190 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 arc 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 v.hicli the living shell is usually buried in the 

 mud, that is, vertical, with the anterior or short side pointing downward. They 

 are very fragilo 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 iu some instances the original colored markings remain upon the 

 shells. Were it not for the occurrence of Gnathodon ciineatus, 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 tho peculiar and highly important 

 distribution of the exialhig Gnathodon, burrowing in myriads in the mud flats near 

 Mobile, and confined to the estuaries of tlie Gulf of IMexico. An occasional water 

 worn valve in the deposit on the Potomac, above described, seemed to indicate that 

 the species lived in that river iu 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. Clary'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 arc frequently connected, and there can be no doubt that hero 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 deiwjsit near tho 

 mouth of the Potomac was of the same period, but more directly communicating 

 M'ith the ocean. The species found here arc as follow ; 



Unirahes. Univalves. 



ActsBon melanoides, Conrad. Buccinum obsoletum, Say. 



Crcpidula convexa. Say. Buccinum trivittalum. Say. 



Crepidula glauca, Say. Nalica duplicala. Say. 



Fusus cinercus. Say. Ranclla caudata. Say. 



Bivalves. Bivalves. 



Area iransrersa. Say. Pandora trilineata. Say. 



Area ponderosa. Say. Petricola pholadiformis. Lam. 



Corbula conlracta. Say. Phnlas caslaic. Lam. 



Gnathodon cuneatus. Gray. Sanguinolaria fusca, Saj% 



Cijlherea Saijana, Conrad. TelLina lusoria, 



Mnctra lateralis. Say. (Psamuiobia lusoria, Say.) 



yiija arenaria. Sokcurtus carib<cus, Lam. 



