728 



FRESH-WATER BIOLOGY 



196 (193) Post-abdomen of moderate length; ventral (upper) margin straight, 



or nearly so; greatest width behind anus 197 



197 (200) Angle of post-abdomen sharp, with cluster of spines at apex. . 198 



198 (199) Teeth at infero-posteal angle of valves; no hook on first foot 

 of female Pleuroxus denticulatus Birge 1877. 



0.1 MM. 



Infero-posteal angle with small tooth- 

 like spines. Post-abdomen moderately 

 long, straight, very little narrowed toward 

 apex; length of post-anal part 1.5 times, 

 or more, that of anal emargination; apex 

 truncate; with cluster of fine, straight 

 denticles at apex and 8-12 anterior to 

 these. Color greenish or yellowish, usu- 

 ally transparent. Length, 9 0.5-0.6 

 mm.; ,0.36 mm. 



Common everywhere in weedy water. 



FIG. 1141. Pleuroxus denticulatus. 



199 (198) Infero-posteal angle rounded; first foot of female with stout hook. 



Pleuroxus hamulatus Birge 1910. 



Infero-posteal angle rounded, without teeth; valves reticulated; also marked by very fine 

 striae, which run nearly longitudinally. Rostrum long, recurved. Keel of labrum small, 



rounded, prolonged. Post-abdomen much 

 like P. denticulatus, but with apex more 

 rounded and denticles not so crowded there. 

 Denticles about 1 2-14. unknown. Color 

 horn-yellow, often dark on dorsal side like P. 

 striatus. Length, 9 ca. 0.6 mm. 



New England and southern states; prob- 

 ably a coastal form; not reported from north 

 central region. Common in pools and 

 weedy waters. 



FIG. 1142. Pleuroxus hamulatus. a, first foot; 

 b, post-abdomen. 



O.I MM. i 



FIG. 1143. Pleuroxus hamulatus. 



200 (197) Angle of post-abdomen rounded 201 



