THE NATIONAL INSTITUTION. 
193 
figure now given is from a much more perfect specimen, which I found on James 
river, Virginia, a few miles below City Point. 
SYNONYMES. 
Ostrea radians , (nob.)—Fossil Shells of Tertiary Formation, pi. 13, fig. 1, (upper 
valve.) 
Ostrea semilunata , (Lea.)—Contributions to Geology, pi. 3, fig. 69, (young.) 
Ostrea divaricata, ( Lea.) Do. do. fig. 70, (young lower valve.) 
Having carefully studied this species in all stages of growth, in several localities, 
the above synonymes are undoubtedly correct. 
PHOLADOMYA. 
Pholadotnya marylandica, pi. 1, fig. 3.—Ovate, profoundly ventricose, with coarse, 
irregular, concentric lines and furrows, and obsolete, rather distant, radii; summit 
of umbo prominent. 
Locality: Piscataway, Prince George’s county, Maryland. 
PHOLAS. 
Pholas petrosa , pi. 2, fig. 4.—-Ovate-acute ; anteriorly profoundly ventricose; 
radii sharp and numerous; dorsal margin obliquely rectilinear from the summit of 
the umbo; base obliquely subrectilinear; posterior side produced, cuneiform. 
Locality■: Occurs with the preceding species. 
MEDIAL TERTIARY SPECIES. 
ISOCARDIA.— (Lam.) 
Isocardia Markoei , pi. 2, fig. 1.—Suborbicular; length and height nearly equal; 
inflated; umbo very prominent, and the beaks profoundly incurved; posterior mar¬ 
gin direct, arched above, nearly straight below, and obtusely angulated at its junc¬ 
tion with the base ; base regularly, not profoundly arched; posterior slope slightly 
sinuous. 
Locality: Captain Hance’s farm, Calvert Cliffs, Maryland. 
This beautiful species is easily distinguished from I. rustica, (I. fraterna, Say,) 
by the much more prominent umbo, and greater curvature of the beak, and in being 
proportionally much shorter. The young shells of the two species are widely un¬ 
like each other in outline, this species being round, and the rustica of a long, ovate 
figure. 
Deshayes considers the I. fraterna, (Say,) to be identical with I. cor. Upon com¬ 
parison, the latter was found to have a far more profound arch or rotundity to its 
base, as well as much greater curvature of beak; but the difference in the young 
shells, of either species, is so profound, that any idea of their identity would be in¬ 
stantly abandoned by comparison. 
Mr. Markoe obtained three or four valves of this shell, and one specimen with 
connected valves, and I have much pleasure in dedicating the species to him, as 
a slight tribute of respect for his talents, zeal, and most generous devotion to sci¬ 
ence. 
