172 SYSTEMATIC PALEONTOLOGY 



less common form than the variety later described by Conrad nnder the 

 name of D. meehii. Rogers's type represents a moderately thin shell with 

 a weak hinge, while the form described by Conrad is a heavy shell with 

 a broad, solid hinge. Every possible gradation between these extremes 

 has been obtained. 



Length, 70 mm.; height, 70 mm. 



Occurrence. — Aquia Foemation. Fort Washington^, Glymont, Clif- 

 ton Beach, Potomac Creek, Liverpool Point, Aquia Creek, Mattawoman 

 Creek, 1 mile northeast of Piscataway, Sheckels Farm near Sonth Eiver, 

 Reedy Rnn (branch of Chickomuxen Creek), 1 mile west of Hardesty", 

 near Hardesty, Brooks Estate near Seat Pleasant, 1 mile northeast of 

 Grimesville, Harrisons Landing, Upper Marlboro, 3 miles west of Pisgah, 

 Fredericktown. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

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



Family CARDIIDAE. 



Genus PROTOCARDIA Bey rich. 



Peotocardia lenis Conrad. 



Plate XXXVI, Figs. 1, la, 3, 2a, 3. 



CardhLm {Protocardia) lenis Conrad, 1855, Proc. Acad. Nat. Sci. Pliila., yoI. vii, 



p. 358. 

 Protocardia virginiana Conrad, 1864, Proc. Acad. Nat. Sci. Phila., voL xv, p. 211. 

 Protocardia virginiana Conrad, 1866, Smith. Misc. Coll. (200), p. 6. 

 Protocardia virginiana Clark, 1895, Johns Hopkins Univ. Circ, voL xv, p. 5. 

 Protocardia virginiana Clur'k, 1896, BulL 141, U. S. Geol. Survey, p. 81, pi. xxvi, 



figs, la-lf. 

 Protocardia lenis Dall, 1900, Trans. Wagner Free Inst. Sci., voL iii, pt. v, p. 1113. 



Description. — " Compared with C. nicolletti; umbo less inflated, pos- 

 terior margin oblique, shell proportionally longer, and the radiating 

 lines 22; in the other 25. The ]30sterior cardinal tooth larger, &c." 

 Conrad; 1855. 



" Cordate, subtriangular, inequilateral, ventricose, thin; radiating lines 

 minute; anterior upper margin very oblique, slightly emarginate, pos- 

 terior side slightly produced, the margin obliquely truncated; post-um- 

 bonal area densely tuberculated on closely arranged striae; posterior car- 



