// 



17 



The palp of the external maxillipecls articulates at or near the inner angle 

 of the merus. Mouth-cavern usually square. 



Chelipeds usually more or less unequal in both sexes. Legs gressorial. 



It is unnecessary, in dealing with the Indian fauna, to criticise any of the 

 subdivisions of this family that have been proposed by other authors. I have, 

 hovrever, examined specimens of a good many exotic Potamonidce, and I find 

 that the family is cut in two by the mandible — or, rather, by the mandil)ular 

 palp. 



In one of these two sections the mandibular palp is of the normal 

 brachyurous form, the terminal point being simple and falciform. Examples 

 of this section are Potamon fliiviatile, Potamonautes perlatus, Geotelphnsa 

 dehaanii, Platytelphusa armata, " Paratelphusa " nUotica, Erimetopus spi?iosus, 

 Trichodactylus quadratus, Dilocarcinus septemdentatus, Valdivia panophis, and 

 Deckenia imitatrix. 



In the other of these two sections the terminal joint of the mandilmlar 

 palp is split from its base into two lobes, which embrace the incisor process 

 of the mandible. Examples of this section are Gecarcinucus jacqiiemontii, 

 Paratelphusa tridentata, Peritelphusa horneensis, Pseudotelphusa dentata, 

 Potamocarcinus lati/rons, "Potamon" hydrodromus, " Potamonautes " /(/c^'Mg- 

 montii ( = Telphusa indica, aiictorum), " Geotelphusa " kevis. 



I must add that I am indebted to Dr W. T. Caiman of the British Museum 

 for the suggestion that the mandibular palp might give the clue to the labyrinth 

 of the Potamonidce. I take this opportunity of expressing my great obligation 

 to Dr Caiman, not only for this hint, which I have found so interesting and so 

 useful, but also for the kindness and patience with which he bore my many 

 inquisitive trespasses upon his valuable time in the course of this work. 



The Indian genera of the family Potamonida' are exhibited in the following 

 table : — 



I. Mandibular palp of three distinct joints; the terminal joint simple, some- 

 times thickened at base for the attachment of a bunch of hairs . Potamon. 

 II. Mandibular palp of two distinct joints ; the terminal joint cut into two lobes 

 which embrace the incisor process of the mandible : — 



1. Front much wider than the orbit ..... Paratelphusa.i r ^^ 



2. Front little or hardly wider than orbit .... Gecarcinucus.^ 



1 N.B. — To examine the palp, the mandible must be raised from its bed, as in repose only the 

 anterior lobe can be seen, overlapping the incisor process. 



