302 SYSTEMATIC PALEONTOLOGY 



Description. — " Subelliptical : beaks medial; anterior margin obliquely 

 truncated, the extremity acutely rounded ; dorsal margins equally oblique ; 

 posterior basal margin obliquely subtruncated ; basal margin nearly 

 straight in the middle and towards the anterior extremity where it is 

 arched; the extremity considerably above the line of the base; posterior 

 side with an oblique narrow fold." Conrad, 1843. 



Shell very thin; surface crossed by fine concentric lines of growth; 

 within, a well-defined ridge extending from the beak obliquely backward 

 to the lower part of the posterior margin and there iDCcoming obsolete. 



Length, 50 mm. ; height, 32 mm, ; diameter, 7.5 mm. 



Occurrence. — Ghoptank Formation. Jones Wharf (rare). Cal- 

 vert Formation. Three miles south of Chesapeake Beach, Lyon's 

 Creek (?). 



Collection. — Maryland Geological Survey. 



Macoma marylandica n. sp. 

 Plate LXXIII, Fig. 7. 



Description. — Shell very thin and fragile; basal and anterior dorsal 

 margins nearly parallel ; anterior side produced and anterior end rounded ; 

 anterior basal margin regularly arched ; posterior dorsal margin declining 

 and meeting basal margin much above the line of the base ; anterior por- 

 tion of shell convex and capacious, especially under the beak and anterior 

 dorsal margin ; posterior portion of shell contracted and somewhat pointed 

 and posterior end gaping ; exterior smooth except for faint growth lines ; 

 cardinal teeth two ; beak not prominent. 



Length, 15 mm.; height, 7.9 mm.; diameter, 1.8 mm. 



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



Collection. — Maryland Geological Survey. 



Superfomily VENERACEA. 

 Family PETRICOLID/E. 



Genus PETRICOLA Lamarck. 

 Section RUPELLARIA Fleuriau. 



Petricola harrisii Dall. 

 Plate LXXIII, Figs. 8, 9. 



Petricola (Rupellaria) Harrisii Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, 

 pt. V, p. 1060, pi. xliii, fig. 1. 



