FAMILY OSTRACID — OSTREA. 169 
GENUS OSTREA. Linneus. Lamarck. 
Animal with the edges of its mantle thick, not adhering, retractile, with numerous short and 
irregularly disposed tentacular appendages. Mouth large, funnel-shaped, furnished with 
two pair of elongated lanceolate appendices. Gills formed by four nearly equal and semi- 
circular leaflets, minutely striated. Vent posterior, with its orifice floating between the 
lobes of the mantle. Shell very irregular, more or less coarsely foliated ; left valve gene- 
rally larger and more concave, adherent ; the right valve smaller, usually flattened, often 
operculiform, moving forwards with age, leaving a groove for the ligament exposed along 
the adhering valve. Hinge without teeth. 
OsTREA BOREALIS. 
PLATE X. FIG. 204, Apur; 203, Youna VARIETY. 
Ostrea borealis, Lamarck, Am. sans vert. Ed. Brux. Vol. 3, p. 82. 
0. id. Gouxp, Invertebrata ef Mass. p. 137. 
Description. Shell variously shaped, but most frequently suborbicular or oblong-ovate, with 
loosely imbricated concentric flakes, becoming obsolete towards the beaks, which are usually 
curved, generally short, but occasionally somewhat elongated. Lower valve concave, with 
coarse rugose folds on the margin; but these are often indistinct. The young under two 
years often strongly costate, with six to eight convex ribs or folds, which extend into processes 
on the margin of the valves, and resembling equestris of Say (See pl. 10, fig. 203). Upper 
valve with a transverse ridge in the hinge, abrupt behind, and sloping gradually into the shell ; 
on the larger valve, this ridge is prolonged backwards. 
Color, Dusky brown, intermixed with green; within pearly white: muscular impression 
purplish. The young, under a year, are reddish, with dusky radiations. 
Length, 5°0-12°0. Width, 3-0 -6°0. 
More than eighty species of oysters are mentioned in the most recent systematic catalogues ; 
but many of these are so nearly allied, as to render it very doubtful whether mere varieties 
have not been described as species. Lamarck attributes three species to the coast of the 
United States ; but we must confess our inability to find more than one, and that one, under 
certain forms, cannot be distinguished from the O. edulis, or Common Oyster of Europe. 
The three American species in Lamarck are thus characterized : 
1. O. borealis. Shell oblong-ovate, whitish, with imbricated undulated plates ; upper valve 
somewhat convex. Length, nearly three inches. Allied to edulis and virginica, but 
distinct from both. New-York. 
2. O. virginica. Shell elongate, whitish, narrow, rather straight, thick-lamellar ; upper valve 
rather plane. As it advances in age, it becomes very thick, and its lower beak becomes 
very long, and with a channel within furrowed transversely : its upper beak tuberous 
within. Length, six inches. Vzurginia. 
Fauna — Parr 6. 22 
