1913-] Stani^ey Kemp : Crustacea Decapoda. 291 



that it is only since the pubUcation of Alcock's memoir that it 

 has been possible to separate the sub-families Potamoninae and 

 Gecarcinucinae in a satisfactory manner. The classification of 

 the great majority of the known species of the family will have 

 to be revised in the light of Alcock's work. 



It might, indeed, be argued that the species allocated to Pota- 

 miscus do not form a natural group and that the character used 

 for distinguishing the sub-genus is one of doubtful value, more 

 especiall}^ in view of the fact that the exopodite of the outer max- 

 illipede shows varying stages of degeneration in Phricotelphusa 

 and Globitelphitsa, sub-genera of the Gecarcinucinae, and that in 

 one of the specimens of a new species of Potamiscus, described 

 below, the lash of the exopod is well developed on both sides, 

 though quite vestigial in the other six examples. 



Two undescribed species of Potamiscus were obtained in the 

 Abor country and another very remarkable form, also hitherto 

 unknown, has recently been found by Mr. F. H. Gravely in the 

 Dawna Hills. Excluding P. tumidulum, Alcock, concerning which 

 additional information is still lacking, five representatives of the 

 sub-genus are now known from India. 



These fall into two very distinct groups. One, which comprises 

 P. annandalei and P. decourcyi, shows clear affinities with Pota- 

 mon, S.S., while the relationships of the three allied forms included 

 in the second group are much less easy to trace. In course of time 

 it may be necessary to restrict the sub-genus to this group, with 

 P. sikkimense, Rathbun, as type, though it will then be very 

 difficult to frame a satisfactory diagnosis which will exclude P. 

 annandalei and its ally. 



For the present it is not proposed to make any alteration of 

 the kind. Alcock's classification has at least the great merit of 

 convenience, no small advantage in a group presenting such diffi- 

 culties as do the Potamonidae, and until the extra-Indian species 

 have been classified on the lines which he has advocated, such ques- 

 tions as the mutual relationships of the different sub-genera are 

 best left untouched. 



The five species may be tabulated as follows : — 



1. Post-orbital crests sharp-edged and 

 undermined . 



a. Post-orbital crests not sepa- 



rated from epigastric crests 

 and continued as a well-defin- 

 ed ridge to the blunt epibran- 

 chial tooth ; crenulate ante- 

 rolateral margin of carapace 

 not sharply pinched off from 

 the general surface . . annandalei, Ale. 



b. Post-orbital crests separated 



from epigastric crests by a 

 groove and broken into tuber- 



