North American Fresh-water Ostracoda. 415 



the soft parts of these Crustacea, with the result to add 

 several new genera, mostly described by Prof. G. 0. Sars. 

 For the convenience of students who follow me I have 

 thought it best to embody synopses of all these genera 

 in the text. 



Of the twenty-two species herein mentioned fourteen 

 are new to America, and twelve I believe to be un- 

 described. Two of these species belong to the genus 

 Limnicythere and to the family Cytheridse, itself new to 

 America. 



It is a pleasure to me to say that whatever value 

 this work may have is due in great part to the kind 

 encouragement I have received from Dr. Forbes, and to 

 his generosity in providing both material and Uterature 

 for my use. 



LIFE HISTORY AND HABITS.* 



Unlike the other groups of Entomostraca, most of 

 which, independent of shores or bottom, swim easily 

 and continuously throughout the waters they inhabit, 

 the Ostracoda commonly occupy a more restricted range 

 in small and shallow waters, or along the bottoms and 

 margius of large lakes and streams. With them, the act 

 of swimming is more laborious because of the structure 

 of the animal and the weight of the shell. In collections 

 of the limuetic plankton, therefore, they are either want- 

 ing, or present only in relatively insignificant numbers. 

 In consequence of the temporary nature of the small 

 bodies of water in which they ai-e usually found, they 

 are much more frequently observed during the spriug 

 months. At this time, in a little short-lived wayside pool 

 or ditch vast numbers may make their appearance, soon 

 to disappear with the drying up of the water. They may 

 also be seen in such situations in the fall. They thrive 

 in both clear and turbid waters, either in the midst of 

 aquatic vegetation or on a bare mud bottom. Most of 



' The text and tables under this head were prepared by Mr. C. A. Hart. 



