726 



FRESH-WATER BIOLOGY 



^■--^^i 



184 (183) 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 . ca. 0.7 mm. 



Louisiana, Texas; in pools and lakes 

 among weeds; not abundant. 



Fig. 1 135. Dunhevedia serrata. a, labrum; 

 h, post -abdomen. 



0.1 MM. 



185 (180) 



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

 or short 186 



186 (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, cf 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: 

 striatus type (P. striatus, hastaius, hamulatus); (2) short, high-arched forms: denticulatus 

 type (P. denticulatus, aduncus, trigonellus, truncatus); (3) the second form with rostrum bent 

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



187 (190) Rostrum bent up in front 188 



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

 margin of valves Pleuroxus procurvatus Birge 1878. 



General form and markings like P. denticulatus. Posterior 

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

 Post-abdomen like P. denticulatus but slightly more broad- 

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

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



Northern states, common in weedy waters. 



Fig. 113b. Pleuroxus procurvatus. 



0.2 MM. 1. 



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 like 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>4MM. 



Fig. ii37« Pleuroxus uncinatus. European specimens. 



