North American Species of Diaptomus. 107 



forceps. All the transitions between these two forms may be 

 found in a series of species. Certain articles of the right 

 ramus may also bear divers characteristic appendages. 

 Finally, the terminal claw and the lateral spine of the last 

 article often present by their form and their position enough 

 peculiarities to greatly facilitate the determination. 



" The right antenna of the male also furnishes some good 

 characters, among which figure in the first rank the append- 

 ages of the antepenultimate article, much varied as to form 

 and size (I>. bacillifer, wierzejskii, coeruleus). 



" Generally speaking, the fifth pair of feet of the females 

 furnish the most important specific characters, although they 

 do not have the same technical value as in the male. At the 

 same time various peculiarities drawn from the conformation 

 of the last cephalothoracic and of the first abdominal seg- 

 ments and from the length of the antennae, aid greatly in the 

 determination." 



.KEY TO THE NORTH AMERICAN SPECIES OP DiaptomUS, BASED ON 

 THE CHARACTERISTICS OF THE MALE. 



1 (15). Antepenultimate article of right antenna without 



hook-like process at tip. 



2 (3). Antepenultimate article of right antenna with narrow 



hyaline lamina. Second basal segment of right fifth 

 foot armed at the inner margin with two hook-like proc- 

 esses, and on the anterior surface at the apical 

 margin, with a hook-like process extending beyond the 

 middle of the first segment of the outer ramus. Inner 

 ramus extending barely beyond the middle of the first 

 segment of the outer ramus ; heavily spined at apex. 

 Marginal spine below the middle of the segment, 

 near the apical angle ; short, stout, much less than 

 half as long as the segment. Terminal hook very 

 stout, longer than preceding segment. Left leg ex- 

 tending about to end of first segment of outer ramus 

 of right leg ; second basal segment tuberculate on 

 inner margin. Inner ramus very long, extending about 

 to middle of last segment of outer ramus ; incurved, 



