ArT. 21. DRAGONFLIES OF BURMA AND LOWER SIAM—LAIDLAW. 3 
the sternite to be used as accessory organ of oviposition, called for 
convenience the ‘“‘dentigerous plate.’”? It is found in a relatively 
undifferentiated and presumably primitive condition in such genera 
as Anaz and Aeschna, and attains its highest development in Gyna- 
cantha where it has become a remarkable forklike structure, the two 
prongs of the fork having a length in most species of somewhere 
about 1 mm. Now, Periaeschna, a genus belonging to the Brachytron 
series, has its dentigerous plate almost identical with that of Gyna- 
cantha; not, it is true, quite so highly developed but still showing an 
identical plan. And further, in Cephalaeschna, also a member of the 
Brachytron series, we find a still less developed type of the same 
structure, again designed on the same ground plan. 
So, were we to use this character as our sole criterion of relation- 
ship, we should be justified in regarding the three genera Celphal- 
aeschna, Periaeschna, and Gynacantha, as representing stages of 
specialization along a single line of descent; and we could reasonably 
argue that Gynacantha had arisen from a different stock to that which 
has given rise to Aeschna and Anaz and to their more immediate 
allies. ; 
But this would of course imply that Gynacantha on the one hand, 
and Aeschna (and Anaz) on the other, had developed independently 
an almost identical system of venation. In any case we are con- 
fronted with a dilemma from which I can see no way of escape. 
Hither Periaeschna and Gynacantha have developed independently 
an almost identical dentigerous plate, or Gunacantha and Aeschna 
have evolved an almost identical system of venation by convergence. 
It is evident that some other character or combination of char- 
acters must be utilized to provide sufficient evidence. Such may be 
found ultimately in the genital structures of the male, in larval char- 
acters, or elsewhere. The necessary material, time, and knowledge 
are, I regret, not available for me. 
EXPLANATION. 
In the following account of the oriental genera I employ the Com- 
stock-Needham nomenclature for the venation. Mr. Williamson’s 
diagrammatic figure of the wing of a dragonfly of the family Aeschnidae 
published in the second paper of this series (p. 270, figs. 1-3) is a 
sufficient guide to the terms employed in the present account. 
It will be noticed that the number of specimens of Aeschnines in 
the collections before me is remarkably small, when compared with 
the great numbers of Libellulinae and Coenagrioninae. Further no 
examples of the Brachytron group are included, so I am compelled to 
deal in a very general fashion with those genera, and to make my 
account of the subfamily a sketch of the oriental members thereof, 
rather than a full or critical account of tbe species.regret, I this 
60466—23—Proc.N.M.vol.62——46 
