333 SYSTEMATIC PALEONTOLOGY 



Cabdium (Cerastoderma) patuxentium n. sp. 

 Plate LXXXVI, Fig. 5. 



Description. — Shell large, moderately thick, elevated; beak elevated, 

 prominent; ribs fine, rounded to flattened, close set and separated by a 

 narrow groove, normally fifty-two or somewhat more — one specimen with 

 but forty-five; cardinal tooth strong, elevated; 'lateral teeth prominent; 

 interior smooth; interior margin not known. 



This species may be easily distinguished by its large number of ribs. 



Approximate measurements are: length, 55 mm.; height, 50 mm.; 

 diameter, 20 nun. 



Occurrence. — Calvert Formation. Truman's Wharf, White's Land- 

 ing (as well-preserved casts of the exterior in the siliceous beds), New 

 Town, Wye Mills. 



Collection. — Maryland Geological Survey. 



Subgenus FKAGUM Botten. 



Cardium (Fragum) medium Linn6. 

 Plate LXXXVI, Figs. 6a, 6b. 



Cardium medium Linne, 1758, Syst. Nat., ed. x, p. 678. 



Cardium medium Linne, 1768, Syst. Nat., ed. xii, p. 1122. 



Hemicardium columha Heilprin, 1887, Trans. Wagner Free Inst. Sci., vol. i, p. 93, 



pL xi, flg. 26. 

 Cardium (Fragum) medium Dall, 1900, Trans. Wagner Free Inst. Sci., vol. iii, pt. v, 



p. 1101. 



Description. — " C. testa subcordata subangulata ; valvulis angulatis 

 sulcatis lasvibus." Linne, 1758. 



This species is distinguished by the amount of impression of the pos- 

 terior area and the elevation of the upper part of the posterior margin 

 projecting from the central part of the depression of the closed valve. 

 The amount of depression varies very much and the range of variation 

 is so complete that this characteristic is difficult to rely upon. 



The Pliocene form of this species was described by Professor Heilprin 

 as Hemicardium columha. He had only two specimens and they happen 

 to be end members of the gradation series. 



Occurrence. — St. Mary's Formation. St. Mary's River. 



Collection. — U. S. National Museum. 



