08 



THOMAS SAY FOUNDATION 



Genus ISOGENUS Newman. 



1833. Isogenus Newman, Km. May., 1:415. 

 1841. Nephelion Pietet, Perlides, p. 144. 



Rather large concolorous grayish stoneflies of north- 

 ward distribution. The prothorax is rectangular with a 

 median yellow stripe. The second segment of the antennae 

 is nearly as long as the first, and succeeding segments are 

 all short, those of the tip attaining but half the length 

 of the basal segment. The ocelli 

 are very minute and widely sep- 

 arated. The middle segment of 

 the tarsus is half as long as the 

 basal segment and about a fifth 

 as long as the third segment. The 

 portion of the wings beyond the 

 cord is long. The subcosta ends 

 at or shortly before the cord. The 

 fork of the median vein is narrow 

 and its tips curve posteriorly at 

 the margin, while the numerous, 

 branches of the radial sector ar- 

 ranged pectinately on the rear- 

 ward side, tend to curve rather 

 strongly at their tips in the op- 

 posite direction (forward). The 

 middle abdominal segments bear 

 large cushion-like pads in pairs 

 upon the dorsal side. The 9th 

 sternite is little prolonged into a 

 subgenital plate. Segment 10 is 

 cleft above for the accommoda- 

 tion of a retractile U-shaped 

 supra-anal process, and the rear 

 angles beside the cleft are pro- 

 duced and recurved into upturned Fig.13. Isogenus colubrinus 

 obtuse, spinulose tipped genital Hag< 



hooks. 



The three species herein included are distinguished 

 from the species of Perla next following by very slight 

 characters. We retain the name in deference to long 

 usage. 



Genotype, Isogenus mbecula Newman (European). 



