SEDENTARY ANNELIDES. Lam. The third order of the class Annelides, Lam. distinguished from the two other orders by the circumstance of the animal being enveloped by a shelly tube which it never entirely leaves. The order is divided into the families Dorsalia, Maldania, Serpulacea, and Amphitrites. Fig. 1 to 13. SEA DATE. The common name for PHOLAS Dactylus in the market, given to it on account of its cylindrical shape. Fig. 35. SEGMENTINA. Flem. NAUTILUS Lacustris, Montagu. Test. Brit. Planorbis nitidus, Drap. tab. 2. Fig. 17 to 19. SEMICORDATE. Half heart-shaped. SEMIDISCOIDAL. Forming the half of a circular disc. SEMILUNAR. Half moon-shaped. SENECTUS. Humph. A genus of "Senectinæ," thus described by Swainson: "Imperforate; the base produced into a broad flat lobe, spire rather elevated and pointed; the whorls convex; aperture perfectly round; not more oblique than _Helix_; inner lip entirely wanting, imperialis. Mart. 180. f. 1790. marmoratus. l. M. 448. f. 1." Sw. p. 348. SEMIPHYLLIDIANA. Lam. The second family of the order Gasteropoda, Lam. the genera of which are distinguished as follows:-- 1. UMBRELLA, round, flat; apex central, muscular impression not interrupted. Fig. 233. 2. PLEUROBRANCHUS, apex lateral, sub-spiral. Fig. 232. SENOCLITA. Schum. CINERAS, Leach. SEPTARIA. Lam. See TEREDO. SEPTUM. (Lat.) An enclosure, applied to the thin plate of Crepidula, fig. 239; also to the plates dividing the chambers of multilocular shells. SERAPHS. Montf. TEREBELLUM convolutum, Lam. Fig. 451. SERPULA. Auct. (_A little serpent._) _Fam._ Surpulacea, Lam.--_Descr._ Tubular, narrow, pointed at the apex, gradually widening towards the aperture, attached irregularly, sometimes spirally, twisted, imbricated; keeled or plain; aperture generally round, with the edge simple, or angulated by the termination of external ribs or keels.--_Obs._ This description is intended to include the genera Serpula, Spirorbis, Vermilia, Galeolaria, &c. The Serpulæ abound in all seas, on rocky shores, at any time covered by water, attached to any kind of marine substance, whether moveable or stationary. The fossil species occur in almost all tertiary strata. Fig. 4 to 7. SERPULACEA. Lam. The fourth family of the order Sedentary Annelides, Lam. containing the following genera of tubular, irregular shells. 1. SERPULA, attached by a small portion of the shell. Fig. 4. 2. SPIRORBIS, attached by the whole length, coiled. Fig. 5. 3. Galeolaria, with the open extremity raised, and the aperture tongue-shaped. Fig. 6. 4. VERMILIA, attached by the whole length, straight or waved. Fig. 7. 5. SPIROGLYPHUS, which hollows a bed in the body to which it is attached. Fig. 8.