1902.] AND ANCIENT GEOGRAPHY . 331 



and water were established at about the beginning of the Miocene. 

 After the destruction of ihe Lemurian bridge in the Eocene, its 

 northeastern portion, India, became part of Asia, while its south- 

 western portion, Madagascar, which at first remained a peninsula 

 of Africa, became an island. 



The application of these geographical results to the distribution 

 of the freshwater Decapods is the following: First, we have to 

 emphasize that before the middle of the Cretaceous it was impos- 

 sible for the genus Astacoides to reach Madagascar. Since the 

 separation of the Asiatic and Australian group of the crayfishes 

 took place in about the Upper Cretaceous and since the morpho- 

 logical differentiation of the Potimobiidce and FarasfacidcB wdiS cot\- 

 nected with this separation, and further, since Astacoides must 

 have immigrated into Madagascar from the Asiatic part of the old 

 Sino- Australian continent, this latter process must have gone on 

 shortly before the completion of this separation, that is to say, 

 about the middle of the Cretaceous. This assumption is supported 

 by the morphological characters of Astacoides^ which are, in a 

 certain degree, intermediate between the present two families and 

 favor the view of an early separation from the original stock. 



Thus there is nolhing that prevents us to assume an immigration 

 of Astacoides from southeastern Asia into Madagascar in the mid- 

 dle of the Cretaceous period. At a later time this does not seem 

 to have happened, since, in this case, we should have different 

 morphological characters in Astacoides, At an earlier time this 

 immigration was impossible, since then India was not connected 

 with the Sino Australian continent. After the Eocene this migra- 

 tion was absolutely impossible, since then the land connection 

 between India and Madagascar had disappeared. 



Although we may thus fix the time of immigration of Asta- 

 coides rather exactly, there arise other questions. We want prin- 

 cipally an explanation of the absence of similar forms in Africa 

 itself, and for the absence of such in India and generally in 

 southeastern Asia. 



Regarding the Potamonince, their presence in Madagascar, and 

 the close relation of the Madagassian forms to East-African, is 

 easily explained by the former connection of Madagascar with 

 Africa. The freshwater crabs of Madagascar thus indicate geo- 

 graphical conditions which are older than Miocene. The presence 



PROG. AMBR. PHILOS. SOC. XLI. 171. V. PRINTED DEC. 20, 1902. 



