252 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Dimensions of female : Length to tip of caudal set®, 3.1 millimeters ; abdomen, with 
set®, 1.16 millimeters; without, 0.07 millimeter; thorax, 1.95 millimeters in length; 
depth, 0.725 millimeter; width, 1 millimeter. 
Male averaging scarcely smaller, but somewhat differently proportioned. Thorax, 
1.85 millimeters in length; depth, 0.58 millimeter; width, 0.08 millimeter; abdomen, 
without set®, 0.745; with set®, 1.35 millimeters. 
Especially abundant in Shoshone Lake, but occurring in other lakes and even in 
pools of some size in Yellowstone Park. 
Diaptomus lintoni, u. sp. (Plate xlii, Figs. 26-28.) 
A large red species occurring commonly with D. shoshone , but distinguishable 
from it at a glance by its different shape, its longer antenn®, its smaller size, and by 
characters derived from the right antenna and the fifth foot of the male. The thorax 
is symmetrically elliptical in shape, broadest at the middle. The posterior angles are 
not produced or bifid, but are each armed with a minute spine. The first segment of 
the abdomen of the female is not especially produced, but bears at its broadest part a 
minute spine on each side. The abdomen itself is very short, its length contained 
about three and one-third times in that of the ceplialothorax. The antenna of the 
female is long and slender, 25-jointed, reaching a little beyond the tip of the abdomen. 
The fifth pair of legs in this sex is similar to those of D. slioshone , but much 
smaller. The inner ramus is not jointed. It is longer than the basal joint of the outer 
ramus, bears two stout plumose set® at its tip, somewhat shorter than the ramus 
itself, and has likewise at its inner tip a patch of small spines or fine hairs. The second 
segment of the outer ramus with its terminal claw is two-thirds as long again as the 
preceding segment, the breadth of the latter two-thirds its length. The third joint is 
indicated by a single long stout seta and one or two smaller ones. 
In the male the geniculate antenna is relatively rather slender, its last two joints 
without special appendages, its penultimate with a slender transparent apical process, 
reaching about to the middle of the succeeding segment, acute at tip, but neither serrate 
nor emarginate. Fifth pair of legs in the male usually without internal ramus 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 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 outwards and has the apex minutely hairy. The basal segment of 
the outer ramus is thick, two-thirds as broad as long, somewhat 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 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 set®; 
depth, 0.42 millimeter. 
