SHANK SHELL. The vulgar name for the shell designated Murex Rapa. It is used in Ceylon for ornamental purposes. SIDEROLITES. Montf. A genus of microscopic Foraminifera. SIGARETUS. Lam. _Fam._ Macrostomata, Lam.--_Descr._ Suborbicular, oblique, haliotoid, thick; spire depressed, consisting of two or three rapidly increasing whorls; aperture wide, entire, modified by the last whorl, the width exceeding the length; columella tortuous; inner lip spread thinly over part of the body whorl; epidermis thin.--_Obs._ This genus is distinguished from Natica, by the width of the aperture, and the absence of the umbilical callosity. It may be known from Stomatia, and Stomatella, by the texture, which in Sigaretus, is never pearly as in Stomatia, the former being partly an internal shell. Fig. 334, S. concavus. Mostly brought from tropical climates. SILIQUA. Megerle. (A husk, or pod.) LEGUMINARIA, Schum. A genus composed of species of SOLEN, Auct. which have an internal rib. Fig. 51, Solen radiatus. SILIQUARIA. Brug. _Fam._ Cricostomata, Bl. Dorsalia, Lam.--_Descr._ Tubular, rugose, spiral near the apex, irregularly twisted near the aperture, with a longitudinal fissure radiating from the apex, and proceeding through all the whorls and sinuosities of the tube.--_Obs._ This genus was included in Serpula by Linnæus, from which, however, it is distinguished by the longitudinal slit, fig. 1. S. anguina. The recent species are found in the sponges with siliceous spiculæ, in the Mediterranean; the fossils in tertiary beds. SIMPLE. (_Simplex_, lat.) Single, entire, uninterrupted, undivided. SIMPLEGAS. Mont. 1, 83. (_Simplex_, simple; [Greek: gastêr], _gaster_, belly.) A genus described by De Blainville, as being discoidal, and having the spire uncovered like AMMONITES, but having the chambers divided, by simple septa, like Nautilus.--_Obs._ The septa of the shell named Simplegas by De Montfort, are evidently sinuous, according to his figure. Fig. 475, S. sulcata. SINISTRAL. (_Sinister_, left.) On the left side. A sinistral shell is a _reversed_ one. The sinistral valve of a bivalve shell may be known, by placing the shell, with its ligamentary or posterior part towards the observer; the sides of the shell will then correspond with his right and left side. SINUOUS. Winding, serpentine. The septa of Ammonites are sinuous. The muscular impression of the mantle, or palleal impression of some bivalve shells, is sinuated near the posterior muscular impression. SINUS. (_Sinus_, a winding, or bay.) A winding or tortuous excavation. The sinus in the outer lip of Strombus, fig. 406; and that in the muscular impression of Venus, will be indicated by the letter _s_.