THE WATER FLEAS (CLADOCERA) 729 



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

 abdomen of male broadened in middle of posl-amd tiari 

 with crescentic dorsal margin. 



Pleuroxiis trigoncllus (O. F. MuUcr) 17S5 



Form of P. dentUulatui tvi>r Infrro- 

 postcal anRlc with 2 or \ .small trrth. ..ftm 

 minute, sometimes wantiriK I'mt aUlo- 

 mcn much as /'. drntuulatut; but i\ui^\ 

 mar^'in slightly convex. I.roadcr Jx-hind 

 anus; ai^x rounde*!; 14-16 marKifwd licn- 

 tides, longer toward apex, hut not <liv 

 tinctly clustered there. ^ fxrt aMnmm 

 is characteristic; hroadene*! ' 

 into a semi-elliptical plate, t 

 set hairs, no spines; jrreatly ; 

 ward aiK'X. forming' a slender prolan. 

 Color yellowish, transparent; j*.-: 

 men often dark. Lcn^h. 9 . o <> "im . 

 $ , 0.4 mm. 



Not common; Maine, \Vi.vx)n<un, Ne- 

 braska; doubtless widely dislribute<l. 



Fig. 1 144. PUuroxus lrif(meUus 



Pa8t-»l«.J<>m«j: 



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

 post-abdomen not crescentic. 



Pleuroxus admicus (Jurinc) iSio. 



9 very closely resembling P. triionrUut. 

 but differing as follows: valves striatc<l; in- 

 fero-posteal angle usually without teeth. iN^st- 

 abdomen shorter, the length of post -anal p*rl 

 hardly e.xceeding anal emargination; dors*! 

 margin slightly arched, with u manorul 

 denticles; ape.\ rounde<i. <J i>ost-alxlomen 

 very diflerent from P. tri^ontUns: narrower 

 than 9. tajx'red toward claws; no dorul en- 

 largement or apical prolongation. Color born- 

 yellow, sometimes t)|)a(iue. Length. 9. ca. 

 0.6 mm.; ^ , ca. 0.45 mm. 



Colorado, California. Among weeds or in 

 pools. 



Fig. 11.(5. Plfuroxus aJumiiu. 



0.1 MM. 



203 (186) Claws with i basal spine [lonilln (most six-cics) . . 230 



Take up the key at the number indicated where one sul)genus is discus.seii as a irniL 



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 • Monelhi mimi . . ^4-' 



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



207 (206) Valves reticulated or not plainly marked. ChyJorus barnyUi 



Chydorus hyhruius . . 2 it, 337 

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



