PSAMMOBIA. Lam. _Fam._ Nymphacea, Lam.--_Descr._ Transverse, oblong, slightly gaping at both ends; hinge with two cardinal teeth in one valve, one in the other; ligament supported upon a prominent fulcrum; muscular impressions two in each valve, sub-orbicular, distant; palleal impression with a large sinus; epidermis thin.--_Obs._ The genus thus described includes PSAMMOTÆA of Lamarck, which, according to him, only differs in the number of teeth, and which he says are but "Psammobies dégenerées." The difference appears to be accidental. This genus differs from Tellina in not having a posterior fold in the margin. Fig. 100. The species are found in temperate and tropical climates. PSAMMOCOLA. Bl. ([Greek: psammos], _psammos_, sand; _cola_, an inhabitant.) A name given by De Blainville to shells of the genus PSAMMOBIA, including PSAMMOTÆA of Lamarck. PSAMMOTÆA. See Lam. PSAMMOBIA. PSEUDOLIVA. Sw. A genus of "Eburninæ," Sw. thus described: "Shell thick, oval, oliviform, ventricose; spire very short, acute; base with two parallel grooves, one of which forms a notch at the base of the outer lip; suture slightly channelled; inner lip very thick, and turning inwards; aperture with an internal canal. Connects the TURBINELLIDÆ with the VOLUTIDÆ. P. plumbea, Chem. 188. f. 1806, 1807." Sw. p. 306. PSILOSTOMATA. Bl. The third family of Aporobranchiata, Bl. containing no genera of shells. PTEROCERAS. Auct. ([Greek: Pteron], _pteron_, a wing; [Greek: keras], _ceras_, horn.) _Fam._ Ailées, Lam.--_Descr._ Turrited, oval, ventricose, thick, tuberculated; spire short; aperture oval, terminating in a lengthened canal at both extremities; outer lip thickened, expanded, produced into horn-shaped, hollow, thickened spires, with an anterior sinus apart from the canal.--_Obs._ This genus, containing the shells commonly called Devil's Claws, Gouty Scorpions, Spiders, &c. is distinguished from Strombus by the digitations of the outer lip. No fossil species are known. Fig. 405, P. aurantiaca. PTEROCYCLOS. Benson. Syn. B. M. p. 133. A genus formed of species of Cyclostoma, Auct. which have "a groove or hole at the hinder part of the mouth." PTEROPODA. Lam. ([Greek: Pteron], _pteron_, a wing; [Greek: pous], _pous_, a foot.) The first order of the class Mollusca, Lam. consisting of molluscs whose organs of locomotion consist of a pair of wing-shaped fins. This order contains the genera Hyalæa, Clio, Cleodora, Spiratella, Cymbulia, and Pneumoderma. To which may be added other genera enumerated in explanation of figures 220 to 226. They may be thus distinguished. 1. ATLANTA. Shaped like Nautilus, symmetrical. Fig. 220. 2. SPIRATELLA. Spiral, not symmetrical. Fig. 224. 3. CRESEIS. Straight, thorn-shaped. Fig. 222.