170 SYSTEMATIC PALEONTOLOGY 



Meretrix pyga Conrad, 1854, Proc. Acad. Nat. Sci. Pbila., voL vii, p. 30. 

 Dione pyga Conrad, 1865, Amer. Jour. Conch., voL i, p. 6. 



Cytherea ovata Clark, 1895, Johns Hopkins Univ. Circ, yoL xv, p. 5. (In part.) 

 Gytherea ovata Clark, 1896, Bull. 141, U. S. Geol. Survey, p. 76 (In part), pi. xvi, 

 flgs. la-1/. 



Description. — " Suboval, ventricose^ thick, with closely arranged ro- 

 bust concentric lines; umbo wide, prominent above; dorsal margin very 

 oblique, rounded; posterior extremity obtusely and obliquely rounded 

 inwards; anterior extremity much narrower than the posterior, and 

 more acutely rounded; lunule lanceolate, defined by a distinct impressed 

 line; cardinal teeth robust. Length, If; height, 1:^." Conrad, 1848. 



The form described by Conrad under this name was found in the 

 Aquia formation, and the name as here used is thus restricted. The 

 specimens are large, thick, ventricose, and more nearly oval than the 

 variety from the ISTanjemoy formation. 



Length, 50 mm.; width, 43 mm. 



Occurrence. — Aquia Fohmation. Potomac Creek, Aquia Creek, Fort 

 Washington, Gl_ymont, Mouth of Paspotansa Creek, Liverpool Point, 

 Sheckel's Farm near South Eiver, Clifton Beach, 2 miles below Potomac 

 Creek, 1 mile northeast of Grimesville, Upper Marlboro, Hardesty, 

 Brooks Estate near Seat Pleasant, 3 miles west of Collington, Eeedy Eun 

 (branch of Chickomuxen Creek), Mattawoman Creek (?), 1 mile south- 

 east of Mason Springs. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 Philadelphia Academy of Natural Sciences, U. S. National Museum. 



Meeetkix sijbimpeessa Conrad. 

 Plate XXXIII, Figs. 5, 5a, 6, 7, 8, 8a, 9, 9a. 



Cytherea subimpj-eKsa Conrad, 1848, Jour. Acad. Nat. Sci. Pbila., 2nd ser., vol. i, 



p. 130, pi. xiv, tig. 26. 

 CytJierea subimpressa H. C. Lea, 1848, Proc. Acad. Nat. Sci. Pbila., vol. iv, p. 99. 

 Meretrix suhimpressa Conrad, 1854, Proc. Acad. Nat. Sci. Phila., vol. vii, p. 30. 

 Cytherea .nMmpre.ssaCla.rk, 1895, Johns Hopkins Univ. Circ, vol. xv, p. 5. 

 Cytherea suhimpreHna Chirk, 1896, Bull. 141, U. S. Geol. Survey, p. 77, pi. xvii, ligs. 



la-\h. 



Description. — " Ovate, slightly ventricose, smooth and polished, with 

 concentric, slightly impressed lines on the anterior side; anterior side 



