268 University of California Publications in Zoology [Vol. 23 



Genus Opisthopus Rathbun 



Carapace usually firm and unyielding, smooth, subquadrilateral or suborbicular ; 

 regions not defined, lateral margins regularly arcuated. Outer maxillipeds with 

 ischium well developed, merus broad; palp three- join ted, last joint articulated 

 on the inner side of the preceding one. Ambulatory legs subequal in length, 

 joints flattened. 



Opisthopus transversus Rathbun 



Plate 44, figures 4 and 5 



Opisthopus transversus Eathbun, Proc. U. S. Nat. Mus., 16, 252, 1893; 

 Holmes, Occas. Papers Calif. Acad. Sei., 7, 97, 1900 ; Eathbun, H. A. E., 

 10, 188, text fig. 95, 1904; Weymouth, Stanford Univ. PubL, Univ. Ser., 

 no. 4, 61, text fig. 9, 1910; Eathbun, Bull. U. S. Nat. Mus., 97, 173, 

 text fig. 110, pi. 37, figs. 4, 5, 1918; Nininger, Jour. Ent. Zool., Pomona 

 Coll., 10, 36, fig. 8, 1918. 



Fig. 158. Opisthopus transversus, 5, dorsal view, X 1% (from Eathbun, U. S. 

 N. M.). 



Characters. — Carapace transverse, convex, thin but not soft and yielding; 

 angles rounded; front deflexed, almost straight when seen from above, with a 

 slight median sulcus; antennules well developed, lodged in deep, diagonal fossae. 

 Ambulatory legs similar in character; joints rather broad, except the dactyls, 

 which are curved and small, a little more than half the length of the propodal 

 joints; second pair of legs the longest, fourth pair the shortest, reaching midway 

 of the propodal joint of the preceding pair. Abdomen of male narrow at base, 

 tapering regularly from the third to the terminal joint or telson, which is sub- 

 quadrilateral; abdomen of female very wide and long, almost covering the maxili- 

 peds. The female may reach maturity while its abdomen is still as narrow as that 

 of the male. 



Dimensions. — Type, female: length of carapace 14 mm., width 18 mm.; male: 

 length of carapace 8.5 mm., width 9.8 mm. 



Color. — Carapace richly spotted with vermilion to deep red, polished almost 

 to pearly smoothness (Nininger). 



Type Locality. — Monterey, California. 



Distribution. — Monterey to San Diego, California (Eathbun). 



Bemarlcs. — Found in Lucapina crenulata, the large keyhole limpet; in siphon 

 of Pholas, the rock-boring mollusk; in mantle cavity of the mussel, Mytilus edulis; 

 and in the common holothurian, Stichopus calif ornicus (Eathbun). Mr. E. P. 

 Chace, of Los Angeles, recently took a female specimen from the gill chamber of 

 a univalve mollusk, Astraea undosa, and numerous others of both sexes from the 

 shells of Schizothaerus. 



