PORNUS. Humph. AMPULLARIA, Lam. PORCELLANA. Adanson. MARGINELLA, Auct. PORODRAGUS. Montf. A genus composed of species of Belemnites, placed by De Blainville in the section characterized as swelled near the apex, and straightened towards the base. POSIDONIA. Brong. A genus formed on the cast of a bivalve shell, common on schists from Dillemberg. POSTERIOR. (_After_, _behind_.) The posterior or hinder part of a bivalve shell, is that in which the siphonal tube of the animal is placed. It is known in the shell, by the direction of the curve in the umbones, which is from the posterior towards the anterior; also by the ligament, which is always placed on the posterior part of the hinge, when it exists only on one side of the umbones; and by the sinus (when there is one) in the palleal impression, which is always near the posterior muscular impression. In some shells, however, it is very difficult for a learner to trace these marks; such bivalves, for instance, as have the ligament spread out on both sides of the umbones; such as are nearly symmetrical, and have the umbones consequently straight, and a single muscular impression near the centre of the valve. The Brachiopodous bivalves have a different position, with relation to the animal, from the other bivalves, so that the hinge line is the posterior extremity, and the part where the valves open, is the anterior. The posterior extremity of the aperture of a spiral univalve shell, is that nearest to the spire. In patelliform shells the anterior and posterior extremities are distinguished by the muscular impression, which is annular, enclosing a central disc in the inner surface of the shell, excepting where it is interrupted by the place where the head of the animal lies, which of course is anterior. The posterior is marked _p._ in fig. 119, and 387. See ANTERIOR. POSTERO-BASAL MARGIN of a bivalve shell is the posterior side of the margin opposite the hinge. POSTERO-DORSAL MARGIN is the posterior side of the hinge. POTAMIS or POTAMIDES. Brong. A genus of fresh-water shells resembling Cerithium in the characters of the aperture, but which may be known from that genus by the thick, horny epidermis with which they are coated. P. muricata, fig. 377. (Cerithium, Sow.) We think that these shells should be placed near MELANIA. POTAMOMYA. A genus of shells resembling Corbula, in every respect except that of being inhabitants of fresh-water. Fig. 498, 499, represents one of these fresh-water Corbulæ.