390 
Vdisconsin Academy of Sciences , Arts and Letters. 
bottom where the vegetation consists mainly of diatoms, outside of the 
growth of weeds. The number of the Cladocera is simply incalculable.. 
I do not think that any shallow water is more filled with crustacean life 
than are the open waters of our lakes. Dredging does not give a fair 
idea of the number of open water individuals. Only surface collecting- 
at night will disclose them. 
Species 26. Macrothrix rosea, Jurine. Plate XIII. Figs. 13,14. 
I have succeeded in finding several specimens of the male of this 
species and have materially increased the accuracy of my knowledge of 
its structure. I found a single male in 1877 which was described in the 
Transactions of the Wisconsin Academy, Vol. IV. p. 90. Since that time 
the male has been seen by Daday,* who gives a figure which, however* 
is so small and shows so little detail that it does not add much to our 
knowledge. 
The male antennules are long and curved, provided with a long an¬ 
terior sense-hair at the base. They are curved toward the median plane 
of the body at the tip and bear the olfactory hairs on a small elevation 
on the anterior side. On the posterior side of the apex is a cluster of 
5-6 long diverging sense-hairs. Daday shows these in his figure, but 
does not mention them in the text. In the possession of this extra 
sense organ, the male M. rosea differs from all other male Cladocera. 
known, including the closely allied Macrothrix laticornis. These sense- 
hairs were not seen by me in my earlier specimen. 
The post abdomen is prolonged into a flexible projection, on whose 
summit the vas deferens opens, just before the very small terminal 
claws. The whole structure thus resembles that of the male Bosmina. 
Species 27. Macrothrix laticornis, Jurine. 
This form, which is usually given as the commonest of European spec¬ 
ies seems very rare here. I have met with not more than a dozen speci¬ 
mens in a season’s collecting, while M. rosea is very abundant in marshes. 
It is at times the predominant cladoceran, while M. laticornis has never 
appeared except in single specimens. 
Species 28. Drepanothrix dentata, Euren. Plate XIII. Figs. 15-17. 
1861. Acantholeheris dentata , Euren, Om markliga Crustaceer af or- 
dningen Cladocera, funna i Dalarne. Of vers, af K. Vet.-akad. 
Forh. 1861, p. 118. Description of female. Tafl. Ill, fig. 2. 
Female. 
1862. Drepanothrix sentigerci, Sars, G. O. Om de i Omegnen af Chris¬ 
tiania jagttagne Crustacea cladocera. Forh. Vid.-Selskab. i 
Christiania, 1862, p. 156. Description of male and female. 
1862. Drepanothrix hamata , Sars., Do. p. 300. Mention only. 
* Daday, E. Crustacea Cladocera Faunae Hungaricse, r p. 106, PI. II, fig. 43. 
