714 SYSTEMATIC PALEONTOLOGY 



margin oblique, much more produced in the right valve than in the left ; 

 lateral margin obliquely truncate ; base line broadly and evenly rounded in 

 the left valve, quite strongly arcuate in the anterior portion of the right 

 but feebly constricted in front of the posterior keel, which extends in the 

 form of a sharply elevated ridge from the umbonal region to the posterior 

 basal margin; area behind the keel sharply differentiated from that in 

 front of it, its lateral margin in the right valve thin and slightly reflected ; 

 external surface of right valve corrugated with fifteen to twenty promi- 

 nent concentric plications about half of which are confined to the umbonal 

 region and become increasingly fine and sharp toward the apices, the other 

 half very coarse and heavy, often somewhat irregular in size and spacing 

 toward the base, but approximately uniform in prominence from the ante- 

 rior margin to just in front of the posterior keel where they abruptly 

 evanesce ; keel and area behind it sculptured only with strong incre- 

 mentals ; left valve smooth excepting for irregular incremental sculpture : 

 ligament internal, supported by a rather prominent lamelliform chondro- 

 phore in the left valve ; resilial pit in right valve quite profound ; dentition 

 restricted to a single, subumbonal, sharply conical tooth in the right valve 

 and a subumbonal socket for its reception in the left; adductor scars not 

 very distinct, quite well up toward the dorsal margins, pallia! sinus broad 

 but not very deep, pallial line much more distant from the base in the 

 right valve than in the left, because of the overlapping ventral margin of 

 the larger valve. 



Dimensions. Right valve, latitude 5 mm., altitude 4 mm.; left valve, 

 latitude 3 mm., altitude 2.7 mm.; maximum diameter of double valves 

 2.5 mm. 



This species is by far the most abundant representative of the genus in 

 the Upper Cretaceous faunas of Maryland, and is one of the most prolific 

 of the smaller bivalves in the Monmouth of Prince George's County. It 

 is readily recognizable by the strong discrepancy of the valves in size, 

 outline and sculpture and by the robust concentric plications upon the 

 disk of the right valve. 



Weathered individuals of this species present a most deceptive appear- 

 ance ; the entire external sculpture and posterior keel and area are decor- 



