Xorth American Sj^ecics of Diaptomus. 143 



"Dimensions of female: Length to tip of caudal setae, 

 3.1 millimeters; abdomen, with setae, 1.16 millimeters, with- 

 out, 0.67 millimeters; thorax, 1.95 millimeters in length; 

 depth, 0.725 millimeter; width, 1 millimeter. 



"Male averaging scarcely smaller, but somewhat differ- 

 ently proportioned: Thorax, 1.85 millimeters in length; 

 depth, 0.58 millimeter; width, 0.08 [1.08]* millimeter; 

 abdomen, without setae, 0.745 millimeter; with setae, 1,35 

 millimeters in length. 



"Especially abundant in Shoshone Lake, but occurring in 

 other lakes and even in pools of some size in Yellowstone 

 Park."+ 



The drawings here given are in some cases the same as 

 those in the original description with unimportant correc- 

 tions or additions, but two new figures (PI. XXYL, Fig. 1, 

 3) have been added. I have not found this species in any 

 collections except those from Yellowstone Park and the Flat- 

 head region, in which it is rather abundant. 



A few points may be added to the original description. In 

 the female the first basal segment of the fifth pair of legs 

 bears a short sharj) spine on the outer margin a short dis- 

 tance above the distal angle. Both spines of the rudiment- 

 ary third segment of the outer ramus are distinctly spinose 

 on the inner margin. The first abdominal segment is almost 

 as long as the remainder of the abdomen ; the second segment 

 very short, about half as long as the succeeding segment or 

 the furca. Furcal rami about one and a half times as long 

 as wide and hairy within. My observations differ from those 

 of Prof. Forbes in that, as a rule, the abdomen of the female 

 is not asymmetrical, the first segment bearing on each side a 

 small tubercle armed with a minute spine. In the male the 

 first abdominal segment is very slightly dilated laterally but 

 unarmed, and about half as long as any one of the five suc- 

 ceeding segments, which differ very little in length. . The 

 furcal rami are fully twice as long as wide and hairy witliin. 



*The 0.08 in the original description is probably a typographical error, since the 

 specimens measured by myself were about 1.08 millimeters in length. 

 tDescription quoted from Forbes, '93. 



