BY R. J. TILLYARD. 775 



two main types, represented at the present day by the two main 

 divisions of the uEschnine stem, viz., the Brachytronini (to 

 which Austroceschna belongs) and the uEschnini. Of these, the 

 jEschnini soon became dominant in all the regions of the earth 

 except the Australian, while the Brachytronini decreased rapidly 

 everywhere except in Australia, where (like the Marsupials) they 

 enjoyed an uninterrupted development, and increased to form the 

 large genus Austroceschna and its allies. Somewhat later, the 

 JSschnincB, spreading rapidly through the Neotropic Region, sent 

 out a few vigorous species down into Archiplata, and reached 

 across into a temperate Antarctica. Finally, a single species, 

 ^Eschna brevistyla, found its way into New Zealand, and also 

 into Tasmania, and crossed the Bassian Isthmus, while it was still 

 large and supplied with running streams. The fact that uEschna 

 brevistyla is so abundant in Tasmania, and becomes rapidly rarer 

 as we go northwards, finally failing to reach the extreme north of 

 the continent, is a strong argument in favour of this supposition, 

 and against the theory, held by Dr. Ris, of the origin of ^Eschna 

 brevistyla from a common parent with the tropical Anaciceschna 

 jaspidea. 



Next, let us turn to the very extraordinary distribution of the 

 Libellulinai of the region under survey. In Southern Victoria, the 

 only really common species are the three species of Diplacodes, 

 which are abundant everywhere along the coast and inland also. 

 Orthetrum caledonicum is abundant in the warmer parts, but gets 

 rare along the colder southern coast. Nannophya dalei and Aus- 

 trothemis nigrescens are distinctly rare. Yet, of all these species, 

 which breed equally freely in still water, only the two rarest occur 

 in Tasmania. We can only conclude that the commoner Diplacodes 

 and Orthetrum, both known to be offshoots from tropical genera, 

 did not reach their present southern limit until after the disappear- 

 ance of the Bassian Isthmus. It would follow, that Nannophya 

 and Austrothemis are much older genera, which were present in 

 Tasmania before the Isthmus disappeared. Nannophya is known 

 to be an archaic genus, with a somewhat discontinuousdistribution. 

 Austrothemis is a puzzle, having no very close allies, and only one 



