1903. ] FRESHWATER CRAB FROM UPPER GUINEA. 43 
appears, on either side of the cardiac and intestinal regions, 
somewhat granular and rugose in the Congo species, but quite 
smooth, though finely punctate, in P. latidactylum. 
In the large aged female from the River Prah the oblique rug 
near the lateral boundaries of the carapace show a tendency to 
disappear. 
In both species the postfrontal ridge is rather prominent, 
smooth, and nowhere granulated. In the type specimen of 
P. africanum it extends in a nearly straight line towards the 
second epibranchial tooth, uniting with that of the right side, 
but ending about 1 mm. short of that on the left. The postfrontal 
ridge of P. latidactylum usually curves slightly forwards mesially 
and at each end; it never unites with the epibranchial teeth, 
ending about 13 or 2 mm. short of the middle of the first epi- 
branchial tooth. It is situated a little more distant from the 
orbits than in P. africanum, so that the furrow behind the orbits 
is somewhat narrower in the latter species. The front has the 
same form and breadth in both, and shows in the middle a broad, 
though shallow bay; both the front and the furrow behind the 
orbits are a little granular in P. africanum, but quite smooth, 
though finely and sparsely punctate, in P. latidactylum. 
The extraorbital and the two epibranchial teeth have a quite 
different form (Pl. IX. figs. 1-3 & 7, 8). In P. africanum 
(Pl. IX. figs. 7, 8) they are more prominent and separated from 
one another by much deeper incisions. The first epibranchial 
tooth is, in this species, a little smaller than the extraorbital, but 
has about the same form; its straight or slightly arcuate outer 
margin makes a right angle with the anterior margin of this 
tooth. In P. latidactylwm, however, the extraorbital tooth and 
the anterior epibranchial one are much less prominent and much 
lower, the anterior margin of the first epibranchial tooth being 
considerably shorter than its outer margin, measuring only one-fifth 
of it. In the aged female from the River Prah the extraorbital 
tooth appears somewhat longer than the epibranchial one, and in 
the two males (Pl. TX. fig. 1) they have about the same length ; 
in the much younger female from Liberia (Pl. IX. fig. 3) the 
epibranchial tooth, however, is almost twice as long as the extra- 
orbital, but in the quite young male from the same locality 
(Pl. TX. fig. 2) the extraorbital tooth appears again once and a 
half as long as the epibranchial. As regards the relative length 
of these teeth, P. latidactylwm presents therefore considerable 
individual differences. In both species the second epibranchial 
tooth passes backward into an arcuate crest, bounding the cephalo- 
thorax laterally; this crest, which in young individuals of P. lati- 
dactylum (Pl. TX. fig. 3) appears distinctly denticulate, the den- 
ticulations or crenulations disappearing in more aged individuals, 
reaches in P. africanum to the posterior boundary of the uro- 
gastric lobes and does not curve inward on to the surface of the 
branchial region: in P. latidactylum, however, it is much shorter, 
