B. Rhoda and Thysanoëssa. 



According to the general conception the genus Rhoda 

 Sim. (Boreophausia G.O.S.), comprising only two species, 

 has the eyes circular and the anterior pairs of thoracic legs 

 quite similar to each other, slender and with thin setae 

 from near the base to the end, while the old genus Thysa- 

 noëssa Brandt is quite dissimilar in aspect, having the 

 eyes higher than broad, divided by a transverse constric- 

 tion into two areas, and the first pair of legs very different 

 from the second, conspicuously stronger and much longer, 

 with fourth and fifth joints elongate and partly naked, 

 sixth and the very short seventh joint furnished with 

 rather strong or even nearly spiniform setae. Holt and 

 Tattersall have even referred Rhoda to their subfamily 

 Euphausinœ, Thysanoëssa to the Nematoscelince. It shall 

 now be show T n that Rhoda inermis Kr. and Thysanoëssa 

 neglecta Kr. (= T. borealis G. O. S.) are variations of the 

 same species. 



Rhoda inermis Kr. is common in the most northern 

 temperate and the boreal Atlantic, in the adjacent area of 

 the Arctic Ocean and not uncommon in the most northern 

 Pacific. The eyes of the adults are never really circular, 

 but feebly narrowed upwards and slightly higher than 

 broad, ovate, and a division into an upper small and a 

 lower large area is sometimes discernible; in half-grown 

 specimens the eyes are somewhat higher than broad and 

 generally distinctly divided into two areas. Two excellent 

 specific characters are well known ; that the carapace has 

 no denticle on its lower margins, while the sixth abdo- 

 minal segment has a well developed spine above at the 

 end. (In nearly all specimens from the North Atlantic the 

 fifth abdominal segment has no dorsal denticle, but among 

 several adult specimens taken off Cape Cod one specimen 

 has such a denticle very small and in another it is well 

 developed ; this denticle seems to exist in all specimens 

 from the North Pacific). 



