714 



FRESH-WATER BIOLOGY 



no (hi) Post-abdomen not bilobed. . M acrothrix laticornis (Junne) 1S20. 



Fig. 



t2. Macrothrix laticornis. 



Form round-ovate. Valves crested, 

 the dorsal edge serrate with fine teeth. 

 Head evenly rounded. Labrum with 

 large triangular process. Antennule 

 broader distally; a setiferous projec- 

 tion on posterior margin near apex; 

 anterior margin with several fine in- 

 cisions and clusters or rows of hairs; 

 olfactory setae conspicuously unequal. 

 Post-abdomen with numerous fine 

 spines and hairs; anus terminal. 

 Claws small. Color grayish white or 

 yellowish. Length, 9. 0-5-o-7 mm.; 

 $ , 0.3-0.4 mm. 



Widely distributed; found in all 

 parts of the country but nowhere very 

 abundant. 



Ill (iio) Post-abdomen bilobed ■. 112 



112 (113) Conspicuous fold or folds of shell of head at cervical sinus. 



Macrothrix montana Birge 1904. 



Form ovoid. Head large; dorsal margin evenly rounded; 

 the shell extended into collar-like folds in front of cervical 

 sinus. Antennules stout, large, enlarged near apex, about 6 

 anterior cross-rows of hairs, and 3-4 stouter posterior setae; 

 olfactory setae unequal. Post-abdomen bilobed. Claws hardly 

 larger than spines. Color transparent, in preserved speci- 

 mens. $ unknown. Length, 9 . ca. 0.55 mm. 



Colorado. 



Fig. 1 1 13. Macrothrix montana. 



113 (112) No such folds. Macrothrix hirsuticornis Norman and Brady 1867. 



Form broadly ovate, not very different from M. laticornis. 

 Antennules broad, flat, bent, varying in form but always enlarged 

 distally; with 6-8 rows of stiff hairs on anterior side; sometimes 

 stout setae on posterior side; olfactory setae unequal. Post-abdo- 

 men large, broad, bilobed; pre-anal part not flattened nor with 

 projection for abdominal setae; numerous small spines and hairs 

 on both anal and pre-anal parts. $ unknown. Length, 9 . o-55 

 mm. 



New England, Colorado. 



Fig. 1 1 14. Macrothrix hirsuticornis. 



