-^1 



PREFACK 



N presenting Avhat may be denominated a final report of the work 

 cnq) done in this state upon the group of Crustacea best repre- 

 sented, and, all things considered, most important, the author must 

 admit that the term '' final " refers only to his own opportunities 

 and the limitation of time imposed by circumstances. 



While a comparatively large proportion of all the species exist- 

 ing within our limits have been examined during the progress of 

 these investigations, there undoubtedly remain many additional 

 and curious forms to reward the search of the student. A great 

 variation in the degree of completeness with which the diflferent 

 genera and species have been treated will be observed, due in part 

 to the circumstances under which they were studied, and frequently 

 to the poverty of material. The entomostracean fauna is quite 

 different at different seasons, and a complete knowledge of even 

 our local fauna requires a long period of observation. Even the 

 dead of winter is a favorable time to study some groups. The late 

 autumn is, perhaps, the most favorable opportunity; for then, in 

 one group, the sexual activities are just at their height, and both 

 sexes may be studied. A number of cladocera are restricted to this 

 season. There are a number (how large it is not yet possible to 

 tell) of species in both groups which are to be sought by night 

 though no phosphorescent species are yet known. Our larger, 

 and, especially, deeper lakes have a quite diff'erent fauna from the 

 shallow pools and rirers. In general, the flowing waters are poor 

 in entomostraca. The cladocera or shelled entomostraca, have here 

 received a large share of attention, and more particularly the Lyn- 

 ceidse, which are the most minute of arthropods. This study has 

 been rewarded with an unexpectedly large number of forms, and a 

 particularly large number of species identical with those of Europe 

 and elsewhere. Prof. Birge is the only American writer who 

 has attempted this group, and his valuable work has made us 

 familiar with the more striking new species. A few new species 

 are included in our list and several varieties hardly yet known in 

 Europe. The remarkable Monospilius is among these. This ani- 

 mal has but a single larval eye in the middle of its forehead, and 



