MARYLAND GEOLOGICAL SURVEY 647 



Type Locality. Snow Hill, North Carolina. 



" The dimensions of a shell of average size, preserving both valves, are : 

 Length 33 mm., height 22.5 mm., thickness 14 mm. Length of the largest 

 individual observed, 14 mm. Shell very oblique and inequilateral, the 

 beaks obtuse, slightly incurved, situated about three-eighths of the entire 

 length of the shell from the anterior extremity. Anterior margin some- 

 what narrowly rounded and passing into the basal margin ; basal margin 

 moderately convex anteriorly, becoming straight or usually slightly con- 

 cave posteriorly; posterior-basal extremity acutely angular; posterior 

 margin rather short, obliquely truncate; postero-dorsal margin straight, 

 except near the beak where it becomes slightly convex, making an angle 

 of about 136 with the truncate posterior margin. Surface of the shell 

 marked with a sharply angular or subcarinate, usually straight, umbonal 

 ridge passing from the beak to the postero-basal extremity of the shell; 

 postero-dorsal slope concave from the umbonal ridge to the cardinal mar- 

 gin, where the shell is sharply inflected to form a large and nearly flat 

 escutcheon ; in front of the umbonal ridge a broad, more or less indefinite 

 depression passes from the beak to the sinuosity in the posterior portion 

 of the ventral margin ; in front of the beak the surface is inflected to form 

 a rather large and broad lunule. Entire surface of the shell covered with 

 strong, concentric lines of growth which are more or less irregular in the 

 strength of their development. Hinge of right valve with a large bifid 

 cardinal tooth directed obliquely backwards from beneath the beak, and a 

 much smaller simple one directed forward; between these two teeth is a 

 deep triangular pit, and behind the posterior one is a much narrower pit ; 

 two large lateral teeth are present, one in front and one behind the beak, 

 the anterior one is nearer the beak with a broad and deep pit between it 

 and the hinge line, the posterior one is more elongate and slender, and is 

 also separated from the hinge line by a deep pit. The hinge of the left 

 valve has two cardinal teeth, a large bifid one immediately beneath the 

 beak and a thin, very oblique one behind, with a large, oblique, triangular 

 pit between the two ; there are two strong lateral teeth, one in front and 

 one behind, the anterior one being nearer the beak and usually stronger 

 but not so much extended longitudinally as the posterior one. Muscular 



