390 Wisconsin 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. Maceotheix kosea, 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. Mackothrix 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. Acantholeberis dentata, Euren, Om markliga Crustaceer af or- 



dningen Cladocera, f unna i Dalarne. Ofvers, af K. Vet.-akad. 

 Forh. 1861, p. 118. Description of female. Tan. Ill, fig. 2. 

 Female. 



1862. Drepanothrix sentigera, 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 Hungarica3,'p. 106, PI. II, fig. 43. 



