NORTH AMERICAN SPECIES OF BRANCHINECTA. 257 



On June, 4, the ice had all disappeared except at the south- 

 east edge, and the warmest temperature was 6. 2 C. In this 

 collection the larvae were well developed, but none of those taken 

 had transformed the swimming antennae. They were larvae of 

 the last stage (Sars, 1896 : 55, PI. VI., 5, 6). At the time of the 

 next collection, June 17, the water temperature was 12. 2 C. 

 and none of these forms were taken in the Birge net. While the 

 mature animal was never captured in this way the larvae of the 

 last stage were easily taken in the net, so that, these forms were 

 probably mature at this time. 



The mature animals were extremely abundant July 12, with a 

 surface temperature of 13. 6 C., and were equally abundant 

 July 29, when the temperature was the same. At this time there 

 was only o.2 C. difference between the surface and bottom tem- 

 peratures. On August 12, with a surface temperature of I5.6 

 C. the BrancJdnecta had disappeared. After a diligent search, 

 one was seen, but although the search was continued no others 

 were found. The bottom collections made at this time were filled 

 with decomposing fragments of Branchinecta. 



Ward (1904 : 139) recorded this form for July 13, 1903, when 

 the surface temperature was 14. 4 C., and they were collected 

 by the writer on August 1 3 of the same year. 



From the above, it is clear that segmentation and most of the 

 embryonic development must have taken place under the ice. 

 the writer has no records of the temperature of the water before 

 the ice broke, but this temperature according to Zchokke (1900 : 

 45) probably did not exceed 2. 2 C. By the time the tempera- 

 ture had risen to 6. 2 C. the larvae were in the last stage of their 

 development, and at 13. 6 C. the animals had become fully ma- 

 ture. They had disappeared when the temperature had risen to 

 i 5. 6 C. This was the warmest temperature recorded for this 

 Lake. The mature forms were never abundant in the open 

 water. They collected about the rocks and near the shore where 

 they could be seen in great numbers swimming on their backs. 



On August 12, 1903, as well as on July 12 and 29, 1904, the 

 setae of the legs and caudal appendages bore numbers of a green 

 alga Cliaracium ambigumn Hern. On July 29, these algae were 

 so numerous that many of the Branchinecta could swim only 



