88 Records of the Indian Museum. [Vol. XIV, 



much resembling those of P. acanthicum, whilst among accepted species 

 of the subgenus Potamon, Miss Kathbun's P. shensiense,'^ from China, 

 may be cited as an instance of a form in which the crenulations of the 

 autero-lateral margm have become spiuiform, though not to the same 

 extent as in the species from the Shan States. 



The species seems then, so far as the antero-lateral border of the 

 carapace is concerned, to have undergone modification on the same 

 lines as P. (Potamon) shensiense and P. {Acmitliotelphusa) nUoticuni 

 and its allies, though it is, in my opinion, clear that it is not closely 

 related to either. Except in the matter of the antero-lateral border 

 P. shensiense shows the closest affinity with P. denticulatum (Milne- 

 Edwards) ; it differs widely from P. acanthicum in the form of the epi- 

 gastric and post-orbital crests. In P. yiiloticum the longitudinal groove 

 on the ischium of the third maxillipedes (found in P. acanthicum and 

 most Potamonidae) is absent, and there are great differences in the form 

 of the carapace and its areolation. 



P. acanthicum must, I think, be regarded as a highly specialized 

 offshoot of the ancestral stock that gave rise to P. andersonianum and 

 its allies. So far as I can discover no closely related forms are known 

 to exist, and the evidence obtained by Dr. Annandale regarding the 

 other elements of the fauna of the Inle system points to the co)iclusion 

 that it evolved in the locality in which it is now found. The develop- 

 ment of spines in place of serrations on the antero-lateral border of the 

 carapace appears to have originated independently in P. acanthicum,, 

 P. shensiense and P. nUoticum, and is thus an instance of convergence.^ 



The colouration of living specimens of P. acanthicum is very striking : 

 Dr. Annandale has kindly supplied me with the following note. " The 

 dorsal surface is black or very dark green, except that the deep groove 

 running across behind the orbits is pale greenish yellow with a blackish 

 margin in front. The upper half of the chelae, including the w^hole of 

 the movable finger, is densely marbled with pale olive and greenish 

 black ; the lower half, including the immovable finger, is pale yellowish. 

 This particoloured character extends to the whole appendage. The 

 articular membrane at the base of the claw is scarlet. The walking legs 



1 Rathbun, Nouv. Arch. Mus. Paris (4), VI, p. 2(32, pi. ix, fig. 8 (1904). 



- There is one other point, not perhaps altogether disassociated from a discussion 

 of the affinities of P. acanthicum, to which I would like to I'cfer. It concerns the status 

 of Acanthotelphusa as defined by Alcock. That the members of this subgenus have been 

 evolved from Potamon, s. s., will I think be generally admitted, but I am not convinced 

 that the dividing line between the two subgenera is rightly placed. The characters of 

 Acanlhotelphusa given by Alcock are two, — " that the antero-lateral borders of the cara- 

 pace are cut into large teeth or spines, and that the ujjper border of the merus of the 

 chelipeds bears a subternnnal spine." This description is in jierfect agreement with the 

 Indian species, but applies less well to Potamon niloticum, the type of the subgenus. 

 In the l<]gyptian species the teeth of the antero-lateral border, as has already been jjointed 

 out, are small and irregular in their disposition and the subterminal spine on the upper 

 border of the merus of the chelipedes is, in females at any rate, non-existent. On the 

 other hand the groove on the surface of the ischium of the third maxillipedes is absent 

 in P. niloticum, as it is in certain of the Indian species, and this is a rare character in 

 Potamonidae. Judging from the limited material at my disposal I am inclined to think 

 that the Indian species referred by Alcock to Acanthotelphusa differ more widely from 

 P. niloticum than the latter does from typical species of Potamon, s. s. If this jiroves 

 to be the case, Acanthotel2>husa must once more revert to the synonymy of Polamo7i, 

 while a new subgeneric name will be necessary for the Indian species and their allies. 

 {Sec Postscriiitum, p. 101.) 



