1902.] AND ANCIENT GEOGRAPHY. 391 



nental ring and in the North American as far as the northern sec- 

 tion of Central America. There they gave place to the freshwater 

 crabs, which extended thence to Northern South America, Africa 

 and Lemuria, where they came again into touch with the cray- 

 fishes. 



The same fact, namely, that crayfishes and crabs are mutually 

 exclusive, holds good for their distribution in recent times. This 

 fact was first pointed out by Milne-Edwards ; it has also been men- 

 tioned by Faxon, and the present writer 1 has used it for the expla- 

 nation of some of the features in their distribution. It seems, 

 therefore, that the crabs are more vigorous and active than the 

 crayfishes, and that wherever they came into actual contact the 

 latter were exterminated by the former. It is true there are some 

 countries from which both types of Decapods have been reported, 

 namely, Mexico, Northeastern Australia and Madagascar ; but we 

 have no report that both are found associated in the same localities 

 and in the same rivers, streams, ponds or lakes, and it is very likely 

 that just in these regions crabs and crayfishes inhabit stations of a 

 different character. The closer investigation of these conditions 

 would be most interesting. 



If we apply this idea, that the presence of crabs forms a biocoen- 

 otic barrier to the crayfishes in the former distribution of both, we 

 obtain the following result : 



The crayfishes are geologically older than the crabs. They ex- 

 isted, in Lower Cretaceous times, in Sino-Australia, and conse- 

 quently also in the region of Southeastern Asia and the Malaysian 

 islands. In the Middle Cretaceous they sent a branch {Astacoides) 

 across India to Madagascar. But in the Upper Cretaceous the 

 freshwater crabs arrived (or originated) in the same region (Lemu- 

 ria) and extended into Southern Asia and the Malaysian Archipel- 

 ago, everywhere exterminating the crayfishes, namely, in India, 

 Southeastern Asia (Farther India and China) and on the islands. 

 They not only acted as a check to the distribution of the cray- 

 fishes, but directly annihilated them. Only in Madagascar Asta- 

 coides survived, probably because in this island it inhabits parts 

 that have not been occupied by the crabs. 2 On the other hand, 



1 See Ortmann, in Zool. Jahrb. Syst., Vol. g, 1896, p. 593 f. 5 and in Bronn's 

 Klassen und Ordnungen, Vol. 5, 1901, p. 1289. 



2 Possibly the large size of Astacoides has something to do with its survival. 

 Astacoides is — aside from some South Australian species — by far the largest type 

 of all crayfishes. 



