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 (3 1 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 m. 3 are 

 lowest in these months, not rising above 33 per cent, and 500 per 



