North American Species of Diaptomus. 101 



evident which if found to be generally true will necessitate a 

 slight modification of the description of the family Centro- 

 pagidce. I refer to the presence of a sense-club on the first 

 segment of the right male antenna. In his monograph 

 Giesbrecht in his description ('92, p. 85), says, "Vordere 

 antennen almlich wie bei den Calaniden gebaut." On another 

 page (42) we find this statement: "The normal number of 

 processes seems to be three for each segment, a proximal 

 seta, a distal seta, and a sensory structure [asthetask], but 

 this triad is never complete on all segments, the sense-club 

 on the first segment being always wanting and the sense-club 

 and proximal seta nearly always absent on the twentieth to 

 the twenty-fourth." In Diaj)tomus stagnalis and D. clavipes 

 a sense-club is present on the first segment. The statement 

 that the inner rami of the fifth pair of feet are "rudimentary, 

 one-segmented, or lacking" will not hold in many species. 

 Taking only those forms among non-American species which 

 were described in de Guerne and Richard ('89b), we find the 

 following with fat'o-segmented inner rami : D. minis, D. 

 lobatus, D. thceli, and D. glacialis, Lilljeborg; D. ccrruleus 

 Fischer, D. gibber Poppe, and D. wierzejskii Richard ; and 

 at least three American species have the inner ramus two- 

 segmented — D. stagnalis Forbes distinctly, and T). eiseni 

 Lillj. and D. albuquerquensis Herrick indistinctly so. 



It is expected that the keys here printed will be used in con- 

 nection with the descriptions and figures, since the species 

 vary within certain limits, and no hard and fast description 

 can be given which will cover the peculiarities of every indi- 

 vidual of a species. Local varietal differences or slight vari- 

 ations in proportion may make a key useless, and in all 

 cases the totality of characters should be considered. A 

 glance at the figures will indeed often be found more helpful 

 than any verbal description. 



Following the usual plan of specific descriptions, the first 

 paragraph, referring to the general appearance of the body, 

 thorax, abdomen, and furca and their relative proportions, 

 applies always to the female unless especially stated other- 

 wise. 



