1902.] AND ANCIENT GEOGEAPHY. 305 



Supposing that this subfamily must have had once a more or less 

 continuous distribution, we are to draw from this the following 

 conclusions as to the geographic conditions of the past : 



i. Africa and India must have been connected once. This connec- 

 tion, however, was not by way of North Africa, Arabia and Persia, 

 and is possibly identical with that from Africa over Madagascar to 

 India, discussed above (see No. 5, p. 295). 



2. Madagascar must once have been a part of Africa. 



3. The Indo- Malaysian Islands, including the Philippine Islands, 

 Loo- Choo Islands and Japan, must have been once connected not only 

 between themselves, but also with New Guinea and North Australia 

 (as indicated by Geothelphusa). On the other hand, the distribution 

 of the typical forms of Potamon indicates that some of these islands 

 (Sumatra, Java, Philippines) were once connected with the continent 

 of Asia. Then, again, by Potamonautes (and Parathelphusa) the 



former continuity of the whole region from India to New Guinea is 

 indicated (see p. 295). // is evident that here repeated and important 

 changes of the mutual connections have taken place at different 



periods of the past. 



The history of the subfamily of Potamonince would then be this : 

 Its centre lies in an Afro-Indian continental mass, which was divided 

 subsequently into two parts, tropical Africa and India. From India 

 the subfamily extended at a very early period over the Sunda 

 Islands, Philippine Islands, which consequently must have formed 

 a part of the continent, and this continental connection extended 

 as far as New Guinea and Australia, but not without repeated inter- 

 ruptions and changes. In the region of unstability and change lies 

 the home of the subgenus Geothelphusa, which was able at a certain 

 time to go as far north as Japan. A separate branch of the sub- 

 genus Potamon was sent out from India westward, which finally 

 reached the Mediterranean countries, where it met in the lower 

 Nile valley a branch of the African subgenus Potamonautes which 

 came down the Nile from the south. 



2. Subfamily: Deckeniinoz. 

 The second subfamily of the Old World, the Deckeniince, contains 

 only one genus, Deckenia Hlgdf. (see Ortmann, 1897, p. 314), of 

 which three species have been described : 

 D. imitatrix Hlgdf. Interior of British East Africa : Taro (Hil- 



