1901.] CRUSTACEAN FROM THE SOUDAN. 103 



The measurements of the detached chelipedes are the follomug: 

 of one of them the pahn and the fingers are respectively 8^ mm., 

 the height of the pahn at the articulation of the dactylus me'asures 

 7g mm ; in the other leg the palm is 7| mm. long, and 7 mm. 

 high at the articulation of the fingers, which measure 9 mm. 



I have also before me a type specimen of Pofamon {Potamoncades) 

 cristatum A. M.-E., from the Paris Museum, a species the habitat 

 of which is still unknown (A. Milne-Edwards in Nouv. Archives 

 du Museum, t. v, p. 180, pi. xi. figs. 1 & 1 a). As this species is still 

 insufficiently known, I will compare it with P. liilgendorfi Pfeffer. 



The carapace of P. cristatum appears someivhat longer in pro- 

 portion to its width than that of P. hilgendorji., and the antero- 

 lateral margins project less laterally, so that the cephalothorax is 

 not so wide. The upper surface appears a little convex from 

 before backwards and the lateral portions of the cervical sutui'e 

 are completely wanting, but the median semicircular part of it is 

 distinct though not very deep. The postfrontal crest passes in a 

 somewhat sinuous line to the Interal margin : that of P. liilgen- 

 dorfi, however, in an almost straight line. The gastric region is 

 also anteriorly, as everywhere, smooth, ivitJiout granulation. The 

 front has the same form in both species, but that of P. cristatum 

 appears someivhat broader in proportion to the distance between, 

 the external orbital angles. The antero-lateral margins are 7nore 

 finely granulate than those of P. hilgendorfi Pfeffer, and that 

 part which is situated between the extraorbital and epibranchial 

 angles appears in P. cristatum less oblique, a little arcuate, and 

 though not toothed makes a distinct angle with the postfrontal 

 ridge when the cephalothorax is looked at from above, whereas in 

 P. hilgendorfi Pfeifer this part passes without any interruption 

 into the rest of the margin. 



The lower margin of the orbits fully agrees '\\\ both species, for 

 also in P. cristatum there is no incision or hiatus near the outer 

 angle. In P. cristatum the ischium-joint of the outer foot-jaws 

 is distinctly furrowed, and this groove runs somewhat closer to the 

 inner than to the outer margin. 



The suborbital and subbranchial regions, together with the 

 grooves that define them, fully agree in both species. 



The chelae of the male of P. cristatum are of equal size and 

 shape. The fingers are somewhat gaping at base, whereas those 

 of P. hilgendorfi Pfeifer are in contact throughout their length ; 

 they are distinctly longer than the palm and deeply furrowed. 

 On the outer surface of the immobile finger two deep grooves are 

 observed near one another, on that of the dactylus three or four. 

 These furrows are less deep in P. hilgendorfi Pfeffer. The mero- 

 podites of the ambulatory legs of P. cristatum finally are more 

 enlarged. 



Measurements of the type of Potamon cristatum A. M.-E. in 

 millimetres : — 



Greatest breadth of the carapace 16| 



Length of the carapace 13 



