518 • 



(111. Dorsal nitTTfrinal setae of the median sill more than twenty in 

 fiill-gi'own nymijhs; only the proximal third of the thii'd antcn- 

 nal segment dark brown ; gills commonly 5.5-6 mm. in length. . 

 civile. 



ec. Lateral keel of the first abdominal segment with two to four 

 heavy setae; axis of the gills ojiatine or slightly smola*. 

 d. Dorsal setae of the median gill extending beyond the middle ; 

 mental setae four, the meso-caudal seta in each row one-half 



as loiiLT or nearly as Ions as the remaining setae. . ( ct/nfliifjerum. 



{ calverti. 



dd. Dorsal setae of the median gill not extending to the middle 



of the gill ; mental setae three, the meso-eaudal seta in each 



row representing a small fourth, but minute and always less 



than one-half the length of the three larger setae, carunculafum. 



aa. Gills with pigment other than in the traeheae. 



b. Tracheal branches of the gills in alga-like patches ; distance from 

 the caudal margins of the compound eyes to the eaudo-latcral angles 

 of the head greater than half the di.stance between the antennal 

 fossae. Abdomen without a median, ventral, black Hmc. 

 c. Dark portion of the base of the gills extending less than half 



their length (Fig. 55) traviatum. 



CO. Dark portion of the base of the gills extending more than half 

 their length. 



d. Length of the gills less than eight times the greatest width; 

 gills of mature nymphs with a prominent hinge just caudad 



of the middle (Fig. 53) cxsulans. 



dd. Length of the gills eight times the greatest width; gills of 

 mature nymphs without a prominent hinge caudad of the 



.middle (Fig. 54) antennatum. 



bb. Ti-acheal branches of tlie jjills not in alga-like ])atches ; distance from 

 the caudal margins of the compound eyes to the caudo-lateral angles 

 of the head less than half the distance lietween the antennal fossae. 

 Abdomen with a median, ventral, black line. 



c. Median gill witli a bunch of setae proximad of the first dark 

 transverse eross-liand : distal cross-bands of the same degree of 

 blackness as the proximal ones (Fig. 56) ; median gill not greatly 

 expanded distad of the first cross-band ; dor.sal setae of the apical 



margins of the abdomiiml terga not prominent signadim. 



CO.. Median gill without a bunch of setae proximad of the first dark 

 transverse cross-band : distal cross-bands faint, and lighter in 

 color than the proximal ones (Fig. 57) ; median gill much ex- 

 panded distad of the first cross-band ; dorsal setae of the apical 

 margins of the abdominal terga very prominent pollutum. 



