48 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 80 



Specific characters of mole. — Body proportionally wider than in 

 the female ; length of anterior body to width as 16 to 9 (as 9 to 4 in 

 the female) ; greatest width at posterior margin of cephalothorax, 

 while in the female it is a short distance in front of that margin. 

 Second segment as wide as the cephalothorax, third segment nar- 

 rowed a little, fourth segment abruptly contracted to three-fifths of 

 the width of the third segment; fifth segment nearly circular in 

 outline, fifth legs longer than in the female. Genital segment ovate, 

 armed on the dorsal surface at each posterior corner with a long 

 seta. Abdomen of four segments, all the same width; caudal rami 

 considerably longer than the anal segment and three times as long 

 as wide. 



First antennae apparently 14-jointed, but with the jointing of the 

 basal portion very indistinct; terminal portion made up of two 

 joints of about the same length. The basal portion of each antenna 

 is bent near its center, and the joint at the flexure is armed at its 

 distal posterior corner with a small spine inside of which is a rather 

 long seta. The second antennae and mouth parts are like those of the 

 female. 



Measurements. — Female: Total length, 0.6 mm.; length of an- 

 terior body, 0.33 mm. ; greatest width, 0.15 mm. ; length of posterior 

 bc^y, 0.27 mm. Male: Total length, 0.55 mm.; length of anterior 

 bod3% 0.32 mm. ; greatest width, 0.18 mm. ; length of posterior body, 

 0.23 mm. 



Occurrence. — Taken at every station in the bay except U, abundant 

 everywhere and sometimes constituting 50 per cent or more of the 

 total catch. More abundant in the inner bay than in the outer, al- 

 though the two largest single hauls of the species were both made 

 in the outer bay. 



Distribution. — Western Pacific near Hong Kong (Giesbrecht) ; 

 Mediterranean (Pesta, Giesbrecht ) ; Atlantic Ocean, latitude 6° S. 

 (Cleve) ; Woods Hole (Fish). 



Remarks. — The peculiar crossing of the long setae on the caudal 

 rami of the female is a notable characteristic of this species. It is 

 one of the smallest copepods in the bay, but also next to the two 

 Acartias it is the most widely distributed. Hence its regular occur- 

 rence and comparative abundance more than offset its diminutive 

 size and make it of great economic importance in the plankton. It 

 was not found by Esterly on the California coast, but its place was 

 taken by Oithona nana, another of Giesbrecht's species. A com- 

 parison of specimens taken at Woods Hole in the summer of 1923 

 with these from Chesapeake Bay shows that the two are the same 

 species. Fish has recorded it as a summer form, while in Chesapeake 

 Bay it was found throughout the entire bay during both summer and 



