MARYLAND GEOLOGICAL SURVEY 581 



low, flattened as a rule, irregular in outline, submargins auriculately 

 produced in the left valve, somewhat thickened but not produced in the 

 right valve ; not sculptured ; muscle impressions elongated, rudely semi- 

 elliptical, concentrically striated. 



The characters of the attached valve of G. vomer are variable, the 

 inflated umbones and smooth external surface constituting perhaps the 

 best diagnostic of indifferently preserved individuals. The ovate outline 

 and the elevated concentric lamina are sufficient to determine even a 

 fragment of the right valve. 



Occurrence. M ATA WAN FORMATION. ? Gibson's Island, ? head of 

 Magothy River, ? Ulmstead Point, Anne Arundel County, Maryland. 

 MONMOUTH FORMATION. Two miles west of Delaware -City on John Hig- 

 gins farm, Briar Point, Post 156, Chesapeake and Delaware Canal, Dela- 

 ware; Bohemia Mills, Cecil County; mouth of Turner's Creek, Kent 

 County; Brightseat, railroad cut west of Seat Pleasant, Brooks estate 

 near Seat Pleasant, Friendly, and McNeys Corners, Prince George's 

 County, Maryland. EANCOCAS FORMATION. Noxontown Pond, Delaware. 



Collections. Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, New Jersey Geological Survey, U. S. National Museum. 



Outside Distribution. Matawan Formation. Marshalltown clay marl, 

 New Jersey. Monmouth Formation. Navesink marl and Red Bank sand, 

 New Jersey. Rancocas Formation. Vincentown limesand and Horners- 

 town marl, New Jersey. Eutaw Formation (Tombigbee sand member). 

 Exogyra ponderosa zone, Lee and Alcorn counties, Mississippi. Eipley 

 Formation. Exogyra costata zone, east-central and northern Mississippi. 

 Extreme top of zone, Union County, Mississippi. Selma Chalk. Exogyra 

 ponderosa zone, Warrior and Tombigbee rivers and Pickens County, Ala- 

 bama; east-central and northern Mississippi and Tennessee. Exogyra 

 costata zone, Sumter and Wilcox counties, Alabama; east-central and 

 northern Mississippi. Nanjemoy Formation. Maryland. Aquia Forma- 

 tion. Maryland. Jackson Formation. Alabama. 



