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PROCEEDINGS OF THE NATIONAL MUSEUM 



Probably none of the three explanations given above can by itself 

 account for the absence of P. abyssorum from the Pacific Ocean. A 

 combination of them may, however, be effective, the shallowness of the 

 Bering Strait and the northward set of the currents preventing all but 

 a small number from entering the Bering Sea, while competition with 

 P. pacifica prevents those that do enter from establishing themselves. 



Table 2. — Depth of the water and occurrence of Parathemisto abyssorum in the 

 Beaufort and Chukchi Seas, 1950 and 1951 Burton Island Cruises combined. 



Subgenus Euthernisto Bovallius 



Dorsal spines present or absent. Flagellum of female antenna 1 

 curved, heavier than in subgenus Parathemisto (except in P. (E.) libel- 

 lula). Carpus of P3-4 usually more expanded than in subgenus 

 Parathemisto. P5 much longer than P6-7 (except in some females 

 of P. (E.) gracilipes). Peduncle of uropod 3 only slightly produced 

 at inner distal end. 



Key to the Species of the Subgenus Euthernisto 



(Modified from Barnard, 1930) 



1. Female antenna 1 slender, straight. Dactyls of P5-7 pectinate at base. 



libellula Lichtenstein 

 Female antenna 1 stouter, hooked. Dactyls of P5-7 unarmed at base . . 2 



2. Inner ramus of uropod 3 serrate only on outer margin. 



gaudichaudii (Gu£rin) 

 Inner ramus of uropod 3 serrate on both margins 3 



3. Inner margin of uropod 3 peduncle smooth throughout. 



gracilipes (Norman) 

 Inner margin of uropod 3 peduncle distally serrulate. 



australis (Stebbing) 



Hurley (1955) discusses in detail the difficulties in distinguishing 

 between australis, gracilipes, and gaudichaudii in collections from New 

 Zealand waters. The three species show clear differences in general 

 appearances, but these are very difficult to set down in words. Hurley 

 notes that P. australis is shorter and stockier than the other species, 



