciliated terminal setae 

 which extend posteriorly to tip of exposed ramus. 



Marsupial plates. — Four pairs present ; those of second pair largest. 



Pleopods. — First pleopods of female with five plumose setae at 

 distal end of each distal branch. 



Types.— B.o\o\j])Q (U.S.N.M. No. 87477) and paratype (U.S.N.M. 

 No. 87478) collected at Tomales Bluff, Tomales Point, Marin County, 

 Calif., south of reef, laminarian zone, on hydroid, August 5, 1948, by 

 Joel W. Hedgpeth. 



Geographical distribution. — Known only from the type locality. 



Material examined. — Holotype and 1 female paratype. 



Remarks. — This species differs from Idarcturus platysoma., Barnard, 

 the genotype of Idarcturus, in having large body spines, which the 

 genotype lacks. 



