Article XV. — Contribution to a Knowledge of the North 

 American FresJi-ioater Ostracoda included in the Fami- 

 lies Cytheridce and Oyprididce. By Richard W.Sharpe, 

 B. S. 



INTRODUCTION. 



The present paper has been prepared in the course 

 of work at the University of IlUnois for the degree of 

 master of science in zoology. In addition to extensive 

 collections of Entomostraca made at the Biological 

 Station of the University of Illinois, situated at Havana, 

 on the Illinois River, I have been able, through the kind- 

 ness of Dr. S. A. Forbes, to examine all the accumula- 

 tions in this group made by the Illinois State Labora- 

 tory of Natural History during the last twenty years, 

 and covering a territory little less than continental. 

 The greater part of the material studied is from the 

 rivers, ponds, and lakes of Illinois and immediately ad- 

 jacent states, but at least a cursory examination has 

 also been made of collections from the Yellowstone 

 National Park and from the lakes of northwestern 

 Montana. 



Although the Ostracoda of Europe have now been ex- 

 tensively studied, but little work has been done upon 

 this order in America. Prof. C. H. Turner, of Clark 

 University, Atlanta, Ga., has, however, paved the way 

 for American students in this field (36, 62, 63, and 64), 

 and scattered descriptions occur in the writings of some 

 others. The earlier practice of distinguishing species by 

 characters derived from the shell alone has had the effect 

 to surround the study of this group with extraordinary 

 difficulties and greatly to complicate the synonymy. 

 More recently much rise lias been ma'lo of the strncturo of 



