MAEYLAND GEOLOGICAL SURVEY 715 



ticated, leaving a high, trigonal subequilateral shell with no trace of con- 

 centric plications or posterior keel. 



Occurrence. MONMOUTH FORMATION". Brightseat, Brooks estate near 

 Seat Pleasant, 1 mile west of Friendly, McNeys Corners, Prince George's 

 County. 



Collections. Maryland Geological Survey, Philadelphia Academy of 

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



Outside Distribution. Mataivan Formation. Merchantville clay marl, 

 Woodbury clay, Wenonah sand, New Jersey. Monmouth Formation. 

 Navesink marl, Eed Bank sand, New Jersey. Black Creek Formation. 

 North and South Carolina. Eutaw Formation (Tombigbee sand member) . 

 Exogyra ponderosa zone, Prentiss County, Mississippi. Mortoniceras 

 subzone, Stewart County, Georgia. Eipley Formation. Exogyra costata 

 zone, Schley County, Georgia ; Eufaula, Alabama ; Union, Tippah and 

 Alcorn counties, Mississippi. Exireme top of zone, Pataula Creek, 

 Georgia. 



CORBULA MONMOUTHENSIS n. sp. 



Plate XLIV, Figs. 4-8 



Description. Shell rather large for the genus, ovate trigonal in out- 

 line, inequilateral and inconspicuously inequivalve; umbones subcentral 

 in position, subequal in the two valves, somewhat flattened upon their 

 summits, the apices acute and prosogyrate ; right valve more inflated than 

 left in the anterior portion, and with a wider posterior area which is 

 angulated near its dorsal margin and slightly reflected over the left valve ; 

 anterior margins of both valves broadly and evenly rounded; posterior 

 dorsal slope more gentle in the right valve than in the left; the lateral 

 margin produced and obtusely angulated in the right, obliquely truncate 

 in the left; base line more strongly arcuate in the larger valve; external 

 sculpture in both valves of very fine, sharp lamella? closely overlapping, 

 the free edges directed toward the umbones, least feeble on the anterior and 

 ventral portions of the disk, very faint in the umbonal region and evan- 

 escent near the posterior keel; ligament internal, supported by a rather 

 inconspicuous lamelliform chondrophore behind the umbone in the left 



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