MARYLAND GEOLOGICAL SURVEY 205 



cenc at Wailes Bluff, St. Mary's County, and at Federalsburg, Caroline 

 County. At the former locality a thick bed packed with the shells is found 

 overlying the distinctly marine clays below. 



This species probably does not appear prior to the Pliocene at which 

 horizon it occurs in Florida. In the Pleistocene it has been found at 

 numerous points from Prince Edward Island to Florida and Texas, and 

 also in California. In the Eecent it is known on the Atlantic coast from 

 Prince Edward Island south to Florida and west to Mexico, on the Pa- 

 cific coast near the head of the Gulf of California. 



Occurrence. — Talbot Formation. Drum Point, Calvert County; 

 Wailes Bluff near Cornfield Harbor, and on shore opposite Solomon's Is- 

 land, St. Mary's County; Federalsburg, Caroline County; and Easton, 

 Talbot County. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 and IT. S. National Museum. 



Superfamily ARCACEA. 



Family. ARC1DAE. 



Subfamily ARC1NAE. 



Genus ARC A (Linne) Lamarck. 



Subgenus NOETIA Gray. 



Arca (Noetia) ponderosa Say. 



Plate LXIV, Figs. 1-6. 



Area ponderosa Say, 1822, Jour. Acad. Nat. Sci. Phila., vol. ii, 1st ser., 



p. 267. 

 Arca ponderosa Holmes, 1858, Post-Pi. Fos. S. C, p. 21, pi. iv, figs. 4, 4a. 

 Arca (Noetia) ponderosa Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, 



pt. iv, p. 633. 



Description. — " Shell somewhat oblique, very thick and ponderous, 

 with from twenty-five to twenty-eight ribs, each marked by an impressed 

 line; interstitial spaces equal to the width of the ribs; umbones very 

 prominent; apices remote from each other, and opposite to the middle of 

 the hinge, spaces between them with longitudinal lines as prominent as 



