324 ORTAIANN — DISTRIBUTION OF DECAPODS [Aprils, 



If the first sharp separation of Australia and Asia belongs to the 

 Upper Cretaceous, it is consequent, for the Parastacidce and 

 Potamobiidce, that their area of distribution, which before the 

 beginning of the Upper Cretaceous extended over the Sino-Austra- 

 lian continent, was cut in two ; of course, the ancestral forms occu- 

 pying this old continent could not possibly have been divided into 

 these two families, and their differentiation was directly connected 

 with this separation of the geographic range. After that, there 

 was a chance for either family to develop, since there was no longer 

 communication between the Asiatic and the Australian stock. This 

 forces us to the conclusion that the ancestors of these two families 

 must have existed before the beginning of the Upper Cretaceous time, 

 and that during the Upper Cretaceous the division into Potamobiidce 

 and Parastacidce took place. It is impossible to place the origin of 

 these families at a later period, since, as we shall see below, any 

 crayfishes of late Mesozoic or early Tertiary age, in any part of the 

 world, belong either to the one or the other family. Although 

 there was at least a partial connection of Asia and Australia in Ter- 

 tiary times, the two families never came into contact again : with 

 the cause of this remarkable fact we shall become acquainted 

 below. 



With reference to the Potamonince, their distribution over the 

 Indo-Malaysian Archipelago is only partly explained by the 

 assumption of a former continuous land bridge. The distribution 

 of the freshwater crabs is by no means simple, and does not extend 

 uniformly from eastern Asia to Australia, but there are numerous 

 complications and peculiarities. In the first line, we have to 

 emphasize the fact that only a single group, which is apparently 

 highly specialized, the subgenus Geothelphusa, reaches the conti- 

 nent of Australia, and that this group (in its typical forms) is 

 restricted to the Indo-Malaysian islands, and is wanting on the 

 Asiatic continent. This is the more remarkable, since this group is 

 most abundant just on the large islands of Sumatra, Java, Borneo, 

 and extends northward over the Philippine and Loo Choo Islands 

 to Japan. On the other hand, we have seen that the typical species 

 of the genus Potamon (subgenus Potamon), which are found in both 

 India and China, reappear in very closely allied forms in Java, 

 Sumatra and the Philippines, but do not pass farther to the East. 

 Then again, the subgenus Potamonautes possesses scattered stations 



