288 ZOO-GEOGRAPHICAL DISTRIBUTION [OH. 



small remnant composed of Holarctic genera and their derivatives. 

 The Tertiary fossil-beds of this region prove, however, that the 

 Megapodagrioninae existed fairly abundantly in Miocene times. 

 We can only conclude that this group was unable to resist a 

 subsequent lowering of the temperature over those areas in which 

 it existed, and so became extinct within the region. 



Fig. 151. Fore-wing (30 mm.) of Heiaerina americana Fabr., ?, 

 Mexico. Original. 



The Palaearctic Region. 



This enormous region includes the whole of Europe, together 

 with the temperate parts of Africa (north of the Tropic of Cancer) 

 and of Asia and the Japanese Archipelago. The boundary is 

 usually drawn along the western watershed of the Indus, thence 

 along the Himalayas into Thibet and China, north of the Yang- 

 tse-kiang watershed. Although the region is of such great extent, 

 it is by far the poorest in Odonata in the whole world. Japan is 

 the only part of it that contains at all an abundant or striking 

 Dragonfly fauna. 



Out of 59 genera known to occur, only 14, or 24 per cent., are 

 peculiar to or entogenic in the region 1 . The principal portion of 

 the fauna is supplied by the development of the twelve Holarctic 

 genera shared in common with Nearctica, viz. Cordulegaster, 

 Gomphus, Ophiogomphus, Boyeria, Somatochlora, Cordulia, Libellula, 

 Sympetrum, Leucorrhinia, Calopteryx, Agrion and Enallagma. The 

 development of Anisoptera is on the whole less marked, of Zygo- 

 ptera more marked, than in Nearctica. There are no peculiar 

 genera of Gomphini, and only two of Corduliinae, Oxygastra 

 (fig. 152) and Epitheca. But in the Zygoptera, there are the 



1 See foot-note on p. 286. 



