420 PROCEEDINGS OF THE NATIONAL MUSEUM vol. ee 



The margins of the free segment of tlie fifth leg are parallel. 

 Both terminal outgrowths are setif orm ; of these, the posterior is much 

 the shorter (fig. 35, J). The sixth legs are represented only by two 

 setae. The ornamentation of the last thoracic segment is a pair 

 of rows of spinules, about thirty spinules comprising the row on 

 either side. The heavily spinous serrations of the posterior margins 

 of the abdominal somites are like those in the female. There is no 

 feature of the caudal rami exhibiting any considerable dimorphism. 

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

 rami, is about 1.1 mm. 



Immature stage: Female copepodid of the fifth stadium. The 

 metasome presents no point of difference in its contours or its com- 

 plement of appendages from the condition of the adult female, ex- 

 cept its slightly smaller size. The urosome is very different from 

 the adult condition. The five segments comprise the last two tho- 

 racic segments and three abdominal segments. Of these latter, the 

 terminal segment is the longest. This proportion anticipates the 

 final molt in which the first abdominal segment becomes fused to the 

 last thoracic segment and a subdivision of the terminal somite re- 

 stores the number of free abdominal segments to three. The two 

 anterior abdominal segments present have the coarse serration of 

 the posterior margins which is characteristic of the adults of the 

 species. The caudal rami are not so long in proportion to width 

 as is the case in the adult, but the general aspect of arrangement of 

 the ornamentation shows no significant difference. 



The fifth legs are just like those of the adult male. This is a strong 

 contrast to the condition in P. columhiae, where in the fifth copepodid 

 this appendage is recognizable as of the female type. The proportion 

 of the length to width of the free segment of the fifth leg is about that 

 seen in the male. The more posterior (medial) of the terminal setae 

 is much shorter than the anterior (lateral) seta. The terminal tho- 

 racic segment bears a small seta at each posterior lateral corner of 

 the ventral side, representing the reduced sixth leg. An additional 

 ornamentation of the segment is a pair of rows of fine, short spinules 

 extending across the ventral surface parallel with and close to the 

 posterior ventral margin of the segment. 



The total length of the body, exclusive of the caudal setae, is 1 mm. 



PARANTHESSIUS SAXIDOMI, new species 



Figure 37 



Specimens examined. — Six adult females, three adult males, twenty- 

 two copepodids of the fourth and fifth stadia obtained from several 

 specimens of Saxidomus nuttallii Conrad, collected in Tomales Bay, 

 Marin County, Calif., May 23, 1943, P. L. 11 Ig. 



