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Show 94 DR. J. G. DE MAN ON A NEW FRESHWATER [Feb. 19, coloured Kashmir race of the Asiatic Ibex inhabits the great Snowy Range of the Himalaya, where the snowfall is heaviest. The darker Baltistan Ibex, on the other hand, is a dweller in a district where the fall of snow is less ; while the Thian-Shan and Siberian race, at least in part of its habitat, is found in arid districts where the snowfall is still more limited. It would thus seem probable that the type of coloration characteristic of each of the four forms of the Asiatic Ibex mentioned above is directly correlated with the environment of each particular race. 3. Description of a new Freshwater Crustacean from the Soudan ; followed by some Remarks on an allied Species. B y Dr. J. G. D E M A N , of lerseke, Zeeland, Holland. [Eeceived January 21, 1901.] (Plate X.) A male specimen of a Crab from the Bahr-el-Gebel, in the Soudan, obtained by Capt. S. S. Flower, F.Z.S., in April 1900, has been sent to m e for examination. Though apparently belonging to a species not yet described, it was, for the sake of certainty, sent successively to Prof. Pfeffer at Hamburg and to Prof. Hilgendorf at Berlin, w h o both informed m e that in their opinion it represented a new species. I therefore venture now to describe it as such. The carapace is very wide, the greatest breadth, just in the middle between the post frontal crest and the transverse groove separating the mesogastric and urogastric regions from one another, being in proportion to the length as 5 : 3. The carapace is rather strongly convex from before backwards, and somewhat convex transversely. The prominent and sharp postfrontal crest extends to the anterolateral margins much as in Potamon (Potamonaules) aubryi. A. M.-E., a type specimen of which, a male from the Gaboon, was kindly sent m e by Prof. Bouvier. The postfrontal ridge is interrupted by the mesogastric suture, that appears roof-like (" dachformig," Hilgendorf, ' Die Land- und Siisswasser-Deka-poden Ostafrikas," 1898, p. 5). From this suture the crest proceeds sinuously towards, but without uuiting with, the lateral margin of the cephalothorax, a narrow suture remaining between the lateral margin and the lateral extremity of the crest, and this lateral extremity for a very short distance curves backwards (Plate X. fig. 3). In Potamon aubryi A. M.-E., on the contrary, the postfrontal ridge unites with the lateral margin of the carapace. W h e n the cephalothorax is looked at from above, the postfrontal crest appears quite smooth, only a few crenulations being observed near the lateral extremities. In a front view (fig. 2) the free edge of the ridge appears finely crenate, the crenulations slightly, though rather irregularly, increasing in size towards the lateral |