ESTOMOSTRACA AND PHYLLOPODA. 245 



mined by these appearances, and the name learned by 

 the Key, in connection with a pocket-lens. The two 

 Entomostracans, Diaptomus, and Canthocamptus, are 

 separated in the Key by the number of the joints in 

 their long antennae. This seems to be a very minute 

 character to use in so artificial a table, but it need not 

 be an annoyance to the beginner, since the antennae of 

 these two common little crustaceans differ so conspicu- 

 ously in size and length that the joints need not be 

 actually counted ; a glance will show which is Cantho- 

 camptus, with its short and rather inconspicuous an- 

 tennae. 



The beak referred to is the front part of the shell 

 extended in a long, usually curved and pointed prolon- 

 gation, containing the eye and portions of the animal's 

 head. 



Key to Genera of Entomdstraca and Phyllopoda, 



1. Legs with flat plates near the body ; animal swim- 



ming on the back (A). 



2. Legs without flat plates (a). 



a. Body enclosed in a bivalve shell (b). 



a. Body not enclosed in a shell (g). 



I. Shell with a sharp posterior spine or a tooth on 

 or near the upper posterior angle (c). 



b. Shell without a posterior spine, or with one to four 



small teeth on the lower posterior angle (d). 



c. Smooth ; spine on the upper angle, or near the 



middle of the border. Ddphnia, 1. 



