MARYLAND GEOLOGICAL SURVEY 659 



STEPHEN SONI n. sp. 

 Plate XXXIX, Figs. 10, 11 



Description. Shell small, equivalve, inequilateral, valves compressed 

 transversely, oblong to subrectangular in outline; lunule narrow, elon- 

 gated, distinctly impressed and further differentiated by the absence of 

 surface sculpture; escutcheon not denned; umbones small, broadly but 

 feebly inflated, the apices flattened, acute and prosogyrate, slightly over- 

 topping the dorsal margin a little front of the median horizontal ; anterior 

 dorsal margin slightly constricted and feebly excavated in front of the 

 umbones ; posterior dorsal margin gently sloping ; anterior lateral margin 

 broadly arcuate, the posterior widely truncate ; base line feebly convex ; 

 external surface sculptured with fine crowded, closely over-lapping con- 

 centric lamella, attached along their ventral margins, the dorsal margins 

 free and slightly raised near the anterior and posterior extremities of the 

 shell ; ligament submarginal, opisthodetic ; hinge armature rather feeble, a 

 single triangular cardinal in the right valve ; posterior cardinal and ante- 

 rior and posterior laterals obsolete; left valve armed with two slender, 

 divergent cardinals, the laterals appearing as short, incipient laminar proc- 

 esses near the distal extremities of the hinge plate; muscle scars unequal, 

 the anterior elongated, the posterior smaller and irregular in outline ; 

 pallial line simple, distinct. 



Dimensions. Altitude 6.5 mm., latitude 7 mm., semi-diameter 2 mm. 



Type Locality. One mile west of Friendly, Prince George's County. 



Named in honor of Dr. L. W. Stephenson of the U. S. Geological Survey. 



Occurrence. Mox MOUTH FORMATION. Brightseat, Brooks estate near 

 Seat Pleasant, 1 mile west of Friendly, Prince George's County. 



Collection. Maryland Geological Survey. 



Genus PHACOIDES Blainville 

 [Manual Malacology, vol. i, 1825, p. 550] 



Type. Lucina jamaicensis Lam. 



Shell more or less lenticular; compressed, as a rule, or slightly tumid; 

 umbones low, subcentral, erect or prosogyrate; sculpture dominantly con- 



Etymology: 0a/c6s, lentil; ei'5-s. like. 



