278 Annals Entomological Society of America [Vol. I, 



of the arculus only in Neophya and certain species of Neocordulia, 

 and in the latter, the anal loop is truncated on the tip and shows 

 a very slight apical widening. 



(2) On the other hand, the anal loop tends elsewhere to become 

 unsymmetrical on its broadly truncated apical end, and to de- 

 velop a long posterior angle toward the hind margin of the wing 

 (fig. 8, a, I). This is the most salient characteristic of William- 

 son's groups I and II, with Pentathemis added to group II. 



In endeavoring to trace the further cleavage of these groups, I 

 have deemed these characters of most importance: (i) the 

 primary differentiation between fore and hind wing; (2) the 

 manner of development of an anal loop; (3) the form of the 

 special braces formed at arculus, triangle and stigma, and (4) the 

 mutual adjustments of the principal branches of veins in the 

 outer field of the wing; and I have used such characters as the 

 presence or absence of crossveins, only when characters like 

 those above mentioned seemed to be wanting. I ha\x largely 

 copied Martin's arrangement of the genera of Williamson's group 

 I. The minor divisions of the key need to be checked by the 

 study of other characters. The venation of the Odonata is at 

 the present time receiving more than its share of attention. 

 There is great need of critical comparative study of other organs, 

 and there is especial need of more knowledge of the immature 

 stages. 



KEY TO THE GENERA OF THE CORDULHNAE, s. lat., OF THE WORLD. 



(Based on venational characters.) 



a) Anal loop compact, little longer than broad, without bisector, delimited 

 distally by a short straight basal segment of vein Cu2; tri- 

 angle of the forewing transversely elongate, and that of the 

 hind wing never retracted toward the arculus. Sub-f.\mily 

 MACROMIINAE. 

 b) Veins M3 and M4 straight or regularly arcuate behind the nodus, 

 c) Median (or basal) space traversed by crossveins; alternate ante- 

 nopals often hypertrophied or thickened Synthemis 



cc) Median space destittite of crossveins; anal loop considerably 



longer (in the axis of the wing) than broad Macromidia 



bb) Veins M3 and M4 distinctly undulate behind the nodus in both 

 wings, 

 c) Triangles and subtriangles of the forewings traversed by crossveins. 

 d) Vein Cu where it bounds the subtriangle of the forewing on the 



proximal side strong and straight Azuma 



dd) Vein Cu where it bounds the subtriangle of the forewing, weak 



and angulate among the cells Epophthalmia 



cc) Triangles and subtriangles of the forewings free from crossveins. 

 d) Triangle of the forewings followed by two rows of cells. . Macromia 



dd) Triangle of the forewings followed by a single row of cells 



Phyllomacronia 



