1638 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. The 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 Europe. 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... 0... 3-20 ee)! . Megops, 168 
b. Shell with hexagonal meshes. 
* Shell with doubly contoured markings. 
{ Head broad, short. ea, o Dele Beck ont epee tae! ELC RIMen ee SE Eree 
Tf Head narrowed, depressed. 
{ Antennulesnormal.. . ... . . . . rotunda, 169 
tt Antennuleselongate.. . . . . . +. alabamensis, 170 
** Shell simply marked. 
t+ Claws with teeth. . . « . reticulata, 170; dentata, 170 
tt Claws without teeth. 
{ Antenne verylong. . .°. . . ... . punctata, 171 
tt Antenne normal or short. 
2 Post-abdomen broad. . laticaudata. 171; consors, 171 
2% Post-abdomen narrow. 
|| Head not angled behind theeye. . quadrangula, 172 
|||| Head abruptly angled behind the eye... . Scitula, 172 
*** Shell opening pointed. ra outs. Qa Jor}he oat SLOSS aes 
e. Shell recticulate with rectangular reece! Oo i OR oe ee tie 
II. ; Head witha curved:“‘hom.? > 7°33 ge oe ee Ete 
[ Not noticed in the key: asperata, 173; minuta, 173; textilis, 173; 
rotundata, 173; echinata, 173. ] 
* Ceriodaphnia megops Sars. 
PLATE XLI, Fia. 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 striation which breaks up into reticulation only near the shell 
margins. This species seems to form the transition toward Simoceph- 
alus with Scapholeberis, which, however, diverges along its own pecu- 
