8 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



remarkable Monospilins 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, oj^portunity 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 theCladocera 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. I 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. Liudahl, through 

 my friend Mr. Oestlund, I am indebted for a like service. But ray obli^ 

 gation is deepest to Professor Eudolph 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 me to observe the 

 great similarity of widely-separated faunte. Mr. Lieberg also sent 

 specimens of Diaptomus stagnalis from saline pools in Dakota. 



