1902. 



AND ANCIENT GEOGRAPHY. 325 



(probably strongly discontinuous) as far as New Guinea, and the 

 same is the case in the genus Parathelphusa. 



For the present, these very strange conditions defy explanation, 

 and especially the eastern boundary of Potamon (sens, strict.) and 

 the western boundary of Geothelphusa are puzzling. But this much 

 we may say, that the distribution of the Potamonince over the Indo- 

 Malaysian Archipelago is apparently due to the varying relations of 

 the different islands between themselves and to the continents dur- 

 ing the Tertiary period, and that it furnishes additional proof for 

 the complexity of the changes that took place during this time in 

 this region. 



Another fact is to be especially mentioned. Among the Pota- 

 monince we do not have such a sharp separation of Australian and 

 Asiatic types as we have found among the crayfishes : on the con- 

 trary, the species of Geothelphusa, found in northern and eastern 

 Australia, are all closely related to those found in New Guinea and 

 on the other islands. Also the different forms of Potamon (sens. 

 strict.), found in Java and Sumatra, are very closely allied to con- 

 tinental species. All this points to the conclusion that the separa- 

 tion of the respective parts from each other, which brought about 

 the present conditions, must be of comparatively recent date, and 

 that at a time not very far remote from the present the distri- 

 bution of land and water in this archipelago must have been con- 

 siderably different from what it is now. Thus it seems that the 

 causes of the distribution of the Potamonince. in the Indo-Malaysian 

 Archipelago are to be sought for in later times, presumably in the 

 Tertiary, and that during this period, and possibly up to a very 

 recent time, conditions prevailed here which — although they may 

 not have amounted to a continuous land bridge — constituted a cer- 

 tain unstable connection between Asia and Australia. Probably 

 there was a maze of larger and smaller islands, channels, straits and 

 the like, which was not permanent in its parts, and changed 

 repeatedly. 1 



Our final result on this question would be the following: South- 

 eastern Asia was connected with Australia in the Jurassic, and 

 probably also at the beginning of the Cretaceous period. In the 



1 According to von Ihering (1894, p. 406), Australia was connected with Asia 

 during the Eocene and Oligocene. Hedley (1899) connects New Guinea with 

 Australia in the later Tertiary ; but a similar connection existed also in the 

 Eocene, and through the latter Oriental elements were brought to New Guinea. 



