Lest of Crustacea Cladocera from Madison, Wis. 389 



lakes are only a mile apart, I have not found the pointed variety in 

 Mendota or the rounded in Wingra. The outlines of the head are very 

 variable, the variations quite closely resembling those represented in 

 D. berolinensis, apicata, and cucullata, although of course this species has 

 the macula nigra. 



The males appear in the latter part of September. The flagellum of 

 the antennule is convex, stout and short, usually little longer than the 

 sense-hairs: The anterior sense-bristle in our specimens lies little nearer 

 the end of the basal portion of the antennule than the head. In this our 

 specimens differ from Eylmann's * description, who says of it, that it is 

 " von der Endborste nicht weit entf ernt. v 



This species is the most abundant in the open waters of the Madison 

 lakes. I have also obtained it from Minnesota and Michigan, showing 

 some variation from our form in each case. 



Species 24. D. kahlbergensis var. retrocurva, Forbes. Plate XIII. 

 Figs. 7,8. 



This form was first described by Forbes * as a distinct species. It is the 

 most extreme Daphnid form yet observed. I cannot agree in the state- 

 ment of Forbes that the large helmeted forms predominate in the smal- 

 ler lakes (1. c. p. 643.) At Madison the forms of D. hyalina and of D. 

 retrocurva in Lake Mendota are much more helmeted than those in Lake 

 Wingra. The former lake is about six miles by four, the latter 

 1M by M mile. D. hyalina in the smaller lake is more like D. apicata, 

 while in Mendota the crest is more developed than is shown by any 

 European descriptions. D. kahlbergensis from Wingra shows the forms 

 typical of that species and of cederstroemii while the full development 

 of the crest only comes in the larger lake. The males of this species ap- 

 pear late in the fall, in the latter part of October and in November. 

 The head is of the kahlbergensis type, sometimes curved up but never 

 showing the extreme development of the female. The antennule has a 

 flagellum a good deal longer than the sense-hairs, curved at the tip and 

 distinctly articulated to the basal part. 



Our specimens do not show the extreme development of the head be- 

 fore birth noted by Forbes (1. c, p. 642). The head in the young is not as 

 much crested as in the adult D. hyalina. This species is always found 

 in company with D. hyalina and is far less numerous. On calm summer 

 nights the water of Lake Mendota swarms with these two species, to- 

 gether with a Cyclops, a Diaptomus, and Leptodora hyalina. They are 

 not abundant close to shore and seem to spend the day in swarms at the 



*Eylmann, E. Beitrag zur Systematik der Europaischen Daphniden, Freiburg i. B. 1886, 

 p. 33. 



* Forbes, S. A. Entomostraca of Lake Michigan and adjacent waters. American Natur- 

 alist. Vol. xvi., p. 642, August, 1882. 



