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PROCEEDINGS OF 
About three miles above the low sandy point which forms the southern extremity 
of the Western peninsula of Maryland, the bank of the Potomac rises to an eleva¬ 
tion of about fifteen feet at its highest point. The fossils are visible in this bank a 
quarter of a mile in uninterrupted extent. The inferior stratum is a lead-colored 
clay, containing great numbers of Mactra lateralis, (Say,) a common recent bivalve 
of the coast, which in many instances appear in nearly vertical veins, having evi-, 
dently fallen into fissures in the clay. Pholas costata is also abundant, and each 
individual remains in the position in which the living shell is usually buried in the 
mud, that is, vertical, with the anterior or short side pointing downward. They 
are very fragile and can rarely be procured entire. Over the clay reposes a bed of 
Ostrea virginiana in sand, in places a foot in thickness. It is nearly horizontal, 
varying from a height of four, to eight or ten feet above high water mark. The 
fossils of this locality, with two exceptions, are common recent species of the At¬ 
lantic coast, and in some instances the original colored markings remain upon the 
shells. Were it not for the occurrence of Gnathodon cuneatus, Mytilus hamatus , 
and Area ponderosa, the group would not vary from that now inhabiting the coast 
as far north as Massachusetts ; but the presence of these three bivalves indicate that 
a climate equivalent to that of Florida prevailed when the shells of this locality 
were living in the sea, I have before alluded to the peculiar and highly important 
distribution of the existing Gnathodon , burrowing in myriads in the mud flats near 
Mobile, and confined to the estuaries of the Gulf of Mexico. An occasional water 
worn valve in the deposit on the Potomac, above described, seemed to indicate that 
the species lived in that river in the upper tertiary period. This conjecture was con¬ 
verted into certainty by an exploration of the shore farther north, which resulted in 
discovering a bed composed exclusively of the Gnathodon , on the land of Mr. Ebb, 
above the mouth of St. Mary’s river. This bed, except that the shells are smaller, 
is precisely similar to those which line the bay shore near Mobile. The valves of 
the shells are frequently connected, and there can be no doubt that here was the 
spot where they lived, and were imbedded; that this was a region of sand flats 
bared at low tide, the water brackish, as it is now, and that the deposit near the 
mouth of the Potomac was of the same period, but more directly communicating 
with the ocean. The species found here are as follow : 
Univalves. 
Actseon melanoides, Conrad. 
Crepidula convexa, Say. 
Crepidula glauca , Say. 
Fusus cinereus, Say. 
Univalves. 
Buecinum ohsoletum, Say. 
Buccinum trivittatvm, Say. 
Natica duplicata, Say. 
Ranella caudata, Say. 
Bivalves* 
Area transversa, Say. 
Area ponderosa, Say. 
Corhula contracta , Say. 
Gnathodon cuneatus, Gray. 
Cyiherea Sayana, Conrad. 
Mactra lateralis, Say. 
Mya arenaria. 
Bivalves. 
Pandora trilineata. Say. 
Petricola pholadiformis , Lam. 
Pholas castata, Lam. 
Sanguinolariafusca, Say. 
Tellina lusoria, 
(Psammobia lusoria, Say.) 
Solecurtus caribccus, Lam. 
