Marsh — JS/orth American Species of Diaptomus. 455 



"Fifth pair of legs in the male usually without internal ru- 

 nius to the right leg, but this ramus sometimes represented by a 

 small rudiment. The limb is usually slender, and its terminal 

 claw short. The basal segment of the outer ramus is nearly 

 as long as the adjacent segment of the pedicel, and the slender 

 second segment of this ramus is fully as long. Long lateral spines 

 are borne near tlie tip of this segment. The terminal claw is 

 about two-thirds as long as the segment, is somewhat abruptly 

 angulated near its base, and slightly recurved at the tip. The 

 inner ramus of the left leg is very stout and long, reaching 

 almost to the tip of the outer ramus, is slightly curved out- 

 wards, and has the apex minutely hairy. The basal segment 

 of the outer ramus is thick, two-thirds as broad as long, some- 

 what inflated within, where it extends dowmvard beyond the 

 articulation mth the second segment as a rounded expansion 

 covered with extremely fine hairs. Second segment of this 

 ramus longer than first, but only half as wide, bearing at its 

 tip, within, a rather small, obliquely projecting cushion cov- 

 ered with cilia, and with two stout terminal spines, one short, 

 blunt, straight, smooth, the other curved and plumose, its 

 length about half that of the segment to which it is attached. 



''The total length of this species is about 2.5 nun., excluding 

 caudal setae; depth, .42 mm. 



"This species is closely related to D. stagnalis Forbes, from 

 which it differs conspicuously by its smaller size, more symmet- 

 rical cephalothorax, without prominent or bifid angles, and 

 longer and more slender antennae, with longer and more slen- 

 der appendage to the antepenultimate segment. 



"In the fifth legs of the female this species differs from D. 

 stagnalis, especially with respect to the inner ramus, which is 

 larger and longer than in the other, lacks the characteristic seg 

 mentation of D. stagnalis, and bears at its tip shorter and 

 broader setae. In the male the terminal claw of the outer 

 ramus of the right fifth leg is much more slender than in 

 D. stagnalis, and the inner ramus is much less developed. 

 The left leg of this pair is different in a number of details, 

 especially in the length and strength of tlie inner ramus and 



