4 TWELFTH AliTNUAL REPORT. 



wears its old covering over the newly-formed shell till the latter 

 is a curious patchwork mass. The attempt has been made to in- 

 corporate 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 meagre 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 

 necessary 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 material 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 per- 

 haps somewhat summarily treated. The excuse must be the con- 

 dition of the synonomy. However, most of the combinations made 

 were the result of careful study of large series from diflPerent locali- 

 ties. The sketches illustrating this paper are photo-printed from 

 the writer's own drawings, and, without the elegance of litho- 

 graphs, serve the purpose of explaining points of structure which 

 cannot be communicated verbally. I am indebted to Prof. A. S. 

 Forbes for very timely aid in bibliography, without which the 

 paper could not have been completed. To Dr. Lindthal, through 

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

 obligation is deepest to Prof. Rudolph Leuckart of Leipzig, who 

 kindly afibrded access to almost a complete set of works on Euro- 

 pean entomostraca. Prof. . 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 en- 

 abling me to observe the great similarity of widely-separated 

 faunse. Mr. Lieberg also sent specimens of Diaptomus stagnalis 

 from saline pools in Dakota. 



