THE WATER FLEAS (CLADOCERA) 



729 



201 (202) Series of marginal denticles longer than anal emargination; post- 



abdomen of male broadened in middle of post-anal part 

 with crescentic dorsal margin. 



Pleuroxus trigonellus (O. F. Muller) 1785. 



Form of P. denticulatus type. Infero- 

 posteal angle with 2 or 3 small teeth, often 

 minute, sometimes wanting. Post-abdo- 

 men much as P. denticulatus; but dorsal 

 margin slightly convex, broader behind 

 anus; apex rounded; 14-16 marginal den- 

 ticles, longer toward apex, but not dis- 

 tinctly clustered there. $ post-abdomen 

 is characteristic; broadened behind anus 

 into a semi-elliptical plate, bearing thick- 

 set hairs, no spines; greatly narrowed to- 

 ward apex, forming a slender prolongation. 

 Color yellowish, transparent; post-abdo- 

 men often dark. Length, j, 0.6 mm.; 

 J , 0.4 mm. 



Not common; Maine, Wisconsin, Ne- 

 braska; doubtless widely distributed. 



Fig. 1144. Pleuroxus trigonellus. Post-abdomen: 

 a. 5; b, $. 



202 (201) Row of marginal denticles about equals anal emargination; male 



post-abdomen not crescentic. 



Pleuroxus aduncus (Jurine) 1820. 



5 very closely resembling P. trigonellus, 

 but differing as follows: valves striated; in- 

 fero-posteal angle usually without teeth. Post- 

 abdomen shorter, the length of post-anal part 

 hardly exceeding anal emargination; dorsal 

 margin slightly arched, with 9-12 marginal 

 denticles; apex roimded. $ post-abdomen 

 very different from P. trigonellus; narrower 

 than 9 , tapered toward claws; no dorsal en- 

 largement or apical prolongation. Color horn- 

 yellow, sometimes opaque. Length, J , ca. 

 0.6 mm.; $ , ca. 0.45 mm. 



Colorado, Cahfomia. Among weeds or in 

 pools. 



0.1 MM. 



Fig. 1145, Pleuroxus aduncus. 



203 (186) Claws with i basal spine. . . . Alonella (most species) . . 230 

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



204(172) Body spherical or broadly ellipsoidal 205 



205 (208) Well marked or small spine at infero-posteal angle 206 



206 (207) Valves conspicuously striated Alonella nana . . 242 



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



207 (206) Valves reticulated or not plainly marked. Chydorus barroisi 



Chydorus hyhridus . . 226, 227 

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



