406 PEECT SLADEN TRUST EXPEDITION. 



which are found in Aldabra, have not been recorded from the Maldives. All these are 

 common species and occur in Africa and on the west coast of India, so that it is possible 

 that some of them have been overlooked by the collectors, but it is unlikely, considering 

 the care with which the collections were made, that this has been the case with them all. 

 On the other hand, Metasesarma rousseanxi, found in the Maldives, was not taken in 

 Aldabra. Since this species is also not recorded from the Seychelles, it seems likely that 

 its southern limit is here reached in the Maldives. The Minikoi fauna is still poorer 

 than that of the Maldives, lacking TJca annulipes, Cardisoma carnifex, and Cosnobita 

 compressus. 



It will be gathered from the above remarks that the land Decapod fauna of the islands 

 of the western Indian Ocean is essentially eastern, and that it shows from south-west to 

 north-east a continuous impoverishment, which culminates in the island of Minikoi, just 

 Avhere a priori the fauna would have been supposed to have been richest, since it is there 

 nearest to the continuous land connection with the east. A fuU discussion of this 

 problem is at present impossible, especially in view of our ignorance of the length of life 

 of the sea-borne larvae by Avhich all the foi*ms in question are spread, and of the lack of 

 an exhaustive list of the fauna of Ceylon, which is a crucial point for the enquiry ; but 

 it is possible that some indication of the solution may be found in the fact that, 

 whereas the south-western portion of the Indian Ocean receives continually the 

 equatorial current from the east, in the north-west the currents vary in direction with 

 the monsoons, so that communication in either direction is intermittent. In this 

 connection the question of the breeding-time of the species may be of importance. 

 Many tropical Decapoda may be found with young at various periods of the year ; but 

 our knowledge on this point is scanty, and it may be that in some cases the principal or 

 only time at which the larvse are set free is one in which the currents in the north- 

 western Indian Ocean are not favourable for their passage from east to west. 



The Chagos, lying in the middle of the eastward-setting equatorial counter current, 

 would receive its fauna from the south and west ; and this suggests that the absence of 

 JBirffus from the Seychelles, due west of the Chagos, may be due to its extermination by 

 human or some other agency. Oeograpsus minikoiensis and Metasesarma rousseauxi 

 appear to belong to the noi'thern portion of the area, and not yet to have reached the 

 southern islands. The general direction of the currents within the western half of the 

 Indian Ocean from south to north would hinder the southern spread of any form which 

 had by any means obtained a footing in the north alone. If it should eventually prove 

 that a solution of the problem is impossible on these lines, the only alternative will be to 

 seek the causes which render the north-western part of the Indian Ocean uninhabitable 

 by a number of forms which flourish in the south-west. ■ 



The following is a list of the species taken in Aldabra : — 



