MARYLAND GEOLOGICAL SURVEY 361 



material is secured it will very probably show the fossil to be at least 

 varietally different from the living species. In this event Dr. Ball's 

 proposed varietal name harrisi will apply. 



Length, 75 mm. ; height, 60 mm. ; diameter, about 14 mm. 



Occurrence. — Calvert Formation. Fairhaven, Lyon's Creek, Plum 

 Point, Chesapeake Beach. 



Collection. — Maryland Geological Survey. 



Family PHOLADOMYACID/E. 



Genus MARGARITARIA Conrad. 



Margaritaria abrupt a (Conrad). 

 Plate XCV, Figs. 5, 6. 



Fholadomya abrupta Conrad, 1832, Fossil Shells of the Tertiary, p. 26, pi. xii. 



Pholadomya abrupta Conrad, 1838, Fossils of the Medial Tertiary, p. 3, pi. i, fig. 4. 



Fholadomya abrupta Tuomey and Holmes, 1856, Pleiocene Fossils of South Caro- 

 lina, p. 101, pi. xxiv, fig. 2. 



Pholadomya abrupta Emmons, 1858, Kept. N. Car. Geol. Survey, p. 300, fig. 231. 



Pholadomya {Margaritaria) abrupta Conrad, 1863, Proe. Acad. Nat. Sci. Phila., vol. 

 xiv, p. 572. 



Margaritaria abrupta Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 12. 



Margaritaria abrupta Dall, 1903, Trans. Wagner Free Inst. Sci., vol. iii, pt. vi, 

 p. 1532. 



Description. — " Oblong oval, much compressed, with from three to five 

 subacute distant ribs or ridges diverging from the apex; one side rather 

 thick and strong, rounded at the extremity ; the opposite side extremely 

 thin, and reflected, with a truncated margin ; muscular and pallial impres- 

 sions distinct." Conrad, 1832. 



The shell is pearly and fragile and is readily identified by the radial 

 ridges crossing the dorsal portion of the shell from the beak to the base. 

 The Survey possesses only some fragments. 



Occurrence. — St. Mary's Formation. Cove Point, St. Mary's River. 

 CtALVERT Formation. Wliite's Landing. 



Collection. — Maryland Geological Survey. 



