85 



outer ramus of the posterior antennae, the much less powerful structure of the 

 posterior maxillipecls, the presence of only a single spine outside the terminal 

 joint of the outer ramus in the 1st pair of legs, and finally, the somewhat different 

 appearance of the last pair of legs in the female. The genus comprises a very 

 great number of species from all parts of the world, amounting to about 80 in all. 

 Some of these species are rather anomalous, and it is therefore very probable, 

 that in future it will be found advisable to subdivide this genus into several 

 distinct genera. The determination of the species is connected with no small 

 difficulty, owing to the imperfect manner in which they have generally been 

 illustrated, most authors having contented themselves with only figuring some ana- 

 tomical details of each species, generally the last pair of legs in the two sexes. 

 To the fauna of Norway belong 7 species, to be described below. Besides some 

 detail-figures, I have given carefully drawn habitus-figures of both sexes of all 

 these, hoping thus to make the Norwegian species at least easily determinable 

 for students. 



34. Diaptomus Castor (Jurine). 



(PI. LVII, LVIII). 



Monoculus castor, Jnvine. Histoire des Monocles qui se trouvent aux environs de Geneve, 

 p. 50, PL IV, figs. 1, 6, PL V, figs. 1, 5, PL VI, figs. 1, 17. 



Syn: Glaucea rubens, Koch. 



Specific Characters. Female. Body somewhat robust, with the anterior 

 division, seen, dorsally, oblong oval in outline, greatest width somewhat in front 

 of the middle, and considerably exceeding y 3 of the length, anterior extremity 

 obtusely rounded, posterior but slightly attenuated. Cervical depression very 

 conspicuous. Lateral expansions of last pedigerous segment not very large, and 

 divided into 2 acute lappets. Urosome distinctly 3-articulate, middle segment well 

 defined, genital segment somewhat asymmetrical and considerably dilated, forming 

 on each side B triangular expansion tipped with a short denticle, the right one 

 somewhat larger than the. left, and placed more posteriorly. Caudal rami com- 

 paratively short, about twice as long as they are broad. Anterior antennae 

 unusually short, when reflexed reaching but slightly beyond the anterior division 

 of the body. Posterior maxillipeds more robust than in most other species, 2nd 

 basal joint rather broad, terminal part much shorter than this joint, and having 

 the anteriorly-curving setae partly spiniform. Last pair of legs with a conspicuous 

 triangular projection on the hind face of the 1st basal joint, inner ramus some- 

 what shorter than the 1st joint of the outer, and distinctly biarticulate. carrying 



