326 ORTMANN — DISTRIBUTION OF DECAPODS [Aprils. 



Upper Cretaceous, a sharp separation between both continents was 

 formed, which continued possibly up to the Eocene. Then the 

 connection was, at least partially, reestablished, but it was of a very 

 changing character, which is expressed by the great complexity in 

 animal distribution. These changing and unstable conditions pre- 

 vailed all through the Tertiary, and up to the present time, and it 

 is hard to trace them under the present imperfect state of our 

 knowledge of the geology of the respective parts. 



The Upper Cretaceous separation of Asia and Australia is 

 expressed in the distribution of the Potamobiidce and Parastacidce : 

 the formerly continuous area of their ancestors, which comprised in 

 the Lower Cretaceous the Sino-Australian continent, was divided, 

 about the middle of the Cretaceous, in a northern (East Asia- 

 Potamobiidie^) and a southern (Austral \z.- Parastacidce) part. The 

 varying conditions of the Tertiary are expressed in the distribution 

 of the Potamonincz ; the details, however, cannot be made out, and 

 further study of the freshwater crabs of these regions, as well as a 

 more thorough study of the geology of these parts, is very desirable. 



3. CONNECTION OF AFRICA AND INDIA. 



The occurrence of crayfishes (genus Astacoides) in Madagascar 

 has led us, as we have seen above (p. 295), to the assumption that 

 there once existed a connection of this island with southern Asia 

 (respectively with the Sino-Australian continent). The same con- 

 nection is suggested by the distribution of the Potamonince, of 

 which the subgenus Potamonautes is found in Africa as well as 

 India. The Madagassian forms of the Potamonina (see above, p. 301) 

 indicate a relation of this island to Africa, while a closer connec- 

 tion with India is not so striking. A genetic connection of the 

 ranges of this subfamily in Africa and India by way of the Nile 

 valley and Syria is improbable, although, geographically, this con- 

 nection actually exists ; this, however, is apparently due to second- 

 ary migrations, different branches of the subfamily, coming from 

 India and Central Africa respectively, meeting in lower Egypt. 



Thus we have to regard Madagascar as a stepping-stone between 

 Africa and India, and, with reference to the PotamonincB, its rela- 

 tion to Africa is closer than that to India. 



This supposed connection is well known in zoogeography under 

 the name of the Lemurian continent. Jacobi (1900, p. 169 ff. ) 

 quite recently has doubted this Lemuria-hypothesis, although he 



