700 SYSTEMATIC PALEONTOLOGY 



them anteriorly and even more sharply posteriorly ; radial sculpture con- 

 fined to striations on the interspaces between the concentric lirae on the 

 medial portion of the disk, appearing posteriorly as six to nine low, 

 radiating lirae, unequal in size and spacing; radial sculpture on anterior 

 portion of shell much finer and sharper ; radials twelve to fifteen in num- 

 ber, approximately uniform in size and spacing, nodulated by the over- 

 riding concentric laminae; ligament external, opisthodetic, mounted on 

 rather a slender nymph which is separated from the rest of the shell by 

 a linear sulcus; hinge of left valve armed with two laminar cardinals, the 

 posterior a little more slender than the anterior, their inner faces flattened 

 and proximate, diverging at a very small angle and subparallel to the 

 dorsal margin; a single moderately robust, posteriorly directed cardinal 

 developed in the right valve with rather a deep pit in front of it and 

 a more shallow one behind it for the reception of the cardinals of the left 

 valve; a feeble elevation on the forward margin of the anterior socket, 

 probably the analogue of the anterior cardinal in the right valve; dorsal 

 margins of right valve bevelled to function as laterals, received in the left 

 by double grooves which are developed at the distal extremities of the 

 hinge plate ; characters of muscle scars and pallial sinus obscure. 



Occurrence. MONMOUTH FORMATION. Brightseat, Brooks estate near 

 Seat Pleasant, Friendly, Prince George's County. 



Collections. Maryland Geological Survey, Philadelphia Ai-ademy of 

 Natural Sciences, New Jersey Geological Survey. 



Outside Distribution. Magothy Formation. Cliffwood clay, New 

 Jersey. Matawan Formation. Merchantville clay marl, Woodlmry clay, 

 Marshalltown clay marl, Wenonah sand, New Jersey. Mon month Forma- 

 tion. Red Bank sand, New Jersey. Black Creek Formation. North and 

 South Carolina. Eutaw Formation (Tombigbee sand member) . Kxogyra 

 ponderosa zone, Morloniceras subzone, Stewart County, Georgia. TUpley 

 Formation. Exogyra costata zone, Quitman County, Georgia ; Kufaula, 

 Alabama ; Union, Tippah and Alcorn counties, Mississippi. Extreme top 

 of zone. Pataula Creek, Georgia. 



