254 GEOL. AND NAT. HIST. SUEVEY 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 strise 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. 



"* i 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 

 Ghydorus, 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 YERUS. 



Beak DOt curved forward. 



A. Shell reticulate liastatlis, 254; (?) strainineus, 255 



B. Shell smooth, except upon the front margin aduuctus, 255 



C. Shell striped. 



a. Shell very long and low. 



* With one tooth below gracilis, 257 



** Without a tooth; female with a hook upon the first foot, lianiatus, 257 



6. Shell high. 



* The whole posterior margin of the shell spined. . . . truncata, 258 

 ** The lower angle of the shell spined. 



t Antennae with eight setae; the anterior margin of the valves 



toothed denticulatus, 256 



1 1 Antennae with seven setae bairdii, 256 



Beak procurved (Rhypophilus). 



A. Shell reticulate (either strongly or quite faintly). . . . personatilS, 259 



B. Shell striped. 



a. The posterior margin of the shell toothed prociirvus, 258 



6. Only the lower angle of the shell toothed uncinatus, 259 



(Not noticed in the key trig-onelliis, 255) 



Pleuroxiis hastatus Sars. 

 Plate LXII, Fig. 16. 

 Pleuroxus Ivevis — Sars. 



Form somewhat oval, dorsal line strougly curved, posterior mar- 

 gin short, with a tooth below; head short, beak very long, straight- 

 ish; shell obscurely reticulate. Post-abdomen very long, narrow, 



