774 STUDY OP THE ODONATA OF TASMANIA, 



exactly that one species, be it noted, that is unable to breed in still 

 water. Again, out of three species of Austrogomphus, only A. 

 guerini, which habitually prefers slowly running water, and occa- 

 sionally breeds in still water, occurs in Tasmania. And again, out 

 of two species of Hemicordulia, that one (H. tau), which breeds in 

 still water is present in Tasmania, while the equally common H 

 anstralice, which breeds only in running water, is absent. 



The evidence, afforded by the above facts, seems to me to point 

 conclusively to the existence, for a very considerable period, of a 

 Bassian Isthmus so narrow or incomplete, that only still-water 

 species were able to pass across it into Tasmania. No permanent, 

 running streams could have been present during the time that these 

 migrations were in progress, or, at the best, they must have been 

 very few and far apart. 



Next let us examine the exceptions to the general rule as pre- 

 sented above. 



Firstly, the species of the genus Austroceschna are all running, 

 water forms, yet three occur in Tasmania, and one of these is 

 peculiar to the island. Coupled with this, is the fact that the 

 running- water species, JEschna brevistyla, is also abundant there. 



The answer to this anomaly, lies in the admittedly great an- 

 tiquity of the jEschnince. As these genera occur on both sides of 

 the present barrier quite abundantly, and their larvae cannot breed 

 in still water (that of jEschna brevistyla prefers slowly running 

 water, but all the species of Austroceschna require fairly fast } 

 running water), it seems fair to argue, that their appearance on 

 the scene took place at an earlier period than that of the other 

 groups in question, at a time when the Bassian Isthmus was large 

 and well-supplied with running streams. The fact that Tasmania 

 also possesses, in the undescribed species of Austroceschna* its 

 only truly autochthonous species, points to the greater antiquity of 

 this genus, compared with those whose species have remained 

 undifferentiated. 



I am of opinion that both A ustroceschna and jEschna arose from 

 a common Mesozoic ancestor, which, first of all, differentiated into 



* The description of this species will shortly be published. 



