622 SYSTEMATIC PALEONTOLOGY 



incurved, prosogyrate ; valves flattening in all directions away from the 

 umbones ; anterior end very short, rounded ; posterior end obliquely pro- 

 duced along the obscurely elevated diagonal from the umbones to the pos- 

 terior ventral margin ; posterior dorsal and lateral areas relatively very 

 wide, their margins forming a somewhat asymmetrical arc connecting the 

 umbones and the base ; ventral margin slightly oblique with a feeble sug- 

 gestion of a mesial constriction; faint concentric sculpture probably 

 developed on external surface; characters of hinge and interior not known. 



Dimensions. Maximum altitude 5 mm., maximum latitude 8 mm., 

 maximum diameter 3.5 mm. 



This small but apparently adult Lithophaga is separated from the co- 

 existent members of the same genus not only by its slight dimensions but 

 even more readily by the very short anterior end and expanded posterior 

 end. In no other species is the area behind the diagonal relatively so 

 wide or so flaring. The peculiar alate aspect thus produced is not repeated 

 in any of the co-existent species. The form is described from a cast of 

 double valves. The type is not unique, but the species has not been 

 observed from any but the type locality. 



Occurrence. MONMOUTH FORMATION. Brightseat, Prince George's 

 County. 



Collection. Maryland Geological Survey. 



LITHOPHAGA TWITCHELLI n. sp. 

 Plate XXXVI, Figs. 12, 13 



Description. Shell nacreous, apparently rather thick, large for the 

 genus, subcylindrical in outline ; umbones inflated, incurved, prosogyrate, 

 proximate, placed within the anterior tenth; shell inflated along the 

 diagonal from the umbones to posterior ventral margin, broadly and 

 shallowly depressed between this obscure carina and the feebly inflated 

 anterior end; anterior lateral margin obscurely truncate, posterior 

 strongly arcuate; dorsal margin very feebly convex; base line somewhat 

 constricted medially ; external surface probably smooth ; characters of 

 interior of shell not known. 



