420 DISCUSSION. 



(A) — Barrier, Bass Straits.* 



Synthemis eustalacta Burm.(S.A., Vic, N. S.W.). 

 ,, tasmanica Tillyard(Tas.). 



{ 



(B) — Barrier, Desert-Belt between E. and W. Australia. 



( Nannodythemis dalei Tillyavd(East Australia and Tas.). 



\ „ occidentalis Tillyard{W. A.). 



( Procordulia jacksoniensis Rarab.(Vic. and Tas.). 



\ „ affinis Selys(W.A.). 



{Austrogomphus a^istralis Selys(S.A. ). 

 „ collaris Selys( VV.A.). 



( Austroceschna parvistigma Selys(East Australia and Tas.). 

 \ „ anacantha Tillyard(W. A.). 



f Argiolestes grisea Selys(Vic. and N. S.W.). 

 \ ,, minima Tillyard(W.A.). 



(Pseudagrion cyane SeIys(East Australia and Tas.). 

 \ „ cceruleum Tillyard(W.A.). 



(C) — Barrier, Southern Pacific. 



{ Phyllopelalia patricid Tillyard, N. S. W. 

 1 [Phyllopetalia and allied subgenera. Chili]. f 



Of these eight pairs, no less than six indicate the desert-belt 

 between Eastern and Western Australia as the determining 

 barrier. It is interesting, therefore, to enquire what proportion 

 of the total Odonate fauna is represented by "geminate " pairs. 

 There are 27 species so far recorded from Western Australia. 

 Of these, one has no close allies; two others belong to a distinct 

 Australian genus, but are not closely allied to any of the Eastern 

 species of that genus; no less tha^i eighteen occur on both sides of 

 the barrier without the slightest modification. The proportion 

 of geminate species, then, in the West Australian Odonata is 

 only two-ninths, or 22 %. About the same number of species 

 occur in the area lying east of the barrier, including the southern 



* See also Hedley, on the " Bassian Isthmus," Proc. Linn. Soc, N. S. 

 Wales, xxviii., 1903, p.876. 



t An interesting barrier, Torres Straits, separating Australia from New 

 Guinea, is not here discussed, owing to lack of data. 



