268 University of California Publications in Zoology [Vou. 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-jointed, 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 Rathbun, Proe. U. S. Nat. Mus., 16, 252, 1893; 
Holmes, Occas. Papers Calif. Acad. Sei., 7, 97, 1900; Rathbun, H. A. E., 
10, 188, text fig. 95, 1904; Weymouth, Stanford Univ. Publ., Univ. Ser., 
no. 4, 61, text fig. 9, 1910; Rathbun, Bull. U. S. Nat. Mus., 97, 173, 
text fig. 110, pl. 37, figs. 4, 5, 1918; Nininger, Jour. Ent. Zool., Pomona 
Coll., 10, 36, fig. 8, 1918. 
Fig. 158. Opisthopus transversus, 2, dorsal view, X 11% (from Rathbun, U. 8. 
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 suleus; 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 (Rathbun). 
Remarks.—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 californicus (Rathbun). Mr. E. P. 
Chace, of Los Angeles, recently took a female specimen from the gill chamber of 
a univalve mollusk, Astraea wndosa, and numerous others of both sexes from the 
shells of Schizothaerus. 
