8 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



additional setae; esthete more than 3 times as long as appendage. 

 2nd antenna 3-merous; 3rd segment with long terminal seta and shorter 

 setae near proximal end. 1st maxilla like that of female, with 2 

 processes. 2nd maxilla 3-merous; 1st segment with 3 rows of fine 

 serrations; 2nd segment with a few serrations distally; 3rd segment 

 with denticulate inner margin, apex like that of female. Maxilliped 

 4-merous; 1st segment longer than combined remaining segments, 

 with longitudinal row of serrations. 



Swimming legs 2 pairs, identical. Exopod with 3 outer spines, 2 

 terminal setae, and 2 inner setae. Endopod lateral margin with a 

 row of spiniform teeth and a subterminal spine; medial margin with 

 6 plumose setae. 



Types. — Holotype female, length .33 mm, from marsupium of 

 adult female Parasterope pollex, length 1.55 mm, station 1907, 

 Hadley Harbor, Woods Hole, Mass., USNM 119122. The other 

 specimens listed on pages 3-4 are paratypes. 



Etymology. — The specific name "monothrix," from the Greek 

 "mono" = one and "thrix"=hair, refers to the single apical seta of the 

 trunk legs and caudal rami. 



Relationships. — We have placed the new species in the genus 

 Sphaeronellopsis in spite of its lacking the following characters that 

 Hansen (1905) considered to be among the most distinctive for the 

 genus (Hansen had only females) : caudal rami fused, genital area 

 with broad anterior protuberance, seminal receptacles long and 

 strongly curved. Because of the absence of these characters, a 

 reasonable case could be made for assigning S. monothrix to Sphaero- 

 nella rather than to Sphaeronellopsis; however, the new species does 

 agree with Sphaeronellopsis littoralis, until now the only species of 

 the genus, in having a 2-merous 1st antenna, no 2nd antenna, a 

 3-merous maxilliped with a short terminal segment, and single 

 terminal setae on the trunk legs and caudal rami. The factor that 

 induced us to place our species in Sphaeronellopsis rather than in 

 Sphaeronella is that both S. littoralis and S. monothrix are parasites 

 of ostracods. Even if S. monothrix were placed in Sphaeronella 

 now, it would probably be transferred to Sphaeronellopsis later, since 

 the species of Sphaeronella are quite diverse, and eventually it will 

 probably be necessary to subdivide the genus. Hansen (1898) may 

 have anticipated such a subdivision, for he divided the species of 

 Sphaeronella according to their morphology into 2 main groups, one 

 made up of parasites of amphipods, the other consisting of parasites of 

 cumaceans. 



We have recently described (Bowman and Kornicker, in press) a 

 choniostomatid from the ostracod Pseudophilomedes ferulana Korn- 

 icker. Only copepodids were found, and their characters agree with 



