254 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
projection of the head-shield, are narrow. The form varies much, but. 
is almost always very strongly convex above, and the posterior margin 
is thus only a fraction of the whole height of the animal. In some 
American species the body is very much elongate, and these also 
depart from the characteristic habitus of the genus in having strong 
longitudinal striz instead of reticulations. The lower posterior shell 
angle has teeth which, in a few cases, extend across the entire posterior 
margin. The post-abdomen is slender, usually truncate and armed 
behind with a single set of sharp teeth on either side; the terminal 
claw has usually two spines and may be serrate. 
The male has a shorter beak, the post-abdomen is more or less modi- 
fied, and the first foot has a powerful hook. The winter eggs frequently 
have a true ephippium; and sometimes this structure is like that of 
Chydorus, toward which the round forms of this genus seem to lead. 
There are upwards of a dozen valid species, several of which are 
American. 
KEY TO PLEUROXUS VERUS. 
Beak not curved forward. 
A. Shellreticulate . . . . . . . hastatus, 254; (?) stramineus, 255. 
B. Shell smooth, except upon the front margin. . . . . . adunctus, 255. 
C. Shell striped. 
a. Shell very long and low. 
* With one tooth below. . . . .) gracilis, 257 
** Without a tooth; female with a ipa upon hie first foot. hamatus, 257 
b. Shell high. 
* The whole posterior margin of the shellspined. . . . truncata, 258 
** The lower angle of the shell spined. 
+ Antenne with eight sete; the anterior margin of the valves 
toothed. eee OM ~ eo. ee he a, Centiculatus; 256 
tf} Antennze ith seven es i hes Teak ey ae MSE ee bn, eed ee ee 
Beak procurved (Rhypophilus). 
A. Shell reticulate (either strongly or quite faintly). . . . personatus, 259 
B. Shell striped. 
a. The posterior margin of the shell toothed. . . . . . procurvus, 258 
b. Only the lower angle of the shell toothed. . . . . . Uuncinatus, 259 
(Not noticed inthekey. . ....... . . . trigonellus, 255) 
Pleuroxus hastatus Sars, 
PLATE LXII, Fie. 16. 
Pleurozus levis—Sars, 
Form somewhat oval, dorsal line strongly curved, posterior mar- 
gin short, with a tooth below; head short, beak very long, straight- 
ish; shell obscurely reticulate. Post-abdomen very long, narrow, 
