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No. XV.— ON THE LAND AND AMPHIBIOUS DECAPODA OF ALDABRA. 



By L. A. BoRRADAiLE, M.A., Lecturer in Zoology in the JJiiloerslty and at 



Selwyii College, Cambridge. 



{Communicated by J. Stanley Gardiner, M.A., F.R.S., F.L.S.) 



Ecad 17th February, 1910. 



The collection on which the following paper is based was obtained last year by Mr. J. C. 

 F. Fryer during a stay of five months in Aldabra. It was made with care, and the list 

 with which it furnishes us is probably almost, if not quite, exhaustive. Freshwater 

 forms are wanting, owing to the lack of fresh water on tbe Lsland, but the collection 

 includes several representatives of the amphibious fauna of the mangrove swamps. 



All the species, of which there are seventeen, belonging to ten genera, are common 

 Indo-Pacific forms, and it is not possible to detect any constant local difference from 

 representatives of the same species taken elsewhere. 



The outstanding feature in the land decapod fauna of Aldabra is the presence of the 

 Robber Crab {Birgus). This allies it to the Chagos and the eastern parts of the Indo- 

 Pacific, and diflFerentiates it from Minikoi *, the Maldives *, and the Seychelles f , which 

 Birgus seems not to have reached. Other indications in the same direction are the 

 presence of Helice, which is not found in the north-western Indian Ocean, and the absence 

 of Geograpsus minikoiensis, which, so far as is at present known, is peculiar to the latter 

 region, where it is probably a local representative of the more eastern G. longitarsis. 



With the above exceptions, there is no important difference between the fauna in 

 question in Aldabra and that in the Seychelles. TJca tetragonum and Varuna litterata, 

 found in the Seychelles, do not appear in the collection from Aldabra, while Metopo- 

 grapsus messor, found in Aldabra, is not yet recorded from the Seychelles ; but in view of 

 the fact that these species are found as far north as the Red Sea, and as far east as the 

 east coast of Africa, it seems likely that either they liave been overlooked in the islands 

 in question, or their absence is due to special, and perhaps even temporary, causes. 

 Somewhat greater discrepancy appears between the list of the land Decapoda of Aldabra 

 and that of those known to occur in the Maldives. The Aldabra list is considerably the 

 larger of the two. Besides the forms mentioned in the last paragraph, Cardisoma 

 hirtipes, Metopograpsus messor, Sesarma quadratum, S. intermedia, and Grapszts grapstis, 



* See Gardiner's ' Fauna of the Maldive8,' vol. i. pp. 64-100. 



t See Trans. Linn, Soc. London, ser. 2, Zool, voL sii. (1907) pp. 63-6S. 



5i* 



