718 SYSTEMATIC PALEONTOLOGY 



C. percompressa is separated from C. terramaria by the much higher 

 compression of the valves and the fewer and coarser concentric plications. 



Occurrence. MONMOUTH FORMATION. Brightseat, ? Brooks estate 

 near Seat Pleasant, Prince George's County. 



Collection. Maryland Geological Survey. 



CORBULA SUBRADIATA n. sp. 



Plate XLIV, Figs. 9-15 



Description. Shell of medium size, rather thin, ovate-trigonal in out- 

 line, moderately inflated, inequilateral, inequivalve, right valve slightly 

 overlapping the left along the posterior half of the base line and the dorsal 

 and posterior lateral margins ; anterior ends of both valves broadly 

 rounded ; posterior end obliquely truncate ; ventral margin arcuate in the 

 right valve and feebly contracted in front of the rostrum, slightly patulous 

 anteriorly in the left valve ; umbones of equal altitude, moderately inflated, 

 flattened upon their summits, incurved, prosogyrate ; posterior keel devel- 

 oped along a line extending from the umbones to the posterior ventral 

 margin, the angulations becoming increasingly acute toward the base; 

 posterior area slightly wider in the right valve than in the left and slightly 

 depressed, obtusely angulated near the dorsal margin of the right valve ; 

 external surface sculptured with fine and irregular concentric undulations 

 which sharpen toward the ventral margin but evanesce toward the umbones 

 and are absent altogether upon the posterior area, excepting near the base ; 

 concentric sculpture overridden by microscopically fine radial striations 

 developed only upon the disk and at the ventral margin of the posterior 

 area ; ligament internal, supported by a laminar chondrophore in the left 

 valve ; resilial pit in the right valve broad and shallow ; cardinal tooth in 

 right valve stout, conical; adductor muscle scars obscure; pallial sinus 

 broad but very shallow. 



Dimensions. Altitude 4.5 mm., latitude 6.5 mm., maximum diameter 

 of double valves 3.2 mm. 



Type Locality. Brooks estate near Seat Pleasant, Prince George's 

 County. 



