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another is observable : at nearly equal distances, four or five rugae are 
disposed, as if the consequence of different periods of growth. 
On viewing the different specimens of ostracites and gryphites, in 
my collection, I cannot help doubting as to the propriety of the forma- 
tion of a distinct genus for this shell. I there find specimens in which 
the beak and the body of the valve possess various degrees of curvature, 
in a series of gradation, from the complete curve of the gryphites to 
the slight turn of the edible oyster. 
CXXXVII. Ostrea. A rough adherent inequivalved bivalve ; the 
hinge without a tooth. The pit of the hinge increasing with age, in the 
larger valve. The cartilage, half internal. One muscular impression. 
The genus Ostrea, of Linnaeus, was doubtlessly much confused, he 
having introduced it in several shells, which, like the Pectens, had a 
full claim to a distinct genus. Bruguiere very properly separated 
from this genus the genera Pectevi and Perna, and introduced into it 
several shells really belonging to this genus, but which had been 
placed by Linnaeus in the genus Mytilus. Lamarck proceeded still 
further, and seems to have reduced this genus within its proper limits, 
by withdrawing from it those shells with which he has constituted the 
genera Vulsella and Gryphea. 
Lamarck describes eighteen species of this genus, as found fossil in the 
environs of Paris : 0 . hellovacina, 0 . hippopus, 0 . deltoidea, 0 . hiau- 
riculata, 0. vesicular is, 0. pseudo -chama, O. linguatula, O. Cochlearia, 
0. longirostris, 0. canalis, 0. crenulata, 0. cyathula, 0. spathulata, 
O. deformis, O.uncinata, 0. Jlahellula, 0. cymhula, O.pectinata. 
Several species of this genus are also found in different parts of the 
continent, as well as of this island. 
The most extraordinary shell of this genus, for size, is the large fossil 
oyster, the recent analogue of which, from Virginia, appears to be de- 
picted by Lister, Hist. Conch. PI. 200, Fig. 34, and PI. 201, Fig. 35. It 
is chiefly found at Heutlingen and Aristorf, in Switzerland. The shell of 
this oyster is sometimes from two inches to two inches and a half in 
