12 



tract, from Hazara to Nepal ; and the Eastern Himalayan tract, from Nepal 

 to the head of the Assam valley. 



No Potainonidce, so far as I know, have been reported from the Tibetan 

 tract. I have collected in the western part of the tract, but did not find any 

 traces of crabs there, and Captain F. H. Stewart's fresh-water collection from 

 the eastern part of the tract contains no crabs. 



But, as regards the eastern and western Himalayan tracts, I would point 

 out that, for Potamonida', they are zoological as well as physical subdivisions. 



(e) Assam, Burma, and the territories east of the Bay of Bengal. This 

 physical division is separated by Blanford into six tracts, namely : Assam, includ- 

 ing the neighbouring hills, Chittagong, and Arakan ; Upper Burma ; Pegu ; 

 Tenasserim, as far south as Mergui ; the Andamans and Nicobars ; and South 

 Tenasserim. Of these the first four form part of Blanford's Transgangetic 

 zoological subregion, and the last is included in his Malayan subregion. Here, 

 again, the appended Geographical List seems to demonstrate that at least the 

 northern part of the Assam tract is a division that can be recognized by its 

 fresh-water crabs as well as by its physical features. 



[With regard to the Andaman-Nicobar tract, I have nothing to say. I 

 have never seen or heard of any Potamonidce in the Andamans. Heller and 

 Burger report Paratelplmsa [Oziotelphusa) hydrodrotmis (Herbst) [= Telphusa 

 leschenaudii, Edw.), from the Nicobars; and if their identification, or, rather, 

 since the identification of this species is hardly open to dispute, if their 

 locality-label is correct, this is something extraordinary, as this species belongs 

 to the Peninsula, Ceylon, and the eastern part of the Indo-Gangetic Plain.] 



To conclude : while allowing that Blanford's zoological subdivisions of 

 the Indian Empire, which are based on the present distribution of vertebrates, 

 suit the Potamonidce in a general way, it is here claimed that that author's 

 physical subdivisions of the area have, as a rule, a much more exact corre- 

 spondence with the tracts in which the constituent groups of the family are 

 concentrated. It may also be added that Blanford himself emphasised, in the 

 case of the vertebrate fauna, certain points which a study of the fresh-water 

 crabs brings into strong relief. These points are as follows : — 



(1) The close connexion between the Himalayas and Burma, and "the 

 evidence in favour of the principal elements in the Himalayan fauna having 

 been derived, probably at no distant period geologically, from the Assam 

 range." I think that the evidence from the fresh-water crabs strongly 

 corroborates this view. 



(2) The identity of the river fauna (strikingly exemplified in the cetacean 

 Platanista, the crocodilian Gavialis, the chelonian Hardella, and the siluroid 

 Sisa) of the Indus and Ganges, and the consequent potency of the claim of the 

 Indo-Gangetic Plain to recognition as a special zoological subregion. 



(3) The difference between the vertebrate fauna of (a) the Eastern and 



