igog.] A. Alcock : New Freshwater Crabs. 247 



as in the mixed assemblage of species to which the name Potamo- 

 nautes has been applied. 



In a large female the carapace is i inch long, i\ inch broad, 

 and a little over | an inch deep. 



From Nemotha, Cachar; nine males and six females. 



2. Varieties of Potamon lugubre, Wood-Mason. 



This species is not a true Potamon (subgenus), since it has the 

 terminal joint of the mandibular palp bifurcate or bilobed as in 

 " Potanionautes" jacquemontii, " Geotelphusa " Icevis, Paratelphusa 

 spinigera and Gecarcinucus jacquemontii. 



It is a ver}^ variable species. In the large series which I have 

 examined I can distinguish five varieties (not including P. masoni- 

 anum which seems to be but a variety) to some or all of which 

 many naturalists, with only single specimens of each before them, 

 might give specific rank. In none of these varieties, however, is 

 there any constancy. 



a. Potamon lugubre, var. edentulum. 



H In this variety the lateral epibranchial tooth is quite obsolete, 

 the antero-lateral borders of the carapace are unusually convex, and 

 the individual regions of the carapace are unusually tumid. 

 From the Naga Hills. 



b. Potamon lugubre, var. harpax. 



Though the carapace is flat on the whole, the individual 

 regions are tumid, the amount of swelling being very variable : the 

 carapace also is not quite so broad as in the type. 



The lateral epibranchial tooth is usually more distinct, and 

 the antero-lateral borders are often less arched than in the type. 



The front is usually less deflexed. 



The spine at the inner angle of the carpus is usually more 

 acute. 



In some large males the hand of the larger cheliped is enor- 

 mously enlarged, the dactylus being much longer than the palm 

 and strongly arched, so that when the fingers are closed only their 

 tips are in contact and a very wide gap is left between them. In 

 the female the fingers of both chelipeds are usually a little longer 

 than in the type. 



From Assam, Cachar, Sylhet, Khasi Hills, Garo Hills, and Naga 

 Hills. 



c. Potamon lugubre, var. nigerrimum. 



As in the variety harpax the carapace is not quite so broad, 

 the lateral epibranchial tooth is a spine, the individual regions of 

 the carapace are tumid, the antero-lateral borders are less arched, 

 and the front is less deflexed. 



