North American Species of Diaptomus. 179 



sense-club; 10 and 11, two long spines, one much thicker 

 than the other, and a process; 12, a long spine, a short 

 spine, and a sense-club; 13, a long spine and a process ; 14, 

 a long spine, a long seta, and a sense-club; 15, a process, 

 a long spine, a short seta, and a sense-club; 16, a process, a 

 long spine, a long seta, and a sense-club ; 17, a plate-like proc- 

 ess, a long and a short spine ; 18, a plate and a short spine ; 

 19, 20, and 21 (usually completely ankylosed but some- 

 times with sutures indistinctly visible), a very long spine, a 

 long seta, and a short cuticular process ; 22 and 23 (com- 

 pletely ankylosed), a narrow hyaline lamina (bisected by a 

 sense hair) and two seta?; 24, two setae; and 25, two long- 

 seta and two short ones, a sense hair, and a sense-club. 



Fifth pair of feet in the male (PL XXXV., Fig. 1) charac- 

 teristic. First basal segment of the right leg produced at the 

 inner apical angle into a process (generally blunt but some- 

 times spine-like) having on the posterior surface a tubercle 

 bearing a short blunt spine. Second basal segment (PI. 

 XXXIV., Fig. 1) armed at the inner margin with two proc- 

 esses, the proximal one broad, prominent, concave toward 

 the apex of the segment ; the other, slightly above the middle 

 of the segment, a mere sharp triangular point. At the outer 

 apical angle is a slight indentation from which springs a del- 

 icate hair, and from the inner apical angle arises the inner 

 ramus. First segment of the outer ramus irregular, about 

 one and a half times as long as broad, with a small sharp tri- 

 angular-point on the inner margin at about the beginning of 

 the distal fifth. On this segment is a structure which is not, to 

 my knowledge, found in any other Diaptomus. This is a hook 

 arising from the middle of the posterior aspect, and reaching 

 to the end of the segment. It is sickle-shaped, perfectly 

 smooth, and although supplied with muscles does not seem to 

 be movable. Second segment subquadrate, about twice as long 

 as wide. The marginal spine is short, almost straight, about 

 a third the length of the segment, delicately serrate within. Ter- 

 minal hook very stout, as long as the two preceding segments, 

 tapering gradually, and slightly recurved at the tip ; armed for 

 the distal two thirds of the inner margin with strong teeth. 



