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 *73 



173 (174, 175) 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, 175) Posterior margin with numerous teeth along whole length. 



Pleuroxus procurvatus, P. truncatus . . 1 88, 191 

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



175(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 species . . 186 



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



178 (177) Rostrum short. Alonella dentifera . . 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 rounded. 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 9 > w ^h 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-like 

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

 9, to 0.5 mm.; <f, 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. 



0.1 MM 



