Lest of Crustacea Claclocera 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 entfernt.” 
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 
by % 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. Tcahlbergensis from Wingra shows the forms 
typical of that species and of cmerstroemii 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 Jcahlbergensis 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 Europaiscken Dapkniden, 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. 
