4 E- B. Williamson 



Dr. Calvert also turned over to me his notes and keys on American species 

 of these genera, loaned me some otherwise unavailable literature, and has 

 given me valuable advice and criticism. Through Air. Herbert Campion I 

 have been able to study one of the males in the British Museum, determined 

 by Kirby as G. subviridis. Through Miss Bertha P. Currie. I have obtained 

 the material of these two genera in the United States National Museum 

 (abbreviated as U. S. N. M. in the text). Mr. W. T. Davis (abbreviated 

 W. T. D. in the text) has kindly loaned me all his material in these genera. 

 In the same way, Mr. Hugo Kahl of the Carnegie Museum (abbreviated 

 Carn. Mus. in the text) has loaned me all the specimens of these genera in 

 the collections at Pittsburgh. Through Dr. C. H. Kennedy I have obtained 

 material belonging to the Ohio State University (abbreviated O. S. U. in 

 the text). And Professor Needham has loaned me the specimens in the 

 collections at Cornell. To Dr. Ris I am indebted not only for the loan of 

 material without which this paper would be much more incomplete than it 

 is, but he gave me permission to describe one species which he had already 

 detected as new in his collection, and he also kindly loaned me some litera- 

 ture and has furnished me with various notes. Mons. Severin kindly 

 sent me Martin's types of G. satyrus, and the specimens labelled triHda in de 

 Sely's collection. These specimens are discussed in detail on pa^es 8-11. 

 A review of the pages which follow will show how incomplete my study 

 would have been without the unselfish cooperation of all these friends. Dr. 

 Kennedy made all the drawings for this i)aper, and Miss Alina L. Winslow 

 all the photographs of the wings. 



The GenKra Triacanthagyna and Gynacantha 



I have not given the question whether the xA.merican species discussed 

 in this paper should be included in one, two, or three genera any particular 

 study and I have no well founded opinion in the matter. ]\Iy knowledge of 

 related species in other regions than the American is very limited, and. of 

 the American species aratrix, for which Forster erected Selysiophlebia, I 

 have seen no specimens. However, in the key to species which follows I 

 have indicated at least some characters by which two groups, previously 

 recognized by other authors, may be rather satisfactorily defined, and I am 

 considering these groups as genera. As to the relationships of these two 

 groups with old world groups, as I have stated, I have no opinion. In Am- 

 erica I think two genera exist, regardless of their world relationships and 

 the names by which they should be known. And Selysiophlebia remains to 

 be studied. A factor that seems to me to have weight in this decision is 

 the fact that in Triacanthagyna two groups have developed, somewhat paral- 

 leling groups in Gynacantha. In Triacanthagyna we have the paler septima 

 with its unconstricted abdomen and the darker brighter trifida group with 

 the male abdomens constricted. In Gynacantha, somewhat resembling the 

 septima group of Triacanthagyna, we have several species of the nervosa 

 group, while a number of darker and more brilliant species may represent 

 the trifida group. At the Fame time it is not implied that Triacanthagyna 

 has the wealth of specific adaptions which we find in Gynacantha. The two 



