MARYLAND GEOLOGICAL SURVEY 717 



sinus very shallow; pallial line rather distant from the base; body cavity 

 deeply excavated. 



Dimensions. Of double valves: Altitude 4.2 mm., latitude 6.4 mm., 

 maximum diameter 2.8 mm. Eight valve of a second specimen : Altitude 

 5 mm., latitude 6.9 mm. 



Type Locality.- -Brightseat, Prince George's County. 



Occurrence. MOXMOUTH FORMATION. Brightseat, ? Friendly, Prince 

 George's County. 



Collection. Maryland Geological Survey. 



CORBULA PERCOMPRESSA 11. Sp. 



Plate XLIV, Figs. 1-3 



Description. Shell small, ovate-trigonal, both valves strongly and 

 almost equally compressed, slightly inequilateral, very slightly inequi- 

 valve ; umbones subcentral, much flattened upon their summits, prosogy- 

 rate; lunule feebly defined, less so in the left valve than in the right; 

 anterior end rather narrow, the dorsal slope moderately steep for the 

 genus ; lateral margin evenly but strongly rounded ; posterior dorsal slope 

 a little steeper than the anterior ; lateral margin obliquely truncate ; base 

 line gently arcute and in the right valve very feebly contracted in front 

 of the keel ; posterior keel acute and persistent from umbones to base, but 

 not very conspicuous because of the compression of the valves; external 

 surface sculptured with about twenty broad but not greatly elevated 

 concentric plications, which become increasingly finer and more crowded 

 toward the umbones, and which override the keel and persist to the 

 posterior dorsal margin but with diminished vigor; ligament internal; 

 chondrophorc, judging by the resilial pit in the right valve, quite narrow 

 but considerably produced; right cardinal tooth rather small; adductor 

 scars rather small, the posterior placed well up under the dorsal margin ; 

 pallial sinus very shallow, pallial line distant from the base. 



Dimensions. Altitude 4.4 mm., latitude 6 mm., diameter of double 

 valves 2.3 mm. 



Type Locality. Brightseat, Prince George's County. 



