BY R. J. TILLYARD. 445 



sector of triangle i-eacliing back basally to slightly beyond the 

 basal postcostal nervule. 



(iii.) Psetidagrion, Cfriagrioii and allies. Wings still more 

 petioled, usually to about half-way between the two antenodals; 

 inferior sector of triangle reaching back basally only just to the 

 base of the basal postcostal nervule. 



(iv.) Telebasis and allies. Wings most petioled of all, the basal 

 postcostal nervule being closer to second antenodal than to first, 

 and the inferior sector of triangle reaching back not so far as, or 

 only just up to the base of the basal postcostal nervule. 



These four groups are roughly of coordinate value, and, subject 

 to a careful study of other characters, foi'm an excellent basis for 

 the subdivision of the legion. It will be noticed that (i.) and (iv.) 

 agree with de Selys' classification, but (ii.) and (iii.) do not.* De 

 Selys unfortunately elevated a unisexual character (the presence 

 or absence of the spine on the end of segment 8 in the female, 

 underneath) into a position of first-rate importance, and thus 

 obscured the obvious affinities between such genera as Agrion 

 and Ischnura, for instance. 



c. The form of the quadrilateral.— This is a useful check to 

 apply to our classification. In the least asthenogenetic forms, 

 the quadrilateral will be found to be most sharply pointed distally 

 (compare the Lestes quadrilateral), and very often this develop- 

 ment is especially marked in the forewing as compared with the 

 hind. In the asthenogenetic offshoots, the tendency is towards a 

 return to the normal quadrilateral, e.g., in Agriocnemis. 



d. The position of the median sector. — In those forms where this 

 vein approaches very nearly to the nodal cross-vein, we have a 

 valuable generic character, and one that is evidence of astheno- 

 genesis. 



e. The position of the second antenodal. — In the less reduced 

 forms, this will be found usually to be in line with the arculus. 

 Originally it was, no doubt, variable in position, but attained its 



* Selys divided the legion into three, main gionps, ^rgrici, .(4<7Wom (con. 

 taiuing ii. and iii.) and Ttlehafiis. 



