732 



219 (2i8) 



FRESH-WATER BIOLOGY 



Dorsal surface not flattened; form usually spherical or broadly 

 ovate. . . 220 



220 (223, 224) Small forms not exceeding 0.5 mm., usually less. . . . 221 



221 (222) Fornices gradually narrowing into rostrum. All olfactory setae on 



end of antennule. Chydorus sphaericus (0. F. Mtiller) 1785. 



Spherical or broadly elliptical. Shell usually reticulated, 

 sometimes smooth (var. nitidus Schoedler), sometimes 

 punctate (var. pundatuslltlWch), or with elevations (var. 

 coelalus Schoedler). Post-abdomen with 8-g marginal 

 denticles. Claws small; proximal basal spine very minute. 

 $ with post-abdomen much emarginate. Color light 

 yellow to dark brown. Length, 9>°-3~o-S mm.; $, 

 0.2 mm. Small limnetic forms constitute var. minor 

 Lilljeborg. 



The commonest of all Cladocera; found all over the 

 world. 



Fig, 1 151. Chydorus sphaericus. 



222 (221) Fornices abruptly narrowed into rostrum. Two olfactory setae 

 on side of antennule. . . . Chydorus piger Sars 1862. 



General form much like C. sphaericus. Ventral margin of valves densely ciliated; valves 

 ordinarily marked by oblique striae, sometimes smooth. Fornices abruptly narrowed at rostrum. 

 Antennule with usual lateral sense seta and two olfactory setae on side. Post-abdomen with 

 8-9 rather long marginal denticles. Claws with 2 basal spines, the proximal one minute. J 

 post-abdomen narrow, but not excavated. Color light to dark yellow. Length, 9 > '^- °'4 

 mm. Rare; reported only from Maine. 



i- 





0.1 MM 



Fig. IIS2. Chydorus piger. Entire specimen and 16wer side of rostrum with antennules, 



223 (220, 224) Larger forms, to 0.8 mm. Antennules short and thick with 

 all olfactory setae terminal. . . . Chydorus latus Sars 1862. 



Much like sphaericus, but larger. Mandible attached 

 some way back of junction of head and valve. Denti- 

 cles of post-abdomen 10-12. Claws sometimes with 

 only I basal spine. Color dark yellow-brown. Length, 

 9 . to 0.7-0.8 mm. 



Rare; Canada, near Lake Erie. 



FtG. 1153. Chydorus latus. 



0.1 MM. 



