778 STUDY OF THE ODONATA OF TASMANIA. 



(2) Of the still-water forms, 79% have passed over. These 

 include all except the most csenogenetic genus of the jEschnince 

 (Anax), and the more recent genera of the Libellulidce (Dip- 

 lacodes, Orthetrum, Hemicordulia). Hence, for a very long period 

 of time, probably during the whole of the Isthmian period, there 

 was not sufficient running water on the isthmus to allow of the 

 passage of running-water forms; but there was a good supply of 

 still water, by means of which an abundant migration of still-water 

 forms flourished. This conclusion will be seen to support, very 

 strongly, Mr. Hedley's contention for a narrow Eastern Isthmus. 



Let us now mvert the problem, and classify our genera on the 

 evidence before us. We may divide them into three groups: — 

 (l)Pre-Isthmian genera. 



(a) Running-water forms that passed over in Pre-Isthmian 

 times : — Austroceschna, jEschna brevistyla (this latter most pro- 

 bably passed from Tasmania into Victoria). 



(b) Still-water forms that passed over during the period of "first 

 elevation" : — \Synthemis. 



(2) Isthmian genera. 



(a) Running-water forms that failed to get across: — Diphlebia, 

 Argiolestes, Synlestes, Austrolestes cingulatus, Nososticta, Iso- 

 sticta, Austrogomphus (except A. guerini), Metathemis(1), Hemi- 

 cordulia australice. 



(b) Still-water forms that succeeded in crossing: — Austrolestes 

 (A. leda, A. annulosus, A. psyche, A. analis), Agrion, Ischnura, 

 Austroayrion, Austrogomphus gue'rini, Procordulia, JVannophya, 

 Austrothemis. 



3. Post-Isthmian genera. 



Still-water forms that have failed to cross: — Anax papuensis 

 Hemicordulia tau (very recent migration only), Diplacodes, Orthe- 

 trum. 



The above classification, though it fails to give us any exact geo 

 logical age in which to place the arrivals of the various genera into 

 the area in question, is still of great value in exhibiting the com- 

 parative ages of the different groups, as shown by their arrivals 

 at their southern limits of distribution. 



