THE NATIONAL INSTITUTION. 



193 



figure now given is from a much more perfect specimen, which I found on Jamee 

 river, Virginia, a few miles below City Point. 



SYNONYMES. 



Osirea radians, (nob.)— Fossil Shells of Tertiary Formation, pi. 13, fig. 1, (upper 



valve.) 



Ostrta semilunata, (Lea.) -Contributions to Geology, pi. 3, fig. G9, (young.) 

 Ostrea divaricaia, (Lovi.) Do. do. fig. 70, (young lower valve.) 



Having carefully studied this species in all stages of growth, in several localities, 



the above synouymos are undoubtedly correct. 



PHOLADOMYA. 



Pholadomya marylandica, pi. 1, fig. 3.-0vate, profoundly ventricose, with coarse, 

 irregular, concentric lines and furrows, and obsolete, rather distant, radu ; summit 

 of umbo prominent. 



Locality : Piscataway, Prince George's county, Maryland. 



PHOLAS. 



Pholas pefrosa, pi. 2, fig. 4.-Ovate-acute ; anteriorly profoundly ventricosc ; 

 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 Markoyi, pi. 2, fig- L-Snborbicular; 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 June, 

 lion with the base; base regularly, not profoundly arched; posterior slope shghtly 



sinuous. 



Locality: Captain Hance's farm, Calvert Cliffs, Maryland. 



This beautiful species is easily distingiiished from I. rustics, (Lfraterna, 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 rusiica of a long, ovate? 



figure. 



Deshayes considers the /. fraterna, (Say,) to be identical with /. cor. Upon com- 

 parison, the latter was found to have a far more profound arch or rotundity to ,t.^ 

 base, as well as much greater curvature of beak; but the difl^crencc in the young 

 shells, of cither species, is so profound, that any idea of their identity would be in- 

 stantly abandoned by comparison. 



Mr. Markoo obtained llireo or four valves of this shell, and one epccimon 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. 



