276 SIPHONABIA, 



by the longitudinal slit. The recent species are found in the 

 sponges with siliceous spiculse, in the Mediterranean ; the fossils 

 in tertiary beds. PL i. fig. 1. 



SIMPLE. {Simplex, lat.) Single, entire, uninterrupted, un- 

 divided. 



SIMPLEGtAS. Mont, 1, 83. {Simplex^ simple ; yaorjjp, 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. — Ohs. The 

 septa of the shell named Simplegas by De Montfort, are evi- 



. dently sinuous, according to his figure. S. sulcata, PI. xxii. 

 fig. 475. 



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, wiU be indicated 

 by the letter s. 



SIPHON, (^ifov, siphon.) A pipe, or tube. A shelly tube 

 passing through the septa of chambered shells. It is said to be 

 dorsal, central, or ventral, according to its situation near the 

 outer, or inner parts of the whorl. See Introduction. 



SIPHON AL SCAR. The name appHed by Mr. Gray to the 

 opening or wiading sinus in the palleal impression of a bivalve 

 shell, in the place where the siphonal tube of the animal passes. 



SIPHONARIA. Sow. (2t<|.o»', siphon.) Fam. PhyUidiana. 

 Lam. Patelloidea, Bl. — Descr. Patelliform, depressed, inclining 

 to oval, ribbed ; apex nearly central, obliquely inclining towards 

 the posterior margin ; muscular impression partly encircling the 



