542 CRETACEOUS PALEONTOLOGY. 



thickness, 14 mm. Length of the largest individual observed, 

 14 mm. Shell very oblique and inequilateral, the beaks obtuse, 

 slightly incurved, situated about three-eights of the entire 

 length of the shell from the anterior extremity. Anterior nar- 

 gin somewhat narrowly rounded and passing into the basal mar- 

 gin; basal margin moderately convex anteriorly, becoming 

 straight or usually slightly concave 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 margin, 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 de- 

 pression 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 sur- 

 face of the shell covered with strong, concentric lines of growth, 

 which are more or less irregular in the strength of their develj 

 opment. 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 sep- 

 arated from the hinge-line by a deep pit. The hinge of the 'left 

 valve has two cardinal teeth, a large bifid one immediately be- 

 neath 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 ex- 

 tended longitudinally as the posterior one. Muscular impres- 



