NO. 2000. TWO FOSSIL INSECTS— COCKERELL. 581 



Triangles with not more than five cells, or when rarely with six, two pairs, the 

 formula 2, 2, 1, 1 11 



10. Anal loop of seven cells Gynacaniha trifida. 



Anal loop of twelve cells Gynacantha from Siam [Williamson photo.]. 



11. M3 and M4 separated by one cell only at margin of wing, but a short distance 



before by two rows of cells, owing to the deflection of M4 from the straight 

 course; cell formula of triangles 2, 1, 1; upper branch of radial sector in a line 

 with stem; Rs separated from supplementary vein below it by only three rows 

 of cells; fork of Rs a short distance before level of beginning of stigma (sub- 

 genus Hesperaeshna, new subgenus) Aeshna californica. 



Mg and M^ separated by at least two cells at margin of wing, so that they are almost 

 or quite as far apart here as at any part of their course; triangles with five cells, 

 at least in anterior wings; Rs separated from supplementary vein below it by 

 four rows of cells, at least at widest part (mainly three rows, with four only at 

 one plate, in Ae. solida) 12 



12. Anal loop jn hind wings very large, with about 15 cells; stigma comparatively 



short Aeshna q/anea [Williamson photo.]. 



Anal loop in hind wings not so large 13 



13. Branches of Cu below triangle separated by a single row of cells for the most part, 



but a few paired cells present, the number variable; upper branch of radial 

 sector making a strong angle with the stem, but the lower branch nearly in a 



straight line with the stem Aeshna solida [fossil at Florissant]. 



Branches of Cu below triangle separated through most of their course by two 

 rows of cells Aeshna eremita. 



Note on Aeshna. — I am unable to find any satisfactory generic characters in the 

 venation to separate Aeshna from Gynacantha. According to E. M. Walker's phylo- 

 genetic tree, Gynacantha is deiived from Aeshna. The number of cells in the triangle 

 in Aeshna varies within specific limits, as was fully shown in American Journal of 

 Science, vol. 26, 1908, p. 74. Walker, in his revision of North American Aeshna, 

 finds the following venational characters valid for groups: 



Anal triangle of males three-celled 1 



Anal triangle of males two-celled; females with fork of Rs decidedly asymmetrical 



at base Cserulea, juncea and clepsydra groups. 



1. Females with fork of Ra nearly symmetrical at base. 



Californica and multicolor groups. 

 Females with fork of Rs decidedly asymmetrical at base Cyanea group. 



For a further discussion of Aeshna venation see Entomological News, Dec, 1908, 

 pp. 455-459.' 



(B) Basal space with cross veins. 



( a ) Radial sector not branched; branches 0/ media leaving arculus at or near middle. 

 Radial sector separated from supplementary vein below it by two rows of cells; stigma 



bounding more than three cells below; triangle of anterior wings with tliree 

 cells Boyeria irene. 



(b) Radial sector branched. 



1 An elaborate table, contrasting the venation of Aeshna, Coryphxschna, Episeschna, Planxschna, and 

 Nasixschria, is given by Williamson in Entomological News, 1903, pp. 5-7. Episeschna has a curiously 

 intermediate, or as it were undecided, venation. Run in the table above, it goes to the vicinity of Hes- 

 perseshna, but with difficulty, because (1) Rs is separated from supplementary vein below it by a partially 

 double row of cells; triangles have five cells; (2) M4 is separated from supplementary vein below it in 

 hind wing by only about three triple cells, the rest being double; (3) the triangle of hind wing has a simple 

 basal cell in Williamson's figure; (4) M3 and M< are separated by one cell at margin of wing, but although 

 there is some doubling higher up in hind wing, Mt is not deflected out of its course. Williamson states 

 that the basal cell of triangle in Epixschna is divided in front wing, and rarely divided in hind wing. He 

 also states that the basal cell of the triangle is very rarely divided in Coryphxschna. 



