768 STUDY OP THE ODONATA OF TASMANIA, 



to divide the era under discussion into three parts, which I shall 

 name and define as follows: — 



A. Pre-Isthmian. — All that period of time, including the early 

 Tertiary period of elevation (Howitt), the period of submergence, 

 and the final re-elevation, up to the formation of a broad land-con- 

 nection, with running rivers, between Tasmania and the mainland. 



Fig.l. — Pre-Isthm[an. 



Fig. 1 gives a hypothetical sketch of the coast-line towards the close 

 of this period. 



B. Isthmian. — The period during which a gradual narrowing 

 of the connection took place, resulting in the formation of the nar- 

 row Isthmus on the east side, as suggested by Hedley(Fig. 2), and 

 up to its final submergence. 



