BIVALVIA. 13 
point or umbo of the shell, and advances with the increase of the animal, apparently 
in an opposite direction to that of the Dimyaria, the additions being made in a some- 
what sloping direction inclining towards the anterior side, the successive advance is 
denoted by the lines of increase, as the fresh layers of calcareous matter are deposited 
by the animal, most distinctly visible in this ligamental area, which may be called 
tripartite. 
The general substance of the shells of this genus is thick, though they are ex- 
ceedingly variable in that character, some specimens of the common Oyster possess a 
solidity of nearly an inch, while others, quite as large, have not a fourth of that 
thickness. 
This is a Marine genus, though many are inhabitants of estuaries, and some will 
live where the water, at low tide, is not very salt. Its presence, in any formation, is 
considered to be indicative of its marine nature; none have yet been found as true 
inhabitants of freshwater. It is a genus of great antiquity, species having been 
figured from various secondary formations from the lias to the chalk, while some 
were natives of the Seas during the Eocene Period, all of which have become extinct. 
l. OstREA EDULIS, Linnaeus. Tab. II, fig. 1, a—e. 
Osrrea. Lister. Hist. Conch., lib. iii, fig. 30, A, and fig. 31, 1687. 
— Knorr. Delices des Yeux, pl. 24**, fig. 2, and pl. 25**, fig. 2, 1766. 
— xEpuLIs. Linn. Syst. Nat., ed. 12, p. 1148, No. 211, 1767. 
— — Poli. Test. Sicil., vol. ii, p. 169, t. 29, fig. 1, 1795. 
= — Mont. Test. Brit., p. 151, 1803. 
— — Broc. Conch. Foss. Subap., p. 562, 1814. 
— — G. B. Sowerby. Genera, No. 6, fig. 1. 
— — De Blainv. Man. Malac., pl. 60, fig. 1, 1825. 
— — Crouch. Int. Lam. Conch., pl. 12, fig. 8, 1827. 
= — Goldf. Pet. Germ., vol. ii, p. 19, t. 78, fig. 4, a—h. 
— — S. Wood. Catalogue, 1840. 
== — G. Sow., Jr. Conch. Man., fig. 180, 1842. 
= — Phil. En. Moll. Sic., vol. i, p. 90; vol. ii, p. 64. 
= — Nyst. Conch. Foss. de Belge, p. 327, pl. 31, fig. 2, a, 6, and pl. 33, 
fig. 2, a’, b', 1844. 
— — Loven. Ind. Moll. Scand., p. 30, 1846. 
— — Ford. and Hanl. Hist. Brit. Moll., vol. ii, p. 307, pl. 54, and pl. T, 
fig. 1, 1850. 
—  pseupDO-EDULIs? Desh. Exp. Sc. de. Morée, p. 231, pl. 5, figs. 3, 4. 
— peEFoRMIS. Parkinson. Org. Rem., vol. iii, pl. 14, fig. 3, 1811. 
— _ Desh. 2d ed. Lam., tom. vii, p. 229, 1839. 
— uippopus. Desh. 2d ed. Lam., tom. vii, p. 219, 1835. 
—  Bosuayr1? Desh. Exped. Scient. de Morea, pl. 3, figs. 6, 7, 1833. 
— parasitica. Turt. Brit. Biv., p. 204, pl. 17, figs. 6, 7, 1822. 
— —_— Flem. Brit. An., p. 392, 1828. 
— —_ Thorpe. Brit. Mar. Conch., p, 120, fig. 108, 1844. 
—  penticutata? Born. Mus. Coes. Vindobon., p. 113, t. 6, figs. 9, 10, 1780. 
