BY R. J. TILLYARD. 777 



Let us next consider the ease of the Synthemina, of which the 

 two still-water species, S. eustalacta and S. macrostigma, are repre- 

 sented in Tasmania, while the running-water species of the genus 

 Metathemis do not occur there. The typical form of S. tasmanica 

 discovered by me at St. Patrick's River, differs very considerably, 

 both in size and colouration, from S. eustalacta. But I also found, 

 at Launceston, breeding in still-water, a form whose colouration 

 was almost exactly that of S. eustalacta, and whose size was inter- 

 mediate between the latter and typical S. tasmanica. We see, 

 therefore, that S. eustalacta crossed over as a still-water form, but 

 that it is now developing into a running-water species on the island, 

 and is assuming, with the change of habit, the darker and duller 

 colouration already attained by Metathemis on the mainland. S. 

 macrostigma, on the other hand, still keeps to the swamps, and 

 shows no variation from the mainland form. 



The Synthemina are admittedly the most archaic of the Cordu- 

 liince. There can be little doubt that they attained their present 

 group-characters at least as early as the beginning of the Tertiary 

 period. It seems probable, therefore, that they may have crossed 

 to Tasmania during the very earliest part of the . Pre-Isthmian 

 period (the period of Howitt's "first elevation"). Such a supposi- 

 tion is in keeping with their comparative rarity on the island, if 

 we see in all the other still-water Libellulidae a more recently 

 arrived band of competitors. 



Metathemis is a specialised offshoot of Synthemis, and only 

 breeds in running water. The fact that this genus keeps to the high 

 lands, and only approaches the coast where it is very hilly, pre- 

 vents us from using it as an argument for the absence of running 

 water on the land-connection during Howitt's "first elevation" 

 period. 



Summing up, we now have the following facts : — 

 (l)Of the running- water forms, only 22% have succeeded in 

 passing from Victoria to Tasmania. These consist of the most 

 archaic forms found in the island (Austroceschna, JEschna). 

 Hence only during the earliest period of the land-connection (Pre- 

 Isthmian) was there sufficient running water for the passage of 

 such forms. 



