THE COPEPOD GENUS PARANTHESSIUS — ILLG 423 



The total length of the body, exclusive of the terminal setae of the 

 caudal rami, measures about 1.7 mm. 



Male: The metasome is not so inflated in outline as that of the 

 female. The segmentation (five joints) is the same in the two sexes. 

 As is usual, the urosome of the male is 6-segmented. The rostrum, 

 antennules, and cephalic mouth parts are not distinctly different in the 

 two sexes. The maxillipeds present the most conspicuous ornamenta- 

 tion among the species considered here (fig. 37, G). A large poste- 

 riorly and medially directed spinous process forms a prolongation of 

 the basal segment. The second segment bears a modified seta, recalling 

 that seen in P. columhiae. Unusually stout and niunerous spinules 

 ornament the medial margin of the second segment (several rows of 

 small spinules on the ventral surface of that segment are not depicted 

 in the illustration because of the confusion of detail that would result) . 



In the swimming legs there is the same development of spinous 

 processes on the endopodites of the first three legs as in the preceding 

 species. 



The fifth leg is much reduced by comparison to the female structure. 

 The basal portion of the free segment is constricted somewhat. No 

 preparation was obtained which preserved both the terminal setae. 

 That observed and figured (fig. 37, H) is the more posterior, and it is 

 a fairly stout and notably shortened spinelike outgrowth. 



The sixth legs are represented by the usual two setae. On the 

 flaplike extension of the body somite are borne two pairs of rows of 

 spinules (recalling P. columhiae). The more anterior row comprises 

 about 22 spinules on each side, the posterior row about 15 spinules on 

 each side. 



The abdomen, except for its characteristically male segmentation, 

 exhibits no other features differentiating it from that of the female. 

 The total length of the body is 1.2 mm. 



PARANTHESSIUS PERPLEXUS, new species 

 FlGTTRES 35, 37 



Specimens examined. — Two adult females, from gills of Saxidomus 

 nuttallii Conrad, in association with numerous specimens of P. saxi- 

 domi. Collected at Tomales Bay, Marin County, Calif., May 23, 1943, 

 P. L. Illg. 



The holotypic female has been designated U. S. N. M. No. 85347. 



Description. — Female: The body configuration is distinctive (fig. 

 35, K) . The 5-segmented metasome is not exceptional, its narrowly 

 oval contour being of the generalized cyclopoid f acies. The urosome 

 is of the usual five segments. The contour of the genital segment is 

 characteristic for the species. An ellipsoid process, with free rounded 



