Marsh — North AmeHcan 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 a 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 i^orth 

 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 extensire 

 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. pallidas. 



It is rather remarkable that a species of such wide distri- 

 bution should show so little variability. 



I have not seen specimens of Pear^e'^5 pijtjinaeus, 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 difi^erence 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 cui-ved somewhat "sharply." His figure, however, 

 shows no sharper curvature than would be true of oregonerisis. 

 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 figiire. Apparently, from 

 his figure, the abdomen of the female is somewhat broader than 

 in the type. These differences, however, if they are recog- 

 nizable dift'ereuces, could hardly be considered more than va- 

 rietal. 



