the Copepod Genus Olthona. 499 



genus seems to be in great measure influenced by the salinity 

 of the water which they inhabit, and, consequently, their 

 correct determination a matter of importance when they occur 

 in collections of plankton made to show the correlation of 

 biological and hydrographical facts, it seems advisable to call 

 attention to the matter in a preliminary note. 



The history of that section of the genus Oithona to which 

 0. plumifera and 0. setigera belong, characterized by a long 

 anteriorly directed rostrum visible in dorsal view, is briefly as 

 follows: — In 1843 Baiid* briefly described and named as 

 Oithona plnmifera a copepod taken at the surface in lat. 3° 24' 

 N., long. 22° 7' W. The description was accompanied by a 

 rude figure of the animal in dorsal view. The only points 

 which can guide us to a discovery of what the animal really 

 was were tlie general form of the body, the length, shown as 

 a line 2 mm. long, tlie length of the tirst antennse, almost 

 equal to the body, and the presence of four plumose setae on 

 either side " attached to the sides of the insect " (in reality 

 they are attached to the second basal joints of the swimming- 

 feet) . 



In 1847 Dana described as xSmJeZ/ascWi^a a specimen taken 

 by the United States Exploring Expedition ; but in 1852, iu 

 a further account of the same collection, he withdrew the 

 name and referred the specimen to 0. plumifera^ Baird. He 

 gave at tlie same time a short description and a few figures of 

 0. plumifera, and also briefly described and figured a second 

 species, 0. setigera, characterized by the setae on tlie second 

 basal joints of the swimming-feet being clavate at their 

 extremities instead of being plumose. Olaus, in 18G3 and 

 1866, described and figured, from Messina and Nizza, 

 0. spinirosfris, which has been gen ei-ally regarded as synony- 

 mous with 0. jjlnmi/era, and which I shall refer to below, 

 • In 1864 Boeck f described from the Christiania Fiord, under 

 the name of 0. spinifrons, a species which several authors 

 (V. Breemen, G. O. Sars) believe to be identical with 

 0. jy/ww«yera, but which Gierfbrecht refers, with a query, to 

 O. similis, Glaus. 



In the ' Challenger ' Eeports, 1883, Brady referred all the 

 specimens of Oithona which he met with to a new species, 

 (J. challengeri. From the figures which he gives it appears 

 to resemble 0. setigera very closely, as the third joint of the 

 exopodite of the first foot bears three outer-edge spines, the 



* ' Zoologist; vol. i. (1843). 



t Vid. Selsk. Forhaudl. Christiania. I only know this paper through 

 Oiesbrecht's summary iu Wiss. Unt. Deutscheu Meere (Kiel, 1882). 



