AQUATIC INVEETEBEATE FAUNA OF WYOMING AND MONTANA. 253 



This species is closely related to D. stagnalis Forbes, from which it differs con- 

 spicuously 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 set:e. 

 In the male the terminal claw of the outer ramns of the right fifth leg is mnch 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, especially in the length and strength of 

 the inner ramus and the length and dissimilarity of the set3 at the end of the outer. 



Common in lakes and pools of Yellowstone Park. 



Named for my friend, and companion on the trip of 1890, Prof. Edwin Linton, of 

 Washington and Jefferson College, Pennsylvania. 

 Diaptomus piscinae, n. sp. (Plate XLI, Fig. 22.) 



A species of medium size and symmetrical proportions, antennae reaching to the 

 tip of the abdomen, cephalothorax broadest about the middle, with fovir distinct 

 sutures, the posterior lateral angles not produced, but armed with two distal spines. 



The right antenna of the male is without appendage to the antepenultimate 

 joint, and the fifth pair of legs of the same sex has the inner ramus well developed on 

 both the right and left sides. The usual length is 1.75 millimeters, the transverse 

 diameter 0.45 millimeter; the abdomen, with furca, is a little more than one- third the 

 length of the cephalothorax. 



The fifth pair of legs of the female is without especially marked characters, except 

 that the inner ramus, which reaches to the tip of the principal segment of the outer, is 

 provided with two long, stout, equal setee more than half as long as the ramus itself. 

 The third joint of the outer ramus is aborted, and bears two short, stout spines, and 

 the joint preceding bears a slender spine outside the base of the last. The terminal 

 claw of this joint is simple and nearly straight, viewed in the usual position. 



In the male the fifth pair of legs has a considerable resemblance to the corre- 

 sponding appendages of 1). leptopus, from which, however, this species differs by its 

 more slender form and by the absence of the autennal hook. The peduncle of the 

 left leg is quadrate and equal in length to the basal segment of the outer ramus, but is 

 nearly twice as wide. The sides of this latter segment are parallel, the inner terminal 

 angle is broadly rounded and minutely ciliate, and to the outer terminal angle is attached 

 the second segment of the ramus. This segment is a trifle shorter than the preceding 

 and less than half as wide, and bears at its tip a stout, blunt, conical spine, whose 

 length is equal to that of the diameter of the ramus, and within this a long flexible 

 hair as long as the ramus itself. The inner ramus of this leg is very long, reaching 

 beyond the middle of the terminal joint of the outer ramus. It is slightly concave 

 towards this ramus and terminates with a broadly rounded or subtruucate, thickly 

 ciliate end, forming an acute outer angle and an obtuse inner one. Seen at right 

 angles to this view, the tip is simply obtusely pointed. 



The right leg of the male is without remarkable distinguishing characters. Basal 

 joint of the outer ramus about two-thirds as long as the peduncle and nearly as wide; 

 second joint slightly longer than the peduncle, equal to the first in width; and the 



