Marsh—North American Species of Diaptomus. 455 
“Fifth pair of legs in the male usually without internal n,- 
mus 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 the 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 downward beyond the 
articulation with 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 mm., 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 the inner ramus and 
