PLEUROTOMA. Lam. _Fam._ Canalifera, Lam. Siphonostomata, Bl.--_Descr._ Fusiform, thick, in general ribbed or striated transversely; aperture oval, terminating anteriorly in an elongated canal; outer lip thin, with a fissure near its spiral extremity; columella smooth, nearly straight. Found principally in tropical climates.--_Obs._ This genus, which nearly resembles Fusus in other respects, may be known by the notch in the outer lip. The species differ in the length of the canal. Swainson has designated this genus a family, thus divided into genera: Brachytoma, in the description of which he says that the spire and aperture are of equal length, including the species strombiformis: Pleurotoma, in which the channel is so much lengthened, as to be little shorter than the spire: Clavatula, having the long narrow slit of Pleurotoma, but with a very short canal: Clavicantha, having the canal equally short, but the sinus or notch, instead of being linear and long, is short and wide; the surface is rough, and the whorls either coronated with prickles, or with compressed nodules resembling spines: Tomella, which has the spire and canal fusiform, but the spire of very few whorls, and the inner lip considerably thickened within where it joins the outer lip. Fig. 379, 389, P. marmorata; 381, P. Strombiformis, (Clavatula, Sw.) PLEUROTOMARIA. Defr. _Fam._ Turbinacea, Lam.--_Descr._ Turbinated, spiral; aperture sub-quadrate, with rounded angles; outer lip with a deep slit near its union with the spire.--_Obs._ This genus, which is only known in a fossil state, abounds in inferior Oolite, Oxford clay, and casts are found in a limestone bed in Norway. The Scissurellæ differ in being very minute shells, and are not so trochiform as the species of Pleurotomaria, P. reticulata, fig. 341. PLICACEA. Lam. A family of the order Trachelipoda, Lam. containing the following genera: 1. PYRAMIDELLA. Pyramidal, with numerous whorls. Fig. 342. 2. TORNATELLA. Cylindrical, with few whorls. Fig. 343, 344. 3. RINGICULA. Margin reflected. Fig. 540, 541. PLICADOMUS. Sw. A sub-genus of Pupa, thus described: "spire moderate, regular and thick, but gradually conic; the tip obtuse; aperture perpendicular; inner lip wanting; outer lip semicircular; the margin dilated and reflected. P. sulcata, Chem. 135, f. 1231, 1232." Sw. p. 332. PLICATED. (_Plicatus_, folded.) Applied to spiral plaits on the columella of some shells. _Ex._ Voluta, fig. 433. Also to the angular bendings in the margins of some bivalve shells. _Ex._ Dendostrea, fig. 181.