FAMILY OSTRACHWE OSTREA. 169 



GENUS OSTREA. Linnatus. 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 wilh 

 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, Addlt ; 203, Yobno variety. 



Ostrea borealis. Lamarck, Am. sans vert. Ed. Brux. Vol. 3, p. 82. 

 0. id. Gohld, Invertebrata of 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 pi. 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. Virginia. 

 Fauna— Part 6. 22 



