Lamellibeanchiata. 403 



becoming moderately convex in the middle and scarcely gibbous 

 above. The ventral umbonal slope is angular, and the shell between 

 it and the margin is concave. Hinge-line short, less than the 

 height of the shell. Beaks anterior, prominent, acute, slightly 

 curving forward. 



Test thin, marked by very fine concentric striae of growth, which 

 are crowded on the anterior side and toward the base. Ligamental 

 area of moderate width, finely striated longitudinally. Cardinal 

 teeth small, diverging, situated under the extremity of the beak. 

 Lateral teeth two, small, oblique, situated just below the posterior 

 extremity of the hinge-line. 



A specimen has a length of 30 mm., and height 18 mm. A more 

 elongate form has a length of 43 mm., and height 20 mm. A small 

 individual has a length of 23 mm. and height 15 mm. 



This form differs from M. Chemungensis in its proportionally shorter 

 form, the young shells have a sub T triangular aspect, the ventral um- 

 bonal slope is always angular, and the body is more distinctly arcuate. 



Formation and localities. In the middle portion of the Chemung 

 group, Tioga and Chemung counties, N. Y. 



Mttilaboa begulaeis. 



Mytilarca regularis, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Setell above the medium size ; body narrowly ovate ; length less than 

 twice the height ; byssal area scarcely depressed ; ventral margin 

 for two-thirds the length nearly straight, thence curving into the 

 posterior margin which is regularly rounded ; dorsal margin gently 

 curving to the extremity of the cardinal line. Left valve gently 

 and regularly convex in the posterior half, convex above and scarcely 

 gibbous on the umbo. Eight valve unknown. Hinge-line straight, 

 short, oblique. Beak nearly erect, acute and abruptly attenuate. 



Test thin, marked by fine elevated concentric striae which are 

 crowded on the ventral and dorsal sides of the valve. Ligamental 

 area narrow, longitudinally striated. 



The specimen described has a length of 48 mm. and height 28 mm. 



As compared with M. Chemungensis this species has a proportionally 

 wider form ; it is more nearly equilateral, the umbonal elevation is 

 less abrupt and more nearly central. It is proportionally higher than 

 M. occiclentalis and less elevated along the umbonal slope. 



Formation and locality. In a sandstone of the Chemung group, 

 near Leon Centre, Cattaraugus county, N. Y. 



