730 FRESH-WATER BIOLOGY 

 208 (205) No spine at infero-posteal angle 



209 



209 (210) Valves with conspicuous projection on antero-ventral margin. 



' Anchistropus Sars 1862. 

 Sole American species . . . Anchistropus minor Birge 1893. 



Form globular. Ventral region tumid anteriorly 

 and ventral margin of valves bent sharply away from 

 each other about one-third way from front and the 

 valve folded out into a hollow groove and tooth, 

 which contains the strong hook of the first foot. 

 Head large, bulging over eye, the fornices broad and 

 forming a sort of flap-like rostrum, which can be 

 closely pressed to the valves. Post-abdomen broad 

 at base, pre-anal angle overhanging; rapidly narrow- 

 ing toward apex, which is prolonged into a lobe; a 

 few marginal spines. Claws with long, slender basal 

 spine, denticulate or smooth. First foot of 9 with 

 strong hook, toothed on concave side, which lies in 

 groove formed by folding of valves. 



In A . minor, groove for hook of first foot near 

 anterior part of valves; hook not large. Color brown- 

 yellow. $ unknown. Length, 9 , ca. 0.35 mm. 



Maine, Michigan, Wisconsin, Louisiana. 



FIG. 1146. Anchistropus minor. 



MM: 



210(209) No such projection 211 



211 (229) Post-abdomen ordinarily short with prominent pre-anal angle. 



Chydorus Leach 1843 . . 212 



Shape spherical or ovate. Posterior angles little marked; infero-posteal angle usually un- 

 armed. Antennules short and thick. Rostrum long and acute. Post-abdomen usually short, 

 broad, rarely long and narrow (C. globosus)', apex rounded; with marginal denticles only or 

 (C. globosus) with very fine lateral fascicles. Claws with 2 basal spines, the proximal often very 

 minute, rarely absent. $ with short rostrum, thick antennule, hook on first foot, post-abdomen 

 often very narrow. 



212 (213) Post-abdomen, long, narrow, Pleuroxus-like. 



Chydorus globosus Baird 1850. 



Almost spherical; valves smooth or reticulated, sometimes 

 striated in front. Post-abdomen with small pre-anal angle; 

 numerous marginal denticles and very fine lateral fascicles. 

 Claws with 2 basal spines, the distal very long and slender. 

 Color bright yellow to dark brown, usually with dark spot in 

 center of valve. Length, 9 > to 0.8 mm.; $ , 0.6 mm. 



Everywhere; in lakes and ponds, among weeds, but never 

 present in very large numbers. 



C. globosus might well be type of a separate genus. The other 

 species fall into 3 groups: (i) The sphaericus group or Chydorus- 

 proper (C. sphaericus, gibbus, piger, latus, ovalis); (2) The 

 favtformis group, similar to (i) but with greatly developed cutic- 

 ular structures (C. faviformis, bicornutus); (3) The barroisi 

 group, with toothed labrum; denticles of post-abdomen shortest 

 in middle of row (C. barroisi, hybridus, poppei). 



FIG. 1147. Chydorus globosus. 



