Maryland Geological Survey 557 



Description. — " Ovato-triangular; lower valve convex, crenated near the 

 hinge ; dorsum marked with delicate striae, radiating with fasciculi from 

 the beak to the margin." — Morton, 1833. 



Type Locality. — ? New Jersey. 



Shell of medium size, extremely variable in outline, usually flattened 

 or feebly convex and more or less constricted and falcate in the umbonal 

 region ; umbones inconspicuous, narrow, as a rule, and well over toward 

 the posterior margin; external surface very finely sculptured radially, 

 the ornamentation usually manifested in the form of linear sulci, fre- 

 quently bifurcating and irregularly spaced ; sculpture occasionally almost 

 or altogether obsolete, particularly in the umbonal region and sometimes 

 taking the form of fine radial corrugations; incremental sculpture quite 

 sharp, especially on weathered surfaces; ligament area narrow, the medial 

 depression ill-defined; submargins sharply crenulated; muscle impression 

 elongated, often pyriform, posterior. 



In 0. plumosa Morton the natural tendency toward variation lias been 

 greatly exaggerated by weathering. The radial sculpture is restricted to 

 a very thin surface layer, which may be decorticated without leaving any 

 apparent scar. The outline, though variable, is usually characterized by 

 a decided constriction in the umbonal region. 



This species, so abundant and widespread in the Gulf and Western 

 Interior, has a meager representation in Maryland. 



Occurrence. — Monmouth Formation. Brightseat, Prince George's 

 County. 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

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



Outside Distribution. — Matawan Formation. Woodbury clay, Marshall- 

 town clay marl, and Wenonah sand, New Jersey. Black Creek Forma I ion. 

 North and South Carolina. Pecdee Sand. North and South Carolina. 

 Eulaw Formation (Tombigbee sand member). Exogyra ponderosa zone, 

 Mortoniceras subzone, Georgia; Eussell County, Alabama. Ripley For- 

 mation. Exogyra ponderosa zone, Georgia ; Bullock County, Alabama ; 

 Clay, Lee and Chickasaw counties, Mississippi. Exogyra costata zone, 

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