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. 1 146. Anchistropus minor. 



210(209) No such projection 211 



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



Chydoriis l^tdiCh. i^^2> ■ ■ 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-\ik.t. 



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, lalus, ovalis); (2) The 

 faviformis 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. 



