TELLINA. 



269 



most cases ; or double, when accompanied by a parallel groove 

 close to it ; marginated, when produced into a ledge by the mat- 

 ter 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, {aw, syn, similar ; jxerpov, metron, proportion.) 

 Both sides alike. Although the term is used thus as one of dis- 

 tinction, it is to be observed that no shells are strictly and per- 

 fectly symmetrical ; even in the Nautilus, the apex verges in a 

 slight degree towards one side of the shell. Two kinds of uni- 

 valve are symmetrical, or nearly so; 1st. Those which are sym- 

 metrically 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. Ammo- 

 nites, 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 Symphynotse 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. 

 TAP ADA. (Gray. Turton. p. 127.) A division of the genus He- 

 lix, 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. — 



