656 SYSTEMATIC PALEONTOLOGY 



spending portion in the opposite valve ; anterior end of the shell broadly 

 rounded; posterior end obliquely produced along an obtuse keel which 

 extends from the umbones to the posterior ventral margin ; area in front 

 of the keel broadly and feebly depressed, area behind it obliquely flattened ; 

 posterior dorsal margin evenly and steeply sloping to a point opposite the 

 pallial line where it is obtusely truncated; ventral margin gently arcu- 

 ate anteriorly, slightly constricted in front of the carina; external 

 surface sculptured in the umbonal region with sharp concentric ridges, 

 close set and regularly spaced, merging ventrally into an irregular incre- 

 mental sculpture; posterior keel smooth excepting for faint incrementals 

 and a few sharp ridges at the very apices of the umbones ; radial sculpture 

 absent; hinge rather frail, three cardinals in the right valve, two in the 

 left; a laminar anterior, a moderately robust medial and an exceedingly 

 thin, laminar posterior cardinal in the right valve, subequal and moder- 

 ately heavy anterior and medial cardinals in the left; inner cardinal faces 

 transversely striated ; posterior dorsal margin of right valve and anterior 

 dorsal margin of left valve bevelled to function as laterals, received in the 

 opposite valve by corresponding grooves ; anterior and posterior muscle and 

 pedal adductor scars distinct; pallial line entire; inner margins very 

 finely crenate. 



The species is conspicuous among all other representatives of the genus 

 in the Upper Cretaceous of Maryland by reason of the alate outline and 

 the sharp concentric sculpture in the umbonal region. 



Occurrence. MATAWAN FORMATION. Ulmstead Point, Anne Arundel 

 County. MONMOUTH FORMATION. Brightseat, railroad cut west of Seat 

 Pleasant, Brooks estate near Seat Pleasant, Friendly, 1 mile west of 

 Friendly, 'McNeys Corners, Prince George's County. 



Collections. Maryland Geological Survey, U. S. National Museum. 



Outside Distribution. Black Creek Formation. North and South 

 Carolina. Ripley Formation. Exogyra costata zone, Eufaula, Alabama ; 

 Owl Creek, Tippah County, Mississippi. Extreme top of zone, Pataula 

 Creek, Georgia; Chattahoochee River, Alabama. 



