COPEPODA FROM CHRISTMAS ISLAND. 191 



5th foot very small with a short terminal and a basal seta. 



Four specimens were found in one gathering. 



This species seems to agree in appearance with Oithona hehes, 

 but differs in the jointing and spinulation of the swimming-feet. 

 In structure it is separated from Paroithona parvula, which it 

 closely resembles, by the reduction in number of the setse on the 

 endopodites of the swimming-feet. Not much reliance can be 

 put on tJae presence or absence of setee on the basal joints of 

 the swimming-feet, as they may have been overlooked in the 

 descriptions of both species. 



The following is an attempt to draAv up a diagnostic table of 

 the species of Oithona and Paroithona of which a recognisable 

 description exists, which will assist in the identification of the 

 species described above. The number following each name refers 

 to the paper in which it was first described. 



1. Rostrum present. 



A. Rostrum antero-ventrally directed, visible in dorsal 



view. 



a. Exop. of 1st foot witli 1.1.2 outer edge spines. 

 aa. 2ud basals of swimming-feet with plumose setae ; 



leiifrth 1'0-1"5 mm O. plumifera (1). 



bb. 2ndbasals with smooth setaj; length 1'0-1'16 mm, O. atlantica (11). 



b. Exop. of 1st foot with 1.1.3 outer edge spines. 



aa. Exop. of 4th foot with 0.0.1 outer edge spines ; 



length r2.5-l'35 mm O.frigida (15). 



bb. Exop. of 4th foot with 1.1.3 outer edge spines ; 



length 'GS-'Tl mm 0. vivida. 



cc. Exop. of 4tli foot with 0.0.2 outer edge spines ; 



length 1'2-1'9 mm O. setigera (8). 



B. Rostrum ventrally directed, not visible in dorsal view. 



a. Exop. of 1st foot with 1.0.2 outer edge spines ; 



length •56-"62 ram O. decipiens. 



b. Exop. of 1st foot with 1.1.2 outer edge spines. 

 aa. Exop. of 2nd foot with 1.0.1 outer edge 



spines; length "yS-'S mm O. similis (6). 



bb. Exop. of 2nd foot with 1.0.2 outer edge 



spines; length ■88-94 mm O.fallax. 



c. Exop. of 1st foot with 1.1.3 outer edge spines. 

 aa. Exop. of 4th foot with 1.1.2 outer edge spines. 



a. Rostrum long, slender, straight ; length 



1-65 mm O. rohusta (13). 



j3. Rostrum short, curved ; length "7 mm O. brevicornis {\Q). 



bb. Exop. of 4th foot with 0.0.1 outer edge 



spines; length 1"07-1'12 mm. O. Iinea7'is {12). 



2. Rostrum absent. 



A. Endop. of 2nd-4th feet three-jointed. 

 a. 5th foot with one terminal seta. 



aa. 4th foot with 1.1.3 outer edge spines ; length 



•38-42 mm O. simplex. 



bb. 4th foot with 1.1.2 outer edge spines. 



a. Cephalothorax much longer than abdomen ; 



length 'So mm O. hebes (13). 



ji. Cephalothorax slightly longer than abdomen ; 



length ■oO-"o3 mm O. nana (13). 



y. Cephalothorax shorter than abdomen ; length 



'88 mm 0. attenuata . 



