262 PALEONTOLOGY OF NEW YORK. 



This species is somewhat intermediate to M. Chemungensis and M. regularis, 

 but is distinguished from the former by its shorter form, and from the latter 

 by its greater convexity and more attenuate beak. This form and the suc- 

 ceeding, were referred to Mytilops, but a careful comparison shows them to lie 

 related to the species here arranged under Mytilarca. 



Formation and locality. In the sandstones of the Chemung group, west of 

 Smethport, Pa. 



Mytilarca gibbosa, n. sp. 



PLATE XXXIII, FIG. 20 ; AND PLATE LXXXVII, FIG. 7. 



Shell of medium size ; body ovate-arcuate, obliquely truncate along the hinge- 

 line ; length less than twice the height ; ventral margin nearly straight for 

 more than three-fourths the length, abruptly curving into the posterior 

 margin, thence gently rounded to the extremity of the hinge-line. 



Left valve very convex; the greatest convexity above the middle. 

 Umbonal region gibbous. Right valve unknown. 



Hinge-line oblique, nearly equal to the height of the shell. Beak small, 

 appressed, arching toward the ventral side. 



Test marked by fine concentric striae, which, at irregular intervals, are 

 crowded into fascicles, leaving varices iipon the surface of the cast. 



Interior unknown. 



The specimen described has a length of 45 mm., and height 26 mm. 



This species is proportionally longer, beak more acute, and much more 

 gibbous than M. lata. 



Formation and locality. In the Upper Chemung group, Napoli, Cattaraugus 

 county, N. Y. 



Mytilarca lata. 



PLATE XXXIII, FIG. 22. 

 Mytilops (Modiola) lata. Hall. Pal. N. Y.. vol. v. i)t. 1. Plates and Explanations: PI. 33, fig. 22 Jan., 1883. 



Shell large ; body broadly sub-elliptical, abruptly narrowing toward the beak 

 on the dorsal side ; length one-third greater than the height ; byssal area 



