North American Species of Diaptomus. 129 



minutely hairy. The basal segment of the outer ramus is 

 thick, two thirds as broad as long, somewhat inflated within, 

 where it extends downward and beyond the articulation with 

 the second segment as a rounded expansion covered with ex- 

 tremely 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 covered with cilia, 

 and with two stout terminal spines, one short, blunt, straight, 

 and 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 millimeters, 

 excluding caudal seta3 ; depth, 0.42 millimeters. 



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

 from which it differs conspicuously by its smaller size, more 

 symmetrical cephalothorax, without prominent or bifid angles, 

 and longer and more slender antennae, with longer and more 

 slender appendage to the antepenultimate segment. 



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

 stagnalis especially with respect to the inner ramus, which is 

 larger and longer than in the other, lacks the characteristic 

 segmentation of stagnalis, and bears at its tip shorter and 

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

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

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

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

 cially in the length and strength of the inner ramus and the 

 length and dissimilarity of the setae at the end of the outer. 



" Common in lakes and pools of Yellowstone Park."* 



This species is one of the three American forms in which 

 the inner ramus is rudimentary or wanting, the other two 

 being Z). sanguineus Forbes and 1). minutus Lilljeborg. It 

 has not been recorded from any' localities outside of those in 

 which it was originally found. 



* Description quoted from Forbes, '93. 



