AQUATIC INVERTEBRATE FAUNA OF WYOMING AND MONTANA. 253 
This species is closely related to T). 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 setae. 
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, especially 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. 
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 four 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 setse more than half as long as the ramus itself. 
The third joint of the outer ramus is aborted, and bears two short, stout s] tines, 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 It. leptopus , from which, however, this species differs by its 
more slender form and by the absence of the antennal 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 subtruncate, 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 
