- 3 7 - 



exopod-bearing joint, unjointed or two-jointed and from 

 more than half as long as to somewhat longer than the 

 exopod ; seventh pair without endopod, while the exopod 

 is somewhat styliform, naked or with setae at the end. 

 Copulatory organs with the spine-shaped process thin and 

 curved as in Thysanopoda, the two other processes of the 

 inner lobe and the lateral process well developed but no 

 additional process. The other characters as in Nematoscelis. 



Remarks. — This genus agrees with Nematoscelis and 

 Stylocheiron and differs from Thysanopoda, Euphausia, 

 Pseudeuphausia and Nematobrachion by the antennular 

 peduncles in both sexes; in the female the two distal joints 

 are slender, the third thinner and generally longer than 

 the second and both without lobes or keels; in the male 

 these two joints show always sexual characters and most 

 frequently especially the third joint is shorter and thicker 

 than in the female, but sometimes second joint is more 

 altered. The sexual difference in the development of sixth 

 pair of legs is very interesting, but the same difference is 

 found in Nematoscelis and Stylocheiron. The genus is clo- 

 sely allied to Nematoscelis, especially to its group A (see 

 below), the main difference being the structure of the first • 

 pair of legs ; some difference is also found in the copula- 

 tory organs, and the females of Thysanoe'ssa do not carry 

 their eggs. 



The geographical distribution of this genus is interest- 

 ing. While every other genus of the order comprising 

 more than a single species has its richest development in 

 the tropical, or (Nyctiphanes) the temperate, areas of the 

 oceans and has no or at most (Euphausia) relatively few 

 representatives in cold seas, the large genus Thysanoe'ssa 

 is essentially found in cold water, living in the Arctic and 

 Antarctic and the colder adjacent transverse belts of the 

 Atlantic and the Pacific, while not a single specimen of 

 the genus has been found in the tropical belt around the 

 globe. 



The following key gives a view of the 8 species hitherto 



(210) 



