Mar&h — North American Species of Diaptomus. 395 



segmented exopoditc. D. shoshone by its size is separated from 

 the rest of the group, but its structural relationships seem to be 

 very close. 



D. minutus is most widely distributed, being found from th^ 

 northern United States to Greenland ind Iceland, but not c-ii 

 the Eastern Continent. D. Ashlandi and D. sicilis, so far as 

 known, are limited to the northern tier of states in the United 

 States. D. Birgei and D. siciloides belong to warmer waters 

 but probably do not occur south of the Ohio river, while D. 

 shoshone is peculiar to the mountain region of the West. D. 

 tenuicaudatus is a recent find, and is, so far, reported from only 

 one locality, in the Saskatchewan region. D. sicilis and D. 

 Ashlandi are distinctly lovers of cold water. In Green lake D. 

 sicilis occurs in the winter months and D. minutus in the smn- 

 mer months. (Marsh, '97, Marsh, '03.) D. tenuicaudatus is 

 considered the most primitive form because of the slender female 

 abdomen without armature, the nearly equal length of the 

 male fifth feet, and the two^segmented endopodite of the 

 left fifth foot of the male. D. sicilis is the most nearly re- 

 lated to D. tenuicaudatus. D. shoshone comes very close to 

 it, but if it is in this line it must have been subjected 

 to peculiarly favorable circumstances of food to have developed 

 such an enormous size. D. Ashlandi might easily have been de- 

 rived from D. tenuicaudatus, but the separation must have 

 taken place at a comparatively remote time. D. siciloides and 

 D. Birgei are somewhat more specialized forms from the same 

 stock as D. sicilis. D. minutus, according to the standard I 

 have set up, is the most specialized of the group; there is a 

 marked difference in the lengths of the fifth feet of the male, 

 and there is a striking reduction of the endopodites in the fifth 

 feet of both sexes. The only noticeable primitive character is 

 the three spines of the second segment of the exopodite of the 

 fifth feet of the female. From its wide distribution we might 

 well think of D. minutus as an early fonn from which the 

 others have been derived, were it not for its specialized charac- 

 teristics. • As it is, we must think of it, perhaps, as not derived 

 from D. tenuicaudatus, but as having a common ancestry with 



