88 Records of the Indian ^l ii.seii in. [Voj,. XlV, 



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

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

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

 antero-lateral margin have become spiniform, though not to the saine 

 extent as in the species from the Shan States. 



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

 carapace is concerned, to have undergone modification on the same 

 lines as P. {Potamon) shensiense and P. {AcantJwtelphusa) ndoticuin 

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

 related to either. Except iii 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. niloticum 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. andersoniamim 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 conclusion 

 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. niloticum, and is thus aii 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 v.'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. Arcli. Mus. Paris (4), VI, p. 2(J2, pi. ix, fig. 8 (1904). 



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

 of the affinities of P. acantliicuin, to which I would like to refer. It conc-erns the .status 

 of Acanttwtelplmsa as doiined by Alcock. That the meniber.s 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 j)laued. Tlie characters of 

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

 pace are cut into large "teeth or sjiincs, and that the uj)per border of the merus of the 

 chehpeds bears a subterminal spine." This description is in jjerfect agreenuMit with the 

 Indian species, but ap])lies less well to PulaiHon inlulicuiii, tiie type of tlie subgenus. 

 In th(> Egyptian species the teeth of the antero-lateral border, as has already been pointed 

 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 si^ecies referred by Alcock to Acanthoieljtlmsa differ more widely from 

 P. niloticuin. than the latter does from typical sjjccies of Potamon, s. s. If this proves 

 to be the case, Acanthoteljitiusa must once more revert to the synonymy of Potamon, 

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

 {See Postscriptum, p. 101.) 



