726 



FRESH-WATER BIOLOGY 



Form more elongated, as dorsal margin is little arched. 



Dunhevedia serrata Daday 1898, 



Usually 2 teeth at infero-posteal 

 angle, a very small posterior and a 

 larger anterior one. Keel of labrum 

 serrate in anterior part, smooth be- 

 hind; about 10-12 serrations, pointing 

 backward. $ unknown. Color yel- 

 low. Length, 9 1 ca. 0.7 mm. 



Louisiana, Texas; in pools and lakes 

 among weeds; not abundant. 



Fig. 113s. Dunhevedui serrata. a, labrum; 

 Z>, post-abdomen. 



18s (180) 



Infero-posteal angle without teeth, or tooth very small; rostrum long 

 or short. ... 186 



t86 (203) Claws with 2 basal spines. 



Pleuroxus Baird 1843 



187 



Rostrum long and pointed, rarely bent forward. Dorsal margin much arched; posterior 

 margin short, usually less than one-half height, rarely toothed along entire length; infero- 

 posteal angle rarely rounded, usually sharp and toothed. Keel of labrum large, usually tongue- 

 shaped; posterior angle prolonged. Post-abdomen with marginal denticles only, d smaller 

 than 9 > with usual characters; post-abdomen varies in different species. 



Three types of form are distinguishable in the genus: (i) relatively long and low species: 

 strialus type (P. striatus, hastalus, hamulatus); (2) short, high-arched forms: denticulaius 

 type {P. deniiculatus, aduncus, trigonellus, Iruncatus); (3) the second form with rostrum bent 

 forward: {P. procurvatus, uncinatus). All species littoral. 



187 (190) Rostrum bent up in front. ... . . . ... 188 



188 (189) Rostrum bent sharply into hook; teeth along whole posterior 



margin of valves. . . Pleuroxus procurvatus Birge 1878. 



General form and markings like P. deniiculatus. Posterior 

 margin of valves with 7-8 teeth along the whole length. 

 Post-abdomen like P. denticulaius but sUghtly more broad- 

 ened behind anus. cT unknown. Color yellowish, transpar- 

 ent or opaque. Length, 9 , ca. 0.5 mm. 



Northern states, common in weedy waters. 



Fig. 1136. Pleuroxus procurvatus. 



0.2 MM. 



189 (188) 



Rostrum merely curved forward; teeth at infero-posteal angle only. 



Pleuroxus uncinatus Baird 1850, 



Infero-posteal angle with 2-4 rather long, curved teeth, 

 sometimes branched. Rostrum long, acute, bent forward. 

 Post-abdomen Uke P. trigonellus, broad, somewhat tapered 

 toward apex; about 13 good-sized marginal denticles. Color 

 dirty gray, or with green or yellow tinge. Length, 9, 

 0.7-0.9 mm.; $ (European), 0.56 mm. 



Nebraska (Fordyce). The species is very close to P. 

 trigonellus, separated by procurved rostrum and large teeth 

 at infero-posteal angle. 



t>,rMM. 



Fig, 1137. Pleuroxus utKtnatus. European specimens. 



