BY R. J. TILLYARD. 893 



with distinct Cicadid characters in their venation might be ex- 

 pected to have existed somewhere in the Trias, but apparently 

 neither in Australia nor in the Northern Hemisphere. 



It would appear proved that the Homoptera became differen- 

 tiated from a single Palseohemipterous stock, of which Prosbole 

 is a representative, in the Middle or Upper Permian. From 

 the same stock the Heteroptera became separated off at a some- 

 what later period, the oldest true Heteroptera known being the 

 Dunstaniidae from the Upper Trias of Ipswich. Thus the 

 Homoptera are older than the Heteroptera, as is evident on 

 morphological as well as palasontological grounds. The separa- 

 tion of the Sternorrhyncha from the Auehenorrhyncha must 

 have taken place before the Upper Trias. The oldest existing 

 family of the Auehenorrhyncha appears to be the Jassidae; the 

 oldest of the Sternorrhyncha are the Psyllidae. Venationally, 

 but not in all other characters, the Jassidae are the older of 

 these two. It may also be pointed out that, after the Jassids 

 became differentiated out, the old main stem of the Auehenorr- 

 hyncha went on, and continued to be represented by many forms 

 which, in certain directions, still preserved archaic characters 

 which the Jassids had lost (e.g., the existence of a well developed 

 subcostal vein) . Thus there is no difficulty in understanding 

 how the Fulgoroidea and the Cercopidae could have arisen later 

 'di point of time than the Jassidae, though preserving certain 

 archaic features which the Jassids had lost. This is the same 

 problem as that which confronts us in studying the Panorpoid 

 Orders, in which it is clear, palaeontologically, that the Mecoptera 

 were the first recent Order to be differentiated out, though they 

 are, in some respects, more highly specialised than other Orders, 

 which arose from the main Panorpoid stem at later dates, such 

 as the Megaloptera . 



The above ideas may be expressed in the Phylogenetic Diagram 

 given herewith in Text-fig. 24. There can be no doubt that 

 further exploration of the Ipswich Beds would yield us more 

 valuable material in the Homoptera, and help to fill in the gaps 

 that are only too evident in the present state of our knowledge. 



