134 AMERICAN MARINE CONCHOLOGY. 
to be an important chayacter, inasmuch as it is also wanting to 
some species of Thracia, although the typical species possesses it. 
Genus LYONSIA, Turton. 
Brit. Bivalves, 35. 1822. 
Osteodesma, Deshayes, Encye. Meth., iii. tab. 1830. 
The animal has a closed mantle; a tongue-shaped, grooved foot, 
byssiferous; very short siphons, which are nearly united, fringed; 
large lips and narrow, triangular palpi. 
Distribution, twelve species—all seas. 
1. L. wyatina, Conrad. Fig. 301. 
(Mya.) Journ. Philad. Acad., vi. 261, t. 11, f. 12. 
Shell thin, fragile, pellucid, transversely elongated; anterior 
side short, rounded; posterior side produced, compressed, trun- 
cated, and reflexed at the end; beaks prominent, inclined for- 
wards; epidermis dirty-white, membranaceous, concentrically 
wrinkled and corrugated by radiated lines. 
Vertical diam. 8.7, transverse 15 mill. 
Whole Coast. 
2. L. AaRENOoSA, Moller. Fig. 302. 
(Pandorina.) Ind. Moll. Greenl. 1842. 
Shell ovate-quadrate, ventricose, opaque-white ; beaks anterior; 
epidermis finely radiately ridged, with frequently adhering parti- 
cles of fine sand. 
Height 7.5, length 12.5 mill. 
New England, northwards. 
Distinguished from ZL. hyalina by its smaller size and quadrate 
form. 
Genus THRACIA, Leach. 
Blainy., Dict. Sc. Nat., xxxii. 347. 1824. 
The mantle of the animal is closed; foot linguiform; siphons 
rather long, separate, with fringed orifices; gills single, thick, 
plaited ; palpi narrow, pointed. 
About twenty species have been described, from northern and 
temperate zones, and ranging from 4 to 110 fathoms. 
1. T. conrapi, Couthouy. Figs. 308, 309. 
Bost. Journ. Nat. Hist., ii. 153, t. 4, f. 2. 
T. declivis, Conrad (not Pennant). Am. Mar. Conch. 44, t. 9, f. 2. 
_T. inflata, J. Sowerby. 1845. (Teste Jeffreys.) 
Shell thin, ventricose, rounded in front, narrowed and subtrun- 
