MARYLAND GEOLOGICAL SURVEY 279 



Description. — This shell varies extremely according to age and posi- 

 tion and has received many names; for mnch fuller synonymy see 

 Binney and Dall {cit. supra). 



As most frequently found in Maryland the shell is roughly quadri- 

 lateral in outline; inequivalve, the right valve overlapping; heak nearly 

 terminal; from it two ridges extend backward, one near the superior 

 dorsal margin and the other, usually more prominent, to the posterior 

 basal angle; exterior surface irregularly undulated and coarsely marked 

 by lines of growth; anterior an'l posterior margins often almost squarely 

 truncated; anterior basal margin at times contracted; teeth obsolete or 

 at times one in either valve. 



Instead of being quadrilateral, the outline may be very greatly modi- 

 fied by the production or curving of some portion of the margin. The 

 ridges running back from the beak are perhaps the most constant char- 

 acter. 



Length, 19 mm.; height, 9 mm.; diameter, 5 mm. 



Occurrence. — St. MARy's Formation. Cove Point, St. Mary's Kiver. 

 Choptank Fokmation. Governor Eun (upper and lower beds), 3 miles 

 south of Governor Kun (upper and lower beds). Flag Pond, Jones Wharf, 

 Turner, Pawpaw Point, St. Leonard Creek, Dover Bridge, Greensboro. 

 Calvert Formation. Chesapeake Beach, 3 miles south of Chesapeake 

 Beach, Plum Point, White's Landing, 3 miles west of Centerville. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 U. S. National Museum. 



Family CORBULID/E. 



Genus CORBULA (Bruguiere) Lamarck. 



Section CORBULA ss. 



Corbula idonea Conrad. 



Plate LXVII, Figs. 1, 2, 3. 



Corbula idonea Conrad, 1833, Amer. Jour. Sci., vol. xxiii, p. 341. 

 Corbula idonea Conrad, 1838, Fossils of the Medial Tertiary, p. 6, pi. x, fig. 6. 

 Corbula idonea Conrad, 1863, Proc. Acad,. Nat. Sci. Phila., vol. xiv, p. 572. 

 Corbula idonea Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 12. 

 Corbula idonea Whitfield, 1894, Mou. xxiv, U. S. Geol. Survey, p. 88, pi. xv, fig. 20. 

 Corbula (Corbula) idonea Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, pt. iv, 

 p. 852. 



