226 BULLETIN 201, UNITED STATES NATIONAL MUSEUM 



is only 3-jointed instead of 4-jointed. M. columbiae also has a more 

 marked rostral plate with an acutely pointed apex. 



Genus DIAMYSIS Czerniavsky 



Diatnysis Czerniavsky, 1882a, pp. 57, 63 ; 1887, p. 84. 

 Pseudomysis Czerniavsky, 1882a, p. 132 (nee Sars, 1879). 

 Euxinomysis Czerniavsky, 1882a, pp. 58, 64, 132. 

 Potamomysis Czerniavsky, 1882a, pp. 58, 64, 129. 



In 1882 Czerniavsky founded the three genera Diamysis, Euxino- 

 mysis, and Potamomysis. In 1915 Zimmer united Diamysis and 

 Euxinomysis (1915a, p. 214), and in 1924 Dershavin (p. 117) showed 

 that Potamomysis should also be regarded as a synonym of Diamysis. 

 In the same year Martynov (1924, p. 210) independently concluded 

 that Euxinomysis and Potamomysis were synonymous. Diamysis, as 

 the first of the three names to be used, is the name which must be 

 applied to the genus. In 1878 Czerniavsky used the name Pseudo- 

 mysis, without definition or description, for specimens of Euxino- 

 mysis mecznikowi in the collection of the University Museum of 

 Zoology at Odessa. In 1879 (b) Sars instituted the genus Pseudo- 

 mysis for a very striking mysid captured by the Norwegian North 

 Polar Expedition, and in 1882 Czerniavsky substituted the name 

 Euxinomysis for his former Pseudomysis, regarding the latter name 

 as preoccupied by Sars. I have been unable to trace whether Czerni- 

 avsky actually published the name Pseudomysis in 1878 or whether 

 it existed only in manuscript. In any case, as a nomen nudum, it 

 can be disregarded. 



DIAMYSIS AMERICANA, new species 



Figure 97 



Description. — Body somewhat slender ; carapace hardly at all pro- 

 duced into a rostral plate, front margin evenly arcuate. 



Eyes of moderate size, total length of cornea and stalk not quite 

 twice the greatest breadth, cornea occupying about one-third of the 

 total eye, hardly broader than the stalk, pigment black. 



Antennular peduncle with the basal joint about equal to the second 

 and third joints combined, setose lobe in the male well developed. 



Antennal peduncle (fig. 97, a) shorter than the antennular, second 

 joint about one-quarter longer than the third. 



Antennal scale (fig. 97, a) nearly twice as long as its own peduncle 

 and extending beyond the antennular peduncle for one-sixth of its 

 length, narrow and lanceolate in shape, six times as long as broad, 

 setose on both margins, with a distinct distal transverse suture. 



