MARYLAND GEOLOGICAL SURVEY 385 



45 degrees with each other; dorsal region smooth; ventral region sculp- 

 tured by rather distant irregularly spaced concentric riblets. 



This is much larger and broader and less acute anteriorly than 

 A. harrisii, and is without the fine parallel lines characteristic of the 

 dorsal region in that species. The specimen had been flattened so that 

 the diameter given below is less than the true diameter. 



Length, about 200 mm. ; maximum width, about 120 mm. ; diameter, 

 25-|-mm, 



Occurrence. — Calvert Formation. Chesapeake Beach. 



Collection. — Maryland Geological Survey. 



Superfamily ARCACEA. 



Family ARCID/E. 



Subfomily ARCIN/E. 



Genus ARCA (Linne) Lamarck. 



Subgenus SCAPHARCA (Gray) Dall. 



Section ANADARA Gray. 



Arca (Scapharca) subrostrata Conrad, 

 Plate CIV, Figs. 2, 3a, 3b. 



Area subrostrata Conrad, 1841, Proc. Acad. Nat. Sci. Phila., vol. i, p. 30. 

 Arca subrostrata Conrad, 1842, Jour. Acad. Nat. Sci. Phila., vol. viii, 1st ser., p. 185. 

 Arca subrostrata Conrad, 1845, Fossils of the Medial Tertiary, p. 58, pi. xxx, fig. 7. 

 Scapharca subrostrata Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 580. 

 Scapharca subrostrata Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 6. 

 Scapharca tenuicardo Conrad, 1869, Amer. Jour. Conch., vol. v, p. 39, pi. ii, fig. 4. 

 Scapharca subrostrata Whitfield, 1894, Mon. xxiv, U. S. Geol. Survey, p. 45, pi. vi, 



figs. 11-13. 

 Scapharca {Anadara) subrostrata Dall, 1S98, Trans. Wagner Free Inst. Sci., vol. iii, 



pt. iv, p. 655. 



Description. — "Ovate; profoundly ventricose; ribs about 30, little 

 prominent, flat, longitudinally sulcated; posterior side produced, cunei- 

 form ; rounded at the extremity ; hinge linear in the middle, teeth obso- 

 lete, except towards the extremities; within slightly sulcated; crenula- 

 tions of the margin sulcated in the middle." Conrad, 1841. 



Cardinal area grooved with numerous somewhat irregular, though 

 nearly parallel grooves; posterior umbonal slope angulated near the 

 25 



