AMERICAN ODONATA. 215 



These characters furnish the basis for the following key : 



1. Second branch of anal vein not distinctly angulate wliere the vein between 



the first and second postanal cells meets it, if this vein meets it at all 

 (Figs. l-3a, pi viii, species 1-16 inclusive; not variable). 

 1'. Second branch of anal vein not forming any of the boundary of the first 

 postanal cell (Figs. 1-2, species 1 and 2, and 4 16 inclusive; variable). 



1''. Only first postanal cell not divided tFigs. 1-lc, species 1 and 4-16 in- 

 clusive; variable). 



2". First and second postanal cells not divided (Figs. Id, 2, not normal 

 in any species studied ; see species 12 and 13). 



3". First, second and third postanal cells not divided (Fig. 2, species 2 ; 

 not variable). 

 2'. Second branch of anal vein forming part of the boundary of the first 

 postanal cell (Figs. 3, 3(«, species 3). 



1". Only first postanal cell not divided (not normal in any species studied ; 

 see species 5-7, 9-11 and 13). 



2". First and second postanal cells not divided (Fig. 3, species 3; variable). 



2. Second branch of anal vein distinctly angulate where the vein between the 



first and second postanal cells invariably meets it (Figs. 4-6, species 



17-29 inclusive ; not variable^ 

 1'. Portion (s. e.) of anal vein bounding first postanal cell longer than the 



portion (i. e) of the second branch of the anal vein bounding the 



same cell (Figs. 4-5a, species 17-27 inclusive ; not variable). 

 1". Second postanal cell divided (Figs. 4-4a, species 17; variable). 

 2". Second postanal cell open (Figs. 5-5(1, 18-27 inclusive; variable). 

 2'. Portion of anal vein bounding first postanal cell shorter than the portion 



of the second branch of the anal vein bounding the same cell (Fi|;. 



6, species 28 and 29 ; not variable). 



On the basis of the ratio of the number of wings like figs. 1 and 

 la, plate viii, to the number like figs lb and Ic, species 1, and 4-16 

 are arranged as follows : pallidus, 2-0 ; quadricolor, 5-1 ; descriptus, 

 8-2 ; furcifer, 3-1 ; exilis, 60-33 ; brevk, 91-49 ; graslinellus, 19- 

 11 ; spicatus, 19-13 ; minutus, 4-3 ; sordidus, 9-9 ; abbreviatiis, 9- 

 16; pulchellus, 2-4; militaris, 0-2; viridijroyis, 0-5. 



On the basis of the ratio of the number of wings like fig. 1 (and 

 Irt, lb, Ic and Id), plate viii, to the number like fig. 3 (with s. pa. 

 divided or not divided), species 1, and 4-16 are arranged as follows : 

 spicatus 32-0 ; abbreviains, 26-0 ; furcifer, 4-0 ; viilitaris, 2-0 ; jjal 

 lidiis, 2-0 ; graslinellus, 30-1 ; exilis, 93-7 ; minutus, 7-1 ; quadri- 

 color, 6-1; brevis, 145-28; sordidus, 18-4; pulchellus, 6-2; de- 

 scriptus, 10-5 ; viridifrons, 5-3. 



On the basis of the ratio of the number of wings like fig. 3, plate 

 viii (with s. pa. divided), to the number like fig. 3 (with s. pa. not 

 divided), species are arranged as follows : exilis, 7-0 ; sordidus, 4-0 ; 



TRANS. \M. ENT. SOC, XXVII. MAY, 1901 



