332 OKTMANN — DISTRIBUTION OF DECAPODS [Aprü3, 



of Deckenia on the Seychelle Islands connects also this group more 

 closely with Africa than with India. Possibly this connection is 

 identical with that over Madagascar, although Deckenia has not 

 been found on the latter island. 



The presence of Potamonince in India, corresponding to the 

 African type (subgenus PotamonauUs), indicates the full develop- 

 ment of the Lemurian peninsula, that is to say, conditions prevail- 

 ing in the oldest Tertiary, if not earlier. Potamonince, represented 

 by forms which resembled the subgenus Potamonautes, must have 

 existed at least in the beginning of the Eocene, and their distri- 

 bution extended over Africa and the Lemurian peninsula, includ- 

 ing India. During the Eocene this range was separated into two 

 parts, an African (to which Madagascar belonged) and an Tndian, 

 and, beginning in the Miocene, the Potamonince had a chance to 

 expand over southern and eastern Asia (Farther India and China 1 ). 

 At the same time they availed themselves of the various and chang- 

 ing connections within the region of the Indo-Mala^sian archipe- 

 lago, occupying the latter and reaching Australia. The opening 

 of this region of dispersal offered to this group a new opportunity 

 for a rich development, and the origin of the subgenera Potamon 

 and Geothelphusa was probably the outcome of it. 



We cannot leave this chapter without saying a few words on the 

 Arabian region of dispersal of Jacobi. This extends from north- 

 eastern Africa across Arabia to India. Jacobi mentions the simi- 

 larity of the Siwalik-fauna of India with the Ethiopian. This, 

 consequently, refers to a very recent period, the later Tertiary. 

 Before this time, in the older Tertiary and in the Mesozoic, this 

 connection is out of question. The Potamonince, which, as we 

 have seen, existed in the older Tertiary, show no trace of this con- 

 nection across Arabia, and, as we shall see below, our knowledge 

 of the ancient geography of these parts is a very fair one. Arabia 

 itself formed originally a part of Africa, and the Red Sea did 

 not exist at all in the earlier Tertiary, it being quite recent (see 

 below). Toward the north, northeast and east Arabia was circum- 



1 There was, possibly, an earlier chance to reach the Sinic continent, in Upper 

 Cretaceous times, and I am inclined to believe that the discontinuous localities 

 of J otamonautes (and Parathelphusa) in the Indo Malaysian archipelago point 

 to an immigration of these forms that precedes in time that of Potamon sens, 

 strict. 



