260 PALEONTOLOGY OF NEW YORK. 



MyTILARCA REGTJLARIS, 11. sp. 



PI-ATK XXXII, YUi. I--'. 



Shell al)Ove 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. Right valve unknown. 



Hinge-line straight, short, olilique. Beak nearly erect, acute and abruptly 

 attenuate. 



Test thin, marked by fine elevated concentric strife wliicli arc crowded on 

 the ventral and dorsal sides of the valve. 



Ligamental area narrow, longitudinally striated. 



Interior unknown. 



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. ocddentalis and less 

 elevated along the umbonal slope. 



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

 Centre, Cattaraugus county, N. Y. 



Mttilarca attenuata. 



PLATE XXXn, FIG. •!«. 



Mytilarca altcnmita. Hall. Pi-elim. Notice Lam. Shells, etc., pt. 2, ii. 23. 18G9. 



" " " Pill. N. v., vol. V, pt. 1. Plates and Explanations : PI. 32, fig. 20. Jan., 1SS3. 



Shell above medium size ; body extremely elongate, sub-cuneiform : length 

 nearly three times the height ; ventral margin nearly straiglit for almost the 

 entire length, with a slight depression at the byssal opening; posterior 



