THE WATER FLEAS (CLADOCERA) 725 



171 (122) Posterior margin of valves considerably less than maximum height. 



172 



All species of Pleuroxus belong here; also Alonella. excisa. and exigua. 



172 (204) Body elongated, form not spherical I73 



173 (174, 17s) Lower part of posterior margin excised or crenulated. 



Alonella excisa, A. exigua . 244,245 

 Turn to the key at the numbers indicated where two species are discussed. 



174 (173, 17s) Posterior margin with numerous teeth along whole length. 



Pleuroxus procurvatus, P. truncatus . 188, 191 

 Turn to the key at the numbers indicated where two species are discussed. 



17s (173, 174) Teeth (if any) only at infero-posteal angle. . . . 176 



176 (179) Infero-posteal angle well marked, ordinarily with teeth. . . 177 



177 (178) Rostrum long. . . . Pleuroxus most sptds?, . 186 

 Take up the key at the number indicated where the genus is discussed as a unit. 



178 (177) Rostrum short . Alonella dcntifera . . 233 



Take up the key at the number indicated where the genus is discussed as a unit. 



N.B. If the rostrum is broad, semi-circular at end, see 167. 



179 (176) Infero-posteal angle rounded 180 



180 (185) With well-marked tooth or teeth 181 



181 (182) Rostrum long, recurved Pleuroxus striatus . . 195 



Turn to the key at the number indicated where the species named is discussed. 



182 (181) Rostrum short Dunhevedia King 1853 . . 183 



General shape roimded. Valves tumid, gaping below; obscurely reticulated; infero-posteal 

 angle rounded, with i or 2 teeth on ventral margin in front of angle. Post-abdomen bent 

 abruptly behind anus; post-anal part thick, somewhat foot-shaped as seen from side, its dorsal 

 (lower) margin lying parallel to ventral margin of valves; with many fine denticles and setae. 

 Claws short, curved, with i basal spine. $ with usual characters; post-abdomen same shape 

 as 5 , with fine hairs only. 



183 (184) Form short and high, as dorsal margin is much arched. 



Dunhevedia setigera (Birge) 1877. 



Keel of labrum produced into a somewhat tongue-Uke 

 form, its ventral margin smooth. Color yellow. Length, 

 9, to 0.5 mm.; cf, ca. 0.36 mm. 



New England and Wisconsin to Colorado, Louisiana, 

 and Texas. Not common; among weeds. Perhaps identi- 

 cal with D. crassa King. 



Fig. 1134. Dunhevedia setigera. 



aiMM 



