282 
Of the total individuals, 40 per cent. were males; 45 per cent., 
females without eggs; and 15 per cent., females with eggs. The 
sexes show no marked or constant seasonal differences in distribu- 
tion. Females with eggs are more abundant in August—October, 
and with spermatophores in the same months. Detached sperma- 
tophores were found until December. 
This species is stated by Herrick (’84) to be distributed in the 
entire Mississippi Valley. Marsh (’93) finds it in Wisconsin, but 
it appears nowhere in the plankton of the Great Lakes. Brewer 
(98) reports it in the backwaters of the Platte in Nebraska, and 
Schacht (’97) states that it is an exceedingly common species in 
central Illinois, and that it has been reported from Wisconsin, Ohio, 
and Minnesota. It thus appears to be limited to the shallow and 
relatively warm waters of the prairie regions of the Mississippi basin. 
Diaptomus siciloides Lilljeborg.—Average number, 10; in 1897, 
350; in 1896, 56; in 1895, 282: and in 1894, 23. As will be seen 
on comparison, these yearly averages are very similar to those of 
the preceding species with the exception that the development of 
D. siciloides is about twice that of D. pallidus in 1895. In other 
particulars its seasonal data so resemble those of D. pallidus as to 
make their discussion in large part a repetition. Its seasonal- 
distribution relations to temperature and hydrographic conditions, 
breeding season, and its tendency toward a pulse-like recurrence in 
coincidence with other Entomostraca are all very similar to these 
features in D. pallidus. The proportions of the sexes differ slightly, 
the males being less numerous (31 per cent.) and egg-bearing females 
more abundant (18 per cent.) than in the previous species. 
This is also an American species, reported thus far only from 
Lake Tulare, Calif., the Illinois River, and waters of Indiana and 
Iowa (Schacht, 97), and by Brewer (’98) in lakes and pools of 
Nebraska. It is thus confined largely to shoal and warm waters. 
Diaptomus spp., immature.—Average number, 19; in 1897, 560; 
in 1896, 158; in 1895, 336; and in 1894, 120. 
The immature individuals of D. pallidus and D. siciloides were 
not distinguished from each other in the records. Young Diaptomus 
presumably belonging to these two species occur in every month 
but March, though but 10 of the 74 records were made in January— 
June. The percentage of occurrences and the numbers per mr vane 
lowest in these months, not rising above 33 per cent. and 500 per 
