MARYLAND GEOLOGICAL SURVEY 335 



tooth of the right valve bifid; anterior muscular impression not pro- 

 foundly elongated." Conrad, 1832. 



Length, 33 mm.; height, 31 mm.; diameter, 8 mm. 



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

 FoEMATiON. Governor Run, 2 miles south of Governor Run, Flag Pond, 

 Jones Wharf, Turner, Dover Bridge, Greensboro. Calvert Formation. 

 Church Hill, Reed's, Fairhaven. 



Collections. — Maryland Geological Survey, Johns Hopkins University. 



DiPLODONTA SHILOHENSIS Dall, 



Plate LXXXIX, Figs. 7, 8. 



Mysia parilis Conrad, 1866, Amer. Jour. Conch., vol. ii, p. 71, pi. iv, Hg. 1. 

 Not Jfysia parilis Conrad, 1860, 1865. 



Mysia parilis Whitfield, 1895, Mon. xxiv, U. S. Geol. Survey, p. 61, pi. ix, figs. 9-13. 

 Diplodonta shilohensis Dall, 1900, Trans. Wagner Free Inst. Sci., vol. iii, pt. v, 

 p. 1184. 



Description. — " Equilateral, nearly circular, ventricose, thin and fra- 

 gile; basal and anterior margin regularly rounded." Conrad, 1866. 



Beaks not prominent, situated just slightly interior to the middle of 

 the valve; shell very globular; curvature of margin very nearly circular; 

 curvature of surface almost siDherical. 



Length, 10 mm.; height, 9 mm.; diameter, 4 mm. 



Occurrence. — Choptank Formation. Jones Wharf, Dover Bridge. 



Collections. — Maryland Geological Survey, Johns Hopkins University. 



Section SPH/tRELLA Conrad. 



Diplodonta subvexa (Conrad). 

 Plate LXXXIX, Figs. 9, 10. 



Sphcerella subvexa Conrad, 1838, Fossils of the Medial Tertiary, p. 18, pi. x, fig. 2. 

 Erycina srtbconvexa d'Orbigny, 18.52, Prod. Pal. Strat., vol. iii, p. 115. 

 Sphcerella subvexa Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 577. 

 Sphcerella subvexa Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 8. 

 Diplodonta (Sphcerella) subvexa Dall, 1900, Trans. Wagner Free Inst. Sci., vol. iii 

 pt. V, p. 1186. •' . , 



Description.—" Shell globose, thin and fragile; disk with fine lines of 

 growth; umbo very prominent, slightly oblique, nearly central; lunule 

 undefined; margins rounded." Conrad, 1838. 



