River-crab from the Transvaal. 



235 



flic present species. Only in one instance is there anything 

 suggestive of a transition from the one type to the other; 

 this is in the ease of three specimens from an unspecifi <1 

 locality in the Transvaal, in which the junction of the post- 

 fronta] ridge with the antero-lateral margin is produced in a 

 small blunt tooth, behind which, however, the margin is only 

 obscurely granulated. 



On any of the current schemes of classification for the 

 family Potamonidffl this species would be generically or sub- 

 g< nerically sej)arated from P. perlatutn. Its antero-lateral 

 teeth are quite as well developed as in P. niloticum (M.-E.), 

 the genotype of AcantJiothelphusa, Ortmann. This name 



Potamon (Potamonautes) icctrreni, so. n., holotype. 

 Outline of one-half of carapace from abuve. 



Vas proposed for a subgenus of Potamon, afterwards merged 

 in I'aratkelphusa by Miss Rathbun, accorded generic rank by 

 Alcock, and now included as a subgenus of Hydroihelphusa 

 by Bouvier (C. K. Acad. Sci. clxv. 1917, p. 620), owing its 

 separation in each case mainly to these antero-lateral teeth. 

 I believe, however, that Acanthothelphuaa cannot be main- 

 tained — at all events, on the ground of this character alone. 

 Just as P. warreni is intimately related to its geographical 

 neighbour P. perlatum, so there is at least a hint that P. ni- 

 loticum may be related, though less closely, to some of the 

 river-crabs of East Africa. A comparison with P. johnstoni 



16* 



