MARYLAND GEOLOGICAL SURVEY 665 



upon the posterior slope, narrower and more distinctly spaced than upon 

 the anterior and medial portions ; incremental sculpture obscure, except- 

 ing for an occasional resting stage near the umbonal margin; ligament 

 external, opisthodetic, mounted on short, thickened nymphs; cardinals 

 two in number in each valve, the anterior very stout and conical, springing 

 from directly beneath the umbones, the posterior mere tubercles at the 

 extremities of the nymphs; short but prominent anterior and posterior 

 laterals developed in the left valve, a double anterior and a single posterior 

 in the right; muscle scars rather indistinct placed high up near the 

 extremities of the dorsal margins, the posterior somewhat elongated ; pal- 

 lial line obscure, ventral and lateral margins sharply serrate. 



This species is the. most abundant representative of its genus within 

 the area under discussion. It is rather smaller than the co-existing 

 species and is best characterized by the flattening of the posterior portion 

 of the shell and the truncation of the posterior lateral margin. The 

 costals are somewhat tubular in structure so that when the outer layer 

 of the shell surface is removed, as it frequently is by weathering, the inter- 

 costal areas appear as flat-topped elevations separated by concave depres- 

 sions. 



Occurrence. MAGOTHY FORMATION. Good Hope Hill, District of 

 Columbia. MATAWAN FORMATION. ? Post 105, Chesapeake and Dela- 

 ware Canal, Delaware; three-quarters of a mile southeast of Ulmstead 

 Point, Ulmstead Point, Gibson's Island, north shore Round Bay, Severn 

 River, Anne Arundel County, Maryland. MONMOUTH FORMATION. 

 Bohemia Mills, Fredericktown, Cecil County; Brightseat, Brooks estate 

 near Seat Pleasant, Friendly, 1 mile west of Friendly, McNeys Corners, 

 Prince George's County, Maryland. 



Collections. Maryland Geological Survey, New Jersey Geological Sur- 

 vey, IT. S. National Museum. 



Outside Distribution. MataiL-an Formation. Wenonah sand, New 

 Jersey. Blade Creel: Formation. North and South Carolina. Eutaw 

 Formation (Tombigbee sand member). Exogyra ponderosa zone, Mor- 

 toniceras subzone, Blufftown, Georgia. Ripley Formation. Exogyra pon- 

 derosa zone, Georgia : Barbour County, Alabama. Exogyra costata zone, 



