168 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



The members of this genus are danger signals from a hygienic point 

 of view, for they frequent water containing decaying matter; as many 

 as 1,400 were counted in a single quart of such water, Tlie genus is 

 particularly preplexing, as the varieties named seem to be hardly 

 entitled to specific rank and are so similar as to require great care to 

 properly distinguish. 



The following artificial key, it is believed, will assist in placing the 

 specimens which may be obtained in America. There seems no reason 

 to doubt that our fauna is very similar to that of north Earope. Of the 

 twelve species here enumerated at least one-third may be synonyms 

 and others of the remainder are with difficulty distinguished. 



KEY TO THE GENUS CERIODAPHNIA. 



I. Head without a ''horn." 



a. Shell irregularly striate lliegops, 168 



b. Shell with hexagonal meshes. 



* Shell with doubly contoured markings. 



t Head broad, short pillcliella, 169 



ft Head narrowed, depressed. 



J Antennules normal rotunda, 169 



tt Antennules elongate alabameiisis, 170 



** Shell simply marked. 



t Claws with teeth reticulata, 170; deutata, 170 



ft Claws without teeth. 



t Antennie very long punctata, 171 



JJ AntenniB normal or short. 



? Post-abdomen broad. . laticaudata. 171; censors, 171 

 1^ Post-abdomen narrow. 



II Head not angled behind the eye. . quadranj^fula, 172 



nil Head abruptly angled behind the eye. . . scitula, 172 



*** Shell opening pointed setcsa, 173 



c. Shell recticulate with rectangular meshes nitida, 173 



II. Head with a curved "horn." cornuta, 174 



[Not noticed in the key: asperata, 173; minuta, 173; textilis, 173; 

 rotuudata, 173; ecliinata, 173 ] 



* Ceriodaplinia meg-ops Sars. 

 Plate XLI, Fig. 20. 

 Birge '78 (cristata). 



This species is one of the largest and most readily distinguished 

 as well as rarest of the genus. Very characteristic is the fine anasto- 

 mosing striatiou which breaks up into reticulation only near the shell 

 margins. This species seems to form the transition toward Shnoceph- 

 alus with Seapholeberis, which, however, diverges along its own pecu- 



