924 DR. W. T. OALMAX OX 



segments. Alcock found three segments distinct in all tlie 

 Indian species of Potcmion, but in his later paper he states tha.t 

 in the Potamoninre the palp has either two or three segments. 

 Fi-om the examination of a number of African species I suspect 

 that it will not be possible to distinguish sharply between those 

 species in which the suture-line between the first and second 

 segments is well marked and those in which it is almost or quite 

 obliterated. 



As regards the generic affinities of the species here discussed, 

 there is nothing to forbid the conclusion that they are closely 

 interrelated. They have, indeed, been placed in two distinct 

 geneia, Potamon and Hydrothelphusa, and the species of the 

 former might not unreasonably be placed in two subgenera, 

 Potamon and Geothelphusa, but the groups indicated by these 

 names are probably more convenient than natural. Both Alcock 

 and Miss Rathbun comment on the close resemblance of Hydro- 

 thelphusa to Potamon, and Miss Kathbun specially mentions its 

 affinity with P. m.adagascariense ; this affinity becomes even more 

 striking when comparison is made with some specimens of 

 Mr. Methuen's collection which I have described above as repre- 

 senting a form of P. tnadagascariense (Series B). Potamon 

 goudoti is considered by Miss Rathbun as forming a link between 

 the subgenera Potamon and Geothelphusa, and the new species 

 described above as P. methueni goes even further in the direction 

 of the latter subgenus. There can, however, be little doubt that 

 Geothelphusa, even as restricted by Alcock, has still very slender 

 claims to be regarded as a natural or monophyletic group. A 

 genei-al softening of the asperities of the carapace, such as we 

 frequently see within the limits of undoubted species, has only to 

 be carried a little further to result in the obliteration of the 

 postorbital and epigastric crests ; and there is no difficulty in 

 supposing that so short a step may have been taken independently 

 by more than one evolving species. This independence is, in 

 fact, implied by Alcock's transference of certain species of "6'eo- 

 thelphusa" to genera of his subfamily Gecarcinucinee, and I think 

 there is some slight evidence of the same thing among those that 

 remain in the Potamoninge. At all events there is no necessity 

 to assume that P. goudoti and P. methueni are more closely 

 allied to the species of " Geotheljohusa " found in Africa and Asia 

 than they are to P. madagascariense. 



Potamon m.adagascariense itself is placed by Miss Rathbun in 

 the group of which P. edule (fluviatile) is the type. While it 

 certainly resembles the species of this group in the characters of 

 the carapace, it differs a little, as already indicated, in the form 

 of the mandibular palp, and also in some other small characters, 

 such as, for instance, the much broader contact of the basal 

 antennal segment with the front. What the exact value of these 

 diffei'ences may be it is at present impossible to say. 



