Marsh—North American Species of Diaptomus. 411 
The outer margin is convex, the inner sinuate and setose. The 
second segment is produced into two finger-like processes, one 
exterior and one interior. The exterior one is finely denticu¬ 
late on the inner margin. The interior process bears & small 
process on its base which projects inwards. The inner surface 
of the segment is setose. 
Length of female, 1.5 mm. Length of male, 1.4 mm. These 
are the figures of Lilljeborg. There is considerable variation 
in the length of specimens from various localities. 
This is one of the most widely distributed of the North 
American species. The type specimens were from Portland, 
Oregon. It is by far the most common species in the lakes of 
Wisconsin and Michigan, and occurs, though not in great num¬ 
bers, in the Great Lakes. It is found in Iowa, northern Illi¬ 
nois and northern Indiana, in Minnesota, and I have speci¬ 
mens from Kinistino, Northwest Territory. No extensive 
collections have been made in the eastern states, but it is found 
in eastern Massachusetts, and I think it probable that this spe¬ 
cies is distributed all over North America north of the latitude 
of northern Illinois. In southern Illinois and in the southern 
states its place seems to be taken by D. pallidus. 
It is rather remarkable that a species of such wide distri¬ 
bution should show so little variability. 
I have not seen specimens of Pearse’s pygmaeus, but I have 
gone over his description and figures very carefully, and I see 
no reason to separate it from oregonensis. The only possible 
points of difference which I can see, are the form of the hook 
-of the exopodite of the male right fifth foot, the form of the 
endopodite of the left fifth foot of the male, and the form of 
the female abdomen. Pearse states in his description that the 
hook is curved somewhat “sharply.” TIis figure, however, 
shows no sharper curvature than would be true of oregonensis. 
The endopodite of the left fifth foot of the male is more pointed 
than in typical oregonensis, but the form of this segment varies, 
and frequently is as sharp as in his figure. Apparently, from 
his figure, the abdomen of the female is somewhat broader than 
in the type. These differences, however, if they are recog¬ 
nizable differences, could hardly be considered more than va¬ 
rietal. 
