PROC. BIOL. SOC. WASH. 101(4), 1988, pp. 832-837 DESCRIPTION OF MEMBRANOBALANUS ROBINAE, A NEW SPECIES OF SPONGE BARNACLE (CIRRIPEDIA, ARCHEOBALANIDAE) FROM BAJA CALIFORNIA, WITH A KEY TO THE GENUS Robert J. Van Syoc Abstract. —The California Academy of Sciences, Department of Invertebrate Zoology and Geology has recently undertaken a joint research program with the Centro de Investigaciones de Ciencia y de Educacion Superior de Ensenada (CICESE). The present paper documents a new species of the sponge barnacle Membranobalanus Pilsbry from Bahia de los Angeles collected during two expeditions in 1984. A key to the genus Membranobalanus is included. Systematic Account Subclass Cirripedia Burmeister, 1834 Order Thoracica Darwin, 1854 Suborder Balanomorpha Pilsbry, 1916 Superfamily Balanoidea (Leach) Newman ifeRoss, 1976 Family Archaeobalanidae Newman & Ross, 1976 Subfamily Archaeobalaninae Newman & Ross, 1976 Ge,rms Membranobalanus Vilsbry, 1916 Membranobalanus robinae, new species //o/o/y/?^. — Complete shell, opercular plates and body preserved in 75% EtOH, California Academy of Sciences (CAS), San Francisco, 061082. Dimensions ofholotype. —Height 4.6 mm; carinorostral diameter 4.0 mm; lateral di-ameter 3.7 mm. Type locality.— ^\2Mon BLA-1 1, 28°54'N, 1 13°30'W, Mexico, Baja California, Gulf of California, Bahia de los Angeles, Vi mile south of Casa Diaz, depth 10 feet, 6 Oct 1984. Material examined. —CAS paratypes 061083, 061084, 061086, one specimen each, and CAS 056184, about 100 speci-mens, Mexico, Baja California, Gulf of Cal-ifornia, Bahia de los Angeles, Punta Gringa, depth 10-15 feet. CAS paratype 061085, Mexico, Baja California, Gulf of California, Bahia de los Angeles, Vi mile south of Casa Diaz, depth 0-10 feet. Paratype distribution. —Faratypes have been deposited at the National Museum of Natural History (USNM 211487), Scripps Institution of Oceanography (SIO C9559), the Santa Barbara Museum of Natural His-tory (SBMNH 35076), the Los Angeles County Museum of Natural History (LACM 84-203.1), as well as the California Acad-emy of Sciences (as listed above). Diagnosis. —SheW thin, white, radii pres-ent, widening near orifice; orifice oval, sum-mits of wall plates nearly level, not deeply toothed; rostrum same length as other com-partmental plates or only slightly longer; base of wall plates rounded giving base a lobed appearance; opercular plates without chitinous lamellae; scutal adductor ridge in-cipient or lacking; tergal spur broad, ex-tending about % of basal margin, and short, only about V^ of basal margin width; tergal articular ridge gently sloped to sharp crest, bending V2 way between apex and basal mar-gin toward and descending to scutal margin then rising along margin to high point near basal margin; outer ramus of Cirrus IV with up to 6 recurved teeth per article; basal ar-ticles of Cirrus IV with several tooth-like spines at distal posterior margin.