19 



The mandibular palp is of the normal brachyurous pattern : all three 

 joints are separately distinguishable, and the sickle-shaped terminal joint is 

 simple, not bilobed or bifurcate, and in repose lies altogether behind the incisor 

 process of the mandible. 



The chelipeds are generally more massive than the legs, and are commonly 

 more or less unequal in both sexes. 



The dactyli of the legs are stout, and are armed with four rows of strong 

 translucent spines. 



In the Indian species of Potamon, besides the simple mandibular palp, and 

 besides the comparatively broad 6th abdominal segment of the adult male, 

 there is a third character, which is of some diagnostic value : and that is, that the 

 cervical groove, when it is distinct in all its course, never runs straight and 

 wide to the lateral epibranchial tooth on either side, but makes more for the 

 inner angle of the external orbital tooth and often cuts the post-orbital crest 

 in that longitude. 



It is not possible, from a consideration of the literature alone, to determine 

 the incidence of this genus. I can only say, regarding Indian species, that 

 it includes selections from the subgenera Potamon, Potamonautes, and Paratel- 

 phusa of Miss Rathbun's great monograph. 



Among the Indian species, four principal groups, or subgenera, can be 

 recognized, as follows : — 



1. Antero-lateral borders of carapace usually finely serrulate or crenulate, 

 without any large spines, except the lateral epibranchial spine : no 

 subterminal spine on the upper border of the merus of the chelipeds : — 



i. Exopodite of external maxillipeds with a long, plumose 

 flagellum : — 



1. Post-orbital crests and lateral epibranchial spine well 

 developed ....... 



2. Post-orbital crests and lateral epibranchial spine ill developed, 

 or obsolete ....... 



ii. Exopodite of external maxillipeds with the flagellum vestigial or 

 absent ........ 



II. Antero-lateral borders of carapace cut into large teeth or armed with 

 sharp spines ; upper border of merus of chelipeds with a subterminal 

 spine ......... 



Potamon. 

 Geotelphusa. 



PoTAMISCUS. 

 ACANTHOTELPHUSA. 





t 



6/ 



Subgenus I.^ — PotAxMON, Ortmann 

 Type : Potamon p>otamios (Oliv.) 



The Indian species of the subgenus may be disposed in five companies, 

 as follows : — 



