THE COPEPOD GENUS PARANTHESSIUS — ILLG 417 



fourth endopodite, this tendency to the production of spinous proc- 

 esses is less extreme, the appropriate corner in this appendage being 

 produced only into a short, thornlike denticle of small dimensions. 

 Somewhat similar, small, thornlike denticles occur on the lateral mar- 

 gin of the distal segment of this ramus, one placed just proximal to 

 the base of each of the two more proximal spines. A further slight 

 deviation in this podomere from the female condition is seen in the 

 relatively greater length and slenderness of its spines. 



The usual dimorphism of the lichomolgid fifth leg is seen in this 

 species. The free segment is shorter and narrower than that of the 

 female (fig. 36, G). The terminal armature is strikingly different, 

 the setae being subequal in proportions with the lateral one slightly 

 the longer. 



The ornamentation of the seventh thoracic segment is typical for 

 the genus (fig, 35, G). The pair of rows of spinules exhibit about 18 

 spinules on each side. The armature of the sixth leg comprises two 

 subequal setae. The caudal rami differ in no essential from those of 

 the female. The total length of the body is about 1.8 mm. 



Immature stage : Female of the fifth copepodid stadium. A single 

 specimen of this stage was available and it w^as not subjected to de- 

 tailed study, as it presented the diagnostic features of the species, 

 slightly modified by the characteristics of the developmental state, 

 as has been described here in detail in the cases of P. columhiae and 

 P. tivelae. The length of the specimen is 1.6 mm. 



PARANTHESSIUS TIVELAE, new species 



Figures 35, 36 



Specimens examined. — Nine adult females, one adult male, thirteen 

 immature specimens obtained from a single specimen of Tivela stul- 

 torwnn (Mawe), collected at La Selva Beach, Monterey Bay, Monterey 

 County, Calif., June 13, 1943, P. L. Illg. 



Types have been assigned the following United States National 

 Museum catalog numbers: Holotypic female, 85346; allotypic male, 

 85345. 



Description. — Female : The proportions of the metasome conform 

 fairly closely to the oval configuration of the typical cyclopoid (fig. 

 35, H). The cephalothorax includes the segment of the maxillipeds; 

 the somites of the four swimming legs comprise the remainder of the 

 5-segmented metasome. The lateral portions of the segments of the 

 swimming legs are produced posteriorly as rounded lobes. 



The urosome is of the usual five segments. Distinctive of the 

 species are the coarsely serrate posterior margins of the abdominal 

 segments, with the exception of the terminal segment. These serra- 



