OCR Text |
Show 1901.] CRUSTACEAN FBOM THE SOUDAN. 95 extremities; but between the orbits the crest appears smooth. The mesogastric suture, 6 m m . long, does not extend to the middle of the space between the postfrontal crest and the transverse groove limiting the mesogastric and the urogastric regions from one another; and this groove, visible immediately behind the middle of the carapace, is very shallow and hardly distinguishable. A little further backwards a similar shallow groove is observed separating the urogastric area from the cardiac. The lateral grooves of the H-shaped figure are somewhat deeper, and likewise the two >-shaped grooves that bound the anterior cardiac region laterally. The lateral portions of the cervical suture, which in other species run obliquely forward and outward, are quite indistinct in Potamon fioweri; their direction, however, is still indicated by impressed punctures, that are somewhat larger than the minute punctures scattered on the upper surface of the carapace ; the latter are very fine, only distinguishable by means of a magnifying-glass, aud rather few in number. The antero-lateral margins of the carapace are strongly arcuate, almost semicircular, bulging out very much laterally ; they extend as far beyond the external orbital angles as the breadth of the orbits. They are defined by a distinctly granulated line that extends backwards as far as the urogastric area. The posterolateral margins are rounded and smooth and appear very slightly concave, when the carapace is looked at obliquely from above. An epibranchial tooth is wanting. The granulated line that defines the antero-lateral margins, posterior to the postfrontal ridge, is formed by fifteen or sixteen rather large granules, that are not sharp, gradually decrease in size backwards, and finally disappear. The distance between the epibranchial angles measures four-fifths, and that between the extraorbital angles about two-thirds of the width of the carapace. The front is somewhat convex longitudinally, but almost straight transversely, and the width of the free border measures one-fourth the breadth of the cephalothorax; the upper surface is smooth, rather closely punctate, and the punctures are slightly larger than those of the upper surface of the carapace. W h e n the latter is looked at from above, the free border of the front appears widely emarginate in the middle; this anterior margin forms very obtuse, though not rounded, angles with the very oblique lateral margins of the front; the latter are somewhat thickened, whereas the transverse external portions of the upper orbital margins are thinner. The sharp, dentiform, outer angles of the orbits are rather prominent and forwardly directed. Between the extraorbital tooth and the epibranchial angle there is a granulated tooth or prominence immediately behind the groove that separates the suborbital and subbranchial areas from one another : this tooth, however, is a little smaller than the extraorbital tooth. The postfrontal crest lies far forwards, so that when the carapace is looked at from above a small portion of the upper margin |