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Show 1901.] CRUSTACEAN FROM THE SOUDAN. 103 The measurements of the detached chelipedes are the following: of one of them the palm and the fingers are respectively 8^ mm., the height of the palm at the articulation of the dactylus measures 7g m m ; in the other leg the palm is 7^ m m . long, and 7 m m. high at the articulation of the fingers, which measure 9 m m. I have also before me a type specimen of Potamon (Potamonautes) cristatum A. M.-E., from the Paris Museum, a species the habitat of which is still unknown (A. Milne-Edwards in Xouv. Archives du Museum, t. v. p. 180, pi. xi. figs. 1 & 1 a). As this species is still insufficiently known, 1 will compare it with P. hilgendorfi Pfeffer. The carapace of P. cristatum appears someivhat longer in proportion to its width than that of P. hilgendorfi, and the anterolateral 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 suture 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 lateral margin : that of P. hilgendorfi, however, in an almost straight line. The gastric region is also anteriorly, as everywhere, smooth, without 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 more 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 Pfeffer this part passes without any interruption into the rest of the margin. The lower margin of the orbits fully agrees in 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 chela? of the male of P. cristatum are of equal size aud shape. The fingers are somewhat gaping at base, whereas those of P. hilgendorfi Pfeffer are in contact throughout their length ; they are distinctly longer than the palm and deeply furrowed. O n 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 meropodites of the ambulatory legs of P. cristatum finally are more enlarged. Measurements of the type of Potamon cristatum A. M.-E. in millimetres :- 3- Greatest breadth of the carapace 16| Length of the carapace 13 |