Marsh—North American Species of Diaptomus. 481 
is a succession of forms in some pools in the spring, Herrick 
has maintained a heterogenetic character of this species, but 
his conclusions have not been verified by other authors. The 
exigencies of the life of the species may explain its variability. 
From the fact that its life is so short, we might expect some lo¬ 
calization of species, inasmuch as there is hardly time for 
dispersion in the ordinary ways, and it is very possible that 
extended study will show that there are very definite local 
varieties. 
Schacht considers D. armatus Herrick a variety of D. san¬ 
guineus. While I think this identification is probably correct, 
I have not ventured to include this among the list of syno¬ 
nyms, for Herrick’s description is so very meager that it is 
impossible to know what form he had before him. 
diaptomus eiseni Lilljeborg. 
Plate XXV, fig. 3. Plate XXVI, figs. 6, 8. 
1889. Diaptomus Eiseni DeGuerne and Richard, p. 44; pi. 
I, figs. 19, 29, 33. 
1895. Diaptomus Eiseni Plerrick and Turner, p. 58; pi. X, 
%. ii. 
1897. Diaptomus Eiseni Schacht, p. 162. 
1898. Diaptomus Eiseni Brewer, p. 128; pi. VII, figs. 9-12. 
Among the largest of the genus. The cephalothorax is 
widest at the posterior part of the head. The first segment 
does not equal in length the rest of the cephalothorax. The 
last two thoracic segments are commonly confluent above, or 
indistinctly separated. The lateral lobes of the last segment, 
seen from above, are, in the female, short, the anterior and 
posterior angles somewhat acute, and the lateral obtuse. The 
lateral spines of the last segment are short and thick. 
The first abdominal segment of the female equals or exceeds 
in length the rest of the abdomen including the furca. It is 
swollen laterally and in front, and bears on the lateral expan¬ 
sions rather large spines which project backwards. The second 
31—S. & A. 
