382 SYSTEMATIC PALEONTOLOGY 



Description. — " Obovate, oblique^, thick, compressed ; superior valve 

 flat; inferior valve convex, with concentric imbricated lamellge which 

 are transversely plicated; beaks broad and prominent; fosset large and 

 defined by broad prominent lateral ridges." Conrad, 1833. 



T'he Maryland specimens are smaller and usually thinner than the 

 original Carolina ones. The ribs are fine and regular on some, on others 

 irregular. The submargin of the upper valve near the beaks is trans- 

 versely striated. This species is often very abundant. 



Length, 100 mm. ; Avidth, 75 mm. ; diameter, 25 mm. 



Occurrence. — St. Mary's Formation. St. Mary's River ( ?). Chop- 

 tank Formation. Governor Eun, 2 miles south of Governor Run, 

 Flag Pond, Jones Wharf, Turner, Cuckold Creek, St. Leonard Creek, 

 Peach Blossom Creek, Dover Bridge. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 U. S. jSTational Museum. 



Ostrea percrassa Conrad. 

 Plate CII, Figs. 1, 2. 



Ostrea percrassa Conrad, 1840, Fossils of the Medial Tertian*, p. 50, pi. xxv, flg-. 1. 

 Ostrea percrassa Conrad, 1863, Proc. Acad. Nat. Sei. Phila., vol. xiv, p. 582. 

 Ostrea percrassa Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 3. 

 Ostrea percrassa Heilprin, 1884, 4th Ann. Rept. U.S. Geol. Survey, p. 313, pi. Ixvii, 



ftg. 3. 

 Ostrea percrassa Whitfield, 1894, Mod. xxiv, U. S. Geol. Survey, p. 20, pl. iii, figs. 



1-4. 

 Ostrea percrassa Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, i)t. iv, p. 683. 



Description. — " Shell extremely thick and ponderous ; hinge very 

 broad; cartilage fosset wide and shallow; muscular impression exhibit- 

 ing a very profound cavity." Conrad, 1840. 



Lower valve convex exteriorly, deeply concave within; upper valve 

 more nearly flat; shell substance of innumerable fine lamellte, often the 

 home of boring forms. 



Length, 110 mm.; width, 95 mm.; diameter, 40 mm. 



Occurrence. — Calvert Formation. Chesapeake Beach, 3 miles south 

 of Chesapeake Beach, Plum Point, Hollin Cliff, Magruder Ferry, Wliite's 

 Landing, Reed's, Fairhaven, near Friendship, Milltown Landing. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 Cornell ITniversity. 



