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390      fWisconsin 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, Eur6n. Plate XIII. Figs. 15-17.
  1861. Acantholeberis dentata, Euren, Om mdrkliga Crustaceer af or-
           dningen Cladocera, funna i Dalarne. Ofvers, af K. Vet.-akad.
           F6rh. 1861, p. 118. Description of female. Tafl. III, fig. 2.
           Female.
  1862. Drepanothrix sentigera, Sars, G. 0. 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 Hungaricoe,'p. 106, PI. I[, fig. 43.
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