8 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
remarkable Monospilius is among these. This animal has but a single 
larval eye in the middle of its forehead, and wears its old covering 
over the newly-formed shell till the latter is a curious patchwork 
mass. The attempt has been made to incorporate a brief description 
of all American species with those found in Minnesota, and also to 
frame keys for the larger genera, so that the place of a species among 
its congeners may, at least approximately, be found. The difficulty 
of framing such keys is very great; for few authors have employed the 
same distinctions in their descriptions, and it is necessary to select 
points sharply distinctive and conspicuous from the often meager 
remainder after striking off scattering particulars. In some cases this 
difficulty has been greatly enhanced by the possibility that some of 
the species should be considered synonyms or varietal forms. The 
tendency to combine questionable forms thus produced it was neces- 
sary to offset by what may seem a too great conservatism. Faulty, 
however, as these keys may be, it is hoped that they will serve a good 
purpose in the extent which they cover. While the limits of this 
work preclude much more than a systematic outline, opportunity is 
taken here and there to admit a word on the anatomy or development. 
Such allusions must be considered simply accidental, for a complete 
treatment of these subjects would require large volumes, and the ma- 
terial will be long in gathering. A larger proportion of the rare 
males of the Cladocera are here referred to than in any previous work 
of equal extent. The genus Cyclops, one of the bugbears to fresh- 
water carcinologists, is perhaps somewhat summarily treated. The 
excuse must be the condition of the synonomy. However, most of the 
combinations made were the result of careful study of large series. 
from different localities. The sketches illustrating this paper are 
photo printed from the writer’s own drawings, and, without the ele- 
gance of lithographs, serve the purpose of explaining points of struc- 
ture which cannot be communicated verbally. J am indebted to Pro- 
fessor A.S. Forbes for very timely aid in bibliography, without which 
the paper could not have been completed. To Dr. Lindahl, through 
my friend Mr. Oestlund, I am indebted fora likeservice. But my obli- 
gation is deepest to Professor Rudolph Leuckart of Leipzig, who kindly 
afforded access to almost a complete set of works on European Ento. 
mostraca. Professor C. W. Hall has collected, at much expenditure of 
time and labor, a set of specimens from different parts of the state, 
which he kindly placed in my hands, thus enabling meto observe the 
great similarity of widely-separated faunz. Mr. Lieberg also sent 
specimens of Diaptomus stagnalis from saline pools in Dakota. 
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