DICCAPOD CRUSTACEA FllOM MADAGASCAR. 925 



3. Geographical Relations of the Madagascar River-Crahs. 



In addition to the species mentioned above, three others have 

 been recorded from Madagascar. The African Potamon {Pota- 

 monautes) depressum was recoi'ded by Lenz and Richtei-s, but its 

 occurrence is doubted by Miss Rathbun. Potamon [Potamon) 

 pittarellii, described by JS'obili (Boll. Mus. Zool. Torino, xx. 

 No. 507, 1905) since the publication of Miss Rathbun's mono- 

 graph, appears to be allied to P. madagascariense, but is, no 

 doubt, a distinct species. Finally, Potamon {Parathelp)husa) 

 antongilense, described by Miss Rathbun, is evidently a remark- 

 able and isolated species, concerning which further information 

 is much to be desired ; as far as can be judged from the shape of 

 the male abdomen, it is not a Parathelplmsa in the sense in which 

 that genus has been restricted by Alcock. 



Leaving aside these three species, and considering only those 

 that are represented in the collections now examined, it is 

 evident that the Potamonid fauna of Madagascar bears no close 

 relation to that of Peninsular India, which, as Alcock has shown, 

 belongs entirely to the subfamily Gecarcinucinfe. This is im- 

 portant, since geologists seem to be agreed that the connection 

 of Madagascar with India existed only at a time (not later than 

 the early Oligocene) when Peninsular India was separated from 

 the continent of Asia. Of the possible affinities with African 

 species it is perhaps unsafe to speak until the rich Potamonid 

 fauna of Africa shall have been examined as thoroughly as that 

 of India has been by Alcock ; but it is worth noting that no 

 typical Potam.on seems to be known from East Africa ; and if, on 

 the one hand, P. madagascariense is the most primitive of the 

 Madagascar species, and, on the other, if it is really allied to the 

 group of P. Jluviatile, then it is hard to guess by what route it 

 can have reached the island. Further than this it does not seem 

 possible at present to go. As I have indicated above, our con- 

 ceptions of the phylogenetic relationships of the species rest, as 

 yet, on a very narrow basis of morphological fact ; and, until 

 that basis is broadened very considerably, the group of River- 

 Crabs, as a whole, appears to me to be a hazardous subject for 

 zoogeographical speculation. 



Part II. — Family PAL^MONiDiE (River- Prawns). 



The Palaemonidfe of Madagascar have already formed the sub- 

 ject of an important memoir by Couti^re (Ann. Sci. Nat. Zool. 

 (8) xii. 1900, pp. 249-342, 5 pis.), on which I have relied mainly 

 for the determination of the four species in Mr. Metliuen's 

 collection. These are as follows : — 



Palcemmi lejyidactylus Hilgendorf. 

 ,, dolichodactyhis Hilgendorf. 

 ,, ritsemce de Man. 

 ,, hildebrandti Hilgendorf. 



Proc. Zool. Soc. — 1913, No LXII. 62 



