Marsh—North American Species of Diaptomus . 399 
the right exopodite; in most cases this runs across the segment. 
In all, except D. saltillinus, there is an oblique ridge on the pos¬ 
terior surface of the second segment of the right exopodite. In 
all, except D. purpureas, the terminal segment of the exopodite 
of the left foot is armed with a digitiform process and a ^len¬ 
der articulated spine; this spine in D. saltillirms is curved. The 
fifth feet of the females, too, resemble each other. In all, the 
exopodites are three-segmented. The endopodites are short, 
and in all, except D. saltillinus, are armed with two rather 
prominent spines. The female abdomens, with the exception of 
D. asymmetricus, resemble each other; in D. dorsalis and D. 
purpureas the first segment is especially long and slender, but 
it is stouter in D. saltillinus and D. albuquerquensis. In D. 
asymmetricus the general form is like that of D. purpureus, 
but there is the marked peculiarity of the lateral process. 
D. albuquerquensis, D. dorsalis and D. saltillinus differ from 
the other species of the group in the peculiar dorsal processes 
of the fifth thoracic segment. 
I have called the group the albuquerquensis group, not be¬ 
cause there is any reason to think that this species is the most 
primitive, but because it was the first described. There would 
seem to be little question of the close affinity of all the members 
of the group. D. saltillinus differs more widely than do the 
others. D. saltillinus, too, is the most northern species, the 
others being distinctly southern, D. albuquerquensis being 
found in Colorado, blew Mexico ‘and Mexico, and D. purpu¬ 
reus and D. asymmetricus in Cuba. 
It is hardly possible, with the present knowledge, even to 
guess at the phylogenetic relationships of the group. 
D. sanguineus and D. Eiseni I am not prepared to locate, 
even tentatively. It should be noticed, perhaps, that D. albu¬ 
querquensis, dorsalis, saltillinus, Trybomi , sanguineus and sig- 
nicauda all have a pronounced dorsal process or hump. This 
may indicate some relationship, but it does not seem clear 
enough to lead to a grouping of these species. 
Of the groups I have formed, it seems to me that the tenui- 
caudatus group is the nearest to the primitive form; it does 
