1902.] AND ANCIENT GEOGRAPHY. 303 



tendency to reduce the postfrontal crest has developed in the East 

 African forms independently from the typical Geothelphusce, and 

 the East African (possibly also the Liberian) species form a pecu- 

 liar branch of Potamonautes. 



The genus Erimetopus of Rathbun is found so far only in the 

 Congo basin. 



Considering the distribution of the subfamily Potamoninoz 

 in general, we see that it is continuous over the whole of 

 tropical Africa, then it extends through the Nile valley into the 

 Mediterranean regions and connects with the Asiatic range, which 

 goes from Syria over Mesopotamia, Persia to India, China and the 

 Malaysian archipelago, over which it finally reaches Northern Austra- 

 lia and Japan. This whole range is practically continuous, only 

 the larger continental islands (disregarding the smaller ones), Mada- 

 gascar and the Sunda Islands, the Philippines, New Guinea and 

 Japan, constituting breaks in the continuity. 



Within this large area, however, we are able to distinguish two 

 main divisions : an African, characterized by the prevalence of the 

 subgenus Potamonautes, the complete lack of the subgenus Pota/non ' 

 (and possibly of Geothelphusa), and an Asiatic-Australian division, 

 characterized by the prevalence of the subgenus Potamon, the pres- 

 ence of Geothelphusa (in its eastern part), and the scarcity of 

 Potamonautes. Both divisions are practically connected by the 

 Nile valley ; this connection, however, does not seem to represent 

 the original condition, but suggests a secondary one, since different 

 types are here associated which are not at all related to each other. 

 Species of Potamonautes, to which subgenus, according to our opin- 

 ion, P. berardi also belongs, migrating northward from the Soudan, 

 have met here in Lower Egypt a species of the subgenus Potamon 

 {P. Jiuviatile), which had migrated westward from India. Both 

 subgenera entered the Nile valley from different directions and 

 accidentally became occupants of the same territory, but the Nile 

 valley is not the route of migration by which African species 

 migrated into Asia or vice versa. 



Aside from this narrow connection, the fauna of freshwater crabs 

 of tropical Africa is very sharply characterized and isolated from 

 Asia, 1 and the fact is worth special mention that North Africa 



1 The peculiarity of the African fauna is emphasized by the doubtful forms of 

 Parathelphusa (or Acanthothelphusa), and by Eri?>ietopns. 



