LAMELLIBRANCHIATA. 263 



slightly inflated, thence nearly straight for almost three-fourths the length 

 of the shell ; posterior margin broadly rounded ; dorsal margin gently curved 

 for two-thirds of the length and then truncated in nearly a direct line to the 

 beak. 



Right valve moderately convex in the posterior part, becoming broadly 

 convex in the middle, and somewhat gibbous in the umbonal region. Left 

 valve unknown. 



Hinge-line oblique ; extent unknown. Beak elevated, erect, acute. 



Test marked by rather coarse, elevated lines of growth. 



Interior unknown. 



The specimen described has a length of 60 mm. and height 40 mm. 



This species somewhat resembles M. regularis but the beak is more anterior, 

 the body wider and more convex, and the truncation of the cardinal margin is 

 much longer. 



Formation and locality. In the Chemung group, Randolph, Cattaraugus 

 county, N. Y. 



Mytilarca occidentalis. 



PLATE XXXIII, FIGS. 3-o ; and PLATE LXXXVII, FIG. U. 



Mytilus occidentalis. White and Whitfield. Proc. Boston Soc. Nat. Hist., vol. viii, p. 297. 1862. 

 Mytilarca occidentaJ is (W. and W.). Hall. Prelim. Notice Lam. Shells, etc., pt. 2, p. 24. 1869. 



" " " Pal. N. Y., vol. V, pt. 1. Plates .and Explanations: PI. 33, 



figs. 3-5. Jan., 1888. 



Shell above the medium size ; body extremely elongate, narrow-ovate ; 

 length more than twice the height ; ventral margin a little inflated at the 

 byssal opening, then gently curving to the posterior margin which is abruptly 

 rounded, and broadly curving along the dorsal side. 



Valves equal, convex in the posterior part, becoming gibbous below the 

 middle and anteriorly ; the greatest convexity is anterior to the middle. 



Hinge-line short. Beaks acute, nearly erect. The umbonal region is 

 narrow and the convexity is continued along the median line to the posterior 

 end of the shell. 



