542 Systematic Paleontology 



subcentral, the lateral margins are straight and converge at an angle of 

 approximately 90°, and the base line is strongly and evenly arcuate. The 

 muscle scars are symmetrically placed near the median horizontal just 

 ventral to the extremities of the hinge and at the base of a triangular 

 depression which wedges out in the umbonal region. The teeth, approxi- 

 mately twenty in number, are small but sharp, and evenly arranged in an 

 arcuate series. The inner margins are quite deeply crenulated. 



The identity of Morton's Prairie Bluff form with Conrad's species from 

 Crosswicks, New Jersey, is rather dubious. The Alabama form is repre- 

 sented chiefly by the shells, the New Jersey by internal casts. Whitfield, 

 however, figures the exterior of a shell from New Jersey, under the name of 

 mortoni, which seems to be more strongly sculptured radially than the 

 Alabama species. He considers the southern race smaller and more nearly 

 equidimensional than the northern, but as both forms show a considerable 

 range of variation in size and relative proportions, this alone would hardly 

 constitute a specific difference. Both Conrad and Meek, who noticed the 

 casts but did not describe them, considered the two species distinct. These 

 men were keen observers and their opinion should not be disregarded 

 until more positive evidence is at hand for the identity of the forms. 



Occurrence. — Matawan Formation. Summit Bridge, Chesapeake and 

 Delaware Canal, Delaware ; Magothy Eiver, Anne Arundel County, Mary- 

 land. Monmouth Formation. Bohemia Mills, and right bank of 

 Bohemia Creek near Scotchman's Creek, Cecil County, Maryland. 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

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



Outside Distribution. — Matawan Formation. Merchantville clay marl, 

 New Jersey. Monmouth Formation. Navesink marl and Tinton beds, 

 New Jersey. Black Creek Formation. North and South Carolina. Eutaw 

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

 ville, Mississippi. Ripley Formation. Exogyra costata zone, Georgia; 

 Eufaula, Alabama. Extreme top of zone, Pataula Creek, Georgia ; Chatta- 

 hoochee Eiver, Alabama. Selma Chalk. Exogyra costata zone, Prairie 

 Bluff, Alabama. 



