502 



On the Copepol Genus Oithona. 



surface, but the exact depth at which it occurred could not 

 be ascertained. 



It will be seen from the above descriptions that 0. setigera 

 and 0. pelagica are at once separable from 0. plumifera and 

 0. atlantica by the possession of three outer-edge spines on 

 the third joint of the exopodite of the first foot, the two latter 

 species possessing only two such spines. The differences 

 between the members of each group are shown in the following 

 table : — 



O. plumifera. 



Length 1 '0-1*6 mm. 



Endopcdite of mandible with 

 three setae. 



Endopodite of first maxilla with 

 minute Beta. 



Outer-edge seta on second basal 

 of first to fourth feet very long, 

 strong, feathered on first to third, 

 smooth on fourth. 



Short Bmooth .-eta on inner edge 

 of second basal of second to fourth 

 feet. 



O. setigera. 



Length 1*5-1*6 mm. 



Outer-edge seta on second basal 

 of first to fourth feet stout, clavate 

 at end. 



O. atlantica. 



Length 10-1-16 mm. 



Endopodite of mandible with 

 four setae. 



Endopodite of first maxilla with 

 moderate seta. 



Outer-edge seta on second basal 

 of first to third feet very slender, 

 smooth, absent on fourth foot. 



Xo seta on inner edge of second 

 basal of second to fourth feet. 



0. pelagica. 



Length 1*36-1*52 mm. 



Outer-edge seta on second basal 

 of first to fourth feet very slender, 

 tapered. 



Thus, while 0. plumifera and 0. atlantica are separated by- 

 several small points of difference, the distinction between 

 0. setigera and 0. pelagica lies only in the presence of clavate 

 or tapered setas on the basals of the swimming-feet. Possibly 

 this distinction may be regarded by some as insignificant ; but 

 until it is shown that the two varieties of setse can occur in 

 specimens from all localities, it ought not to be disregarded. 



It is very probable that both 0. atlantica and 0. pelagica 

 have been already described, but it is difficult to discover 

 under what name. Boeck, in his description of 0. spini- 

 frons, does not mention any point which is not common to 

 both species, and his name must accordingly lapse for 

 uncertainty. Claus, in describing 0. spinirostris, does not 

 mention the number of outer-edge spines on the exopodite of 

 the first foot, but in his figure of the first maxilla he shows a 

 moderately long seta on the endopodite and a seta on the 

 second inner lobe. This last character makes it probable that 

 he refers either to 0. setigera or 0. pelagica (if either of 



