500 Mr. G. P. Favran on 



first maxilla lias a moderate seta on the endopodite and the 

 mandible four setae on the endopodite. The figure of the 

 mandible (pi. xl. fig. 12) is wrongly referred to Zaun goodsiri 

 both in the explanation of the plate and the heading to the 

 species in the text, and there is no reference to it in the 

 description of 0. challengeri. There can be no doubt, how- 

 ever, that it really belongs to 0. challengeri, as the mandible 

 of Z. goodsiri is of a quite different form. 



Finally, Giesbrecht, in 1892*, redescribed and figured, for 

 the first time in sufficient detail, both 0. plumifera and 

 0. setigera, the former of which he had met with in collections 

 from the Mediterranean and from the Atlantic, Pacific, and 

 Indian Oceans, and the latter from the Pacific. 



He relied for his identification on the presence of plumose 

 or clavate outer-edge setaj on the second basal of the swim- 

 ming-feet of the respective species. 



The following is a brief description of the two species 

 which, in addition to 0. similis and 0. nana, occur off the 

 west coast of Ireland. The allusions to 0. plumifera and 

 O. setigera must be taken as referring to Giesbrecht's 

 description of those species. 



Oithona atlantica, sp. n. 



Female. — Length 1'0-1'lG mm. 



Oephalothorax of the same form as in 0. plumifera, the 

 rostrum being as in that species in dorsal and lateral view. 

 Abdomen five-jointed, the proportional length of the joints 

 and the furca being 8 : 20 : 10 : 9 : 11 : 7. Furcal setoe as in 

 (). plumifera. 



First antenna reaching almost to the end of the body. 



Second antenna as in 0. plumifera. 



Mandible as in 0. plumifera, except that the endopodite 

 bears four subequal setae instead of three. 



First maxilla as in 0. plumifera, except that the endopodite 

 bears a seta about three times as long as itself. The second 

 inner lobe is only just indicated and bears no seta. 



Second maxilla and maxillipede as in 0. plumifera. 



First foot as in 0. plumifera, with two outer-edge spines 

 on the third joint of the exopodite ; the outer-edge seta of 

 the second basal is, however, more slender and apparently not 

 feathered. 



Second and third feet without an inner-edge seta on the 

 first basal joint ; the second basal joint has a very slender 



* » Fauna u. Flora Golf. Neapel ' (1892). 



