1909.] A. Ai^coCK : New Freshwater Crabs. 249 



No. 3. 



The Potamonidse of the Indian fauna are included in three 

 principal genera, Potanion, Paratelphusa, and Gecarcinucus. 



In the genus Potamon the terminal joint of the mandibular 

 palp is simple ; the 6th segment of the abdomen of the adult male 

 is short and broad, its length never being equal to its distal (least) 

 breadth; and the cervical groove, when it is distinct, runs towards 

 the external orbital tooth on either side. 



In the genus Paratelphusa the terminal joint of the mandibular 

 palp is bilobed, the anterior lobe (which is broadly oval) over- 

 hanging the ventral surface of the mandible, the posterior lobe 

 (which is falciform) lying behind the incisor process; the 6th 

 segment of the abdomen of the adult male is a longish joint, its 

 length hardly ever being less, and usually being more, than its 

 disial breadth; and the cervical groove when distinct usually runs 

 towards the lateral epibranchial tooth on either side. 



In the genus Gecarcinucus the terminal joint of the mandi- 

 bular palp is bifurcate as in Paratelphusa ; the 6th segment of the 

 abdomen of the adult male is broad as in Potanion, but is narrowed 

 distally in a way of its own ; and the front is particularly narrow 

 in the adult. 



The Indian species of the genus Potamon can be grouped in 

 four maniples or subgenera as follows : — 



1. Subgenus Potamon. The antero-lateral borders of the 

 carapace are crenulate or serrulate, not multispinous : there is no 

 spine at the far end of the upper border of the merus of the 

 chelipeds: the exopodite of the external maxillipeds carries a 

 strong plumose flagellum : the post-orbital crests and lateral epi- 

 branchial spine of the carapace are well developed. — Type Potamon 

 fluviatile, Latr. 



2. Subgenus Geotelphusa. As Potamon, but the post-orbital 

 crests and lateral epibranchial spine are ill-developed or obsolete. — 

 Type G. obtusipes, Stimpson. 



3. Subgenus Potamiscus. As Potamon, but the flagellum of 

 the exopodite of the external maxillipeds is absent or vestigial. — 

 Type P. annandalii , A. A. 



4. Subgenus Paratelphusula, nov. As Potamon, but the 

 antero-lateral borders of the carapace are cut into large teeth or 

 spines^ and there is a strong subterminal spine on the upper 

 border of the merus of the chelipeds. — Type P. day ana (W.-M.). 



[N.B. — The species of this subgenus of Potamon have been 

 confused with Paratelphusa (type P. tridentata) which is a widely 

 different form.] 



The Indian species of the genus Paratelphusa can be grouped 

 in six subgenera, as follows : — 



I. Subgenus Paratelphusa. The antero-lateral borders of the 

 carapace are usuallj' cut into large teeth or spines, and there is 

 always a strong subterminal spine on the upper border of the 



