158 On a new River- Crab from Yunnan. 



of carapace, clielipeds, and legs is abundantly marbled witli 

 orange-red. 



This species is distinguislied from all the Asiatic represen- 

 tatives of the genus hitherto described by the possession of 

 more than four antero-lateral teeth on the carapace and by 

 liaving two spines instead of one on the carpus of the 

 clielipeds. Further, the antero-lateral teeth are much more 

 slender and spiniform and their number is variable, whereas 

 Wood-Mason remarks that in this genus these teeth " in 

 point of number and form are as constant- for the several 

 species as are those of the Portunidai" *. In all tliese points 

 the new form approaches tlie African group of species referred 

 to Parathelphusa by A. Milne-Edwards f, but regarded as 

 forming a subgenus of Potamon, under the name Acantho- 

 ihelphusa, by Ortmann |. But in some, if not all, of the 

 African species the merus of the chellpeds has a spine on the 

 anterior and none on the upper edge, and, further, most of 

 them have a strong and continuous postfrontal crest. Ac- 

 cording to Ortmann's arrangement, the chief character sepa- 

 rating Parathelphusa from Potamon (with its subgenus 

 Acanthothdphiisa) is the less deflexed front. This, however, 

 is an ill-defined and variable character, and in some un- 

 doubted species oi Parathelphusa (e. g. P. Dayana), as in the 

 present form, the front is quite as much deflexed as in many 

 species of Potamon. 



Had the ])resent species been found in Africa there can be 

 little doubt that most carcinologists would have referred it to 

 Acanthothelp)husa. Nevertheless, I do not believe that it is 

 necessary to assume any special relationship between it and 

 the African species. The differences separating it from some 

 of thS other Asiatic species of Parathelphusa are compara- 

 tively small, and the distance is not great even to some of 

 those species of Potamon in which the antero-lateral margin 

 is coarsely granulated, as in P. Afkinsonianum and P. denti- 

 culatum. Throughout the whole family of the Potamonidse 

 the generic and specific characters are often very elusive, 

 phylogenttic conclusions are more than usually hazardous, 

 and our knowledge of the group appears to be somewhat 

 inadequate to bear the weight of the geographical speculations 

 which Ortmann has recently based upon it. 



* Wood-Mason, Ann. & ISra<r. Nat. Hist. (4) xvii. 1876, p. 120. 



t A. Milne-Edwaids, Hull. Soc. Pliilom. Paris, x. 1886, p. 143; and 

 Ann. Sci. Nat. (7) iv. 1887, p. 140. 



I Oitmann, Zool. Jabrb. Abth. f. Syst. x. 1898, p. 300. It is remark- 

 able tbat not only in tbis, but also in a later paper (Proc. Amer. Pbil. 

 Soc. xli. 1902, p. 300), Ortmann ajipears to have overlooked tbe second 

 of Milne-Edwavds's papers fjuutud above, iu which the African species 

 are figured. 



