SUTURE. (_Sutura_, lat.) A seam, stitch, joining together. Applied particularly to the line which marks the joining of the whorls of the spire. The suture is distinguished as _simple_, as in most cases; or _double_, when accompanied by a parallel groove close to it; _marginated_, when produced into a ledge by the matter which fills up and covers it; _obsolete_, when it is filled up so as not to be visible, as in the case of Ancillaria. SYLVICOLA. Humph. CYCLOSTOMA, Lam. SYMMETRICAL, ([Greek: sun], _syn_, similar; [Greek: metron], _metron_, proportion.) Both sides alike. Although the term is used thus as one of distinction, it is to be observed that no shells are strictly and perfectly symmetrical; even in the Nautilus, the apex verges in a slight degree towards one side of the shell. Two kinds of univalve are symmetrical, or nearly so; 1st. Those which are symmetrically convolute, as the Nautilacea and the Ammonacea, which are spiral; 2nd. Those which are not spiral, but simply conical, as the patelliform shells. Bivalves belonging to the Brachiopoda are also symmetrical. _Ex._ Patella, fig. 229. Ammonites, fig. 478. SYMPHYNOTA. Lea. A genus of Nayades, in which Mr. Lea proposed to include species of the genus UNIO, the valves of which are connate, or united at the dorsal margin. We believe that this distinction, as a genus, has been abandoned by its author. The fact is, that all the Uniones are Symphynotæ when in a young state. In Unio Alatus, (fig. 147) and Dipsas plicatus, (fig. 142) it will be observed that the valves have not separated at the dorsal edge, but are broken lower down. TAPADA. (Gray. Turton. p. 127.) A division of the genus HELIX, containing HELIX aperta, Auct. or the Tapada snail. TAPES. Schum. PULLASTRA. Sow.? TECTUS. Montf. A genus composed of species of the genus Trochus, having elevated, conical spires, and columella notched or truncated by a spiral fold. Fig. 359. Trochus maculatus, presents an example. TELEBOIS. Montf. A genus of microscopic Foraminifera. TELESCOPIUM. Montf. CERITHIUM Telescopium, Auct. fig. 378. TELLINA. Linn. _Fam._ Nymphacea, Lam. Conchacea, Bl.--_Descr._ Sub-equivalve, inequilateral, compressed, rounded anteriorly, slightly beaked or angulated posteriorly, the posterior ventral margin having a flexuosity; hinge with two cardinal and generally two lateral teeth in each valve; muscular impressions, two in each valve, remote; palleal impression with a large sinus.--_Obs._ The fold or bending in the posterior margin distinguishes this genus from others which it nearly resembles. It is composed of some bivalves of great beauty and variety, which are found in nearly all climates. Fig. 105, T. radiata, 106, T. lingua-felis.