North American Species 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."t 



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. XXVI. , 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 sharp 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 within. 



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

 sperinflns measured by myself were about 1.08 millimeters in length. 



tDescription quoted from Forbes, '93. 



