404 Thirty-fifth Report ok the State Museum. 



Mytilarca simplex. 



Mytilarca (Modiola) simplex, Hall. Pal. N. Y., vol. v, pt. 1. Plates and 

 Explanations : PI. 33, figs. 19, 21 (20 in error). Jan., 1883. 



Shell of medium, size; body elongate-ovate, with an acute apex; 

 length nearly twice the height ; byssal area depressed ; margin below 

 nearly straight for two-thirds the length ; posterior margin abruptly 

 rounded ; dorsal margin gently curving to the hinge-line. Valves 

 equal, moderately convex in the posterior part, and gibbous on the 

 umbo. The line of greatest convexity is on the ventral side of the 

 axis. Hinge-line short. Beaks sub-erect, acute, abruptly attenuate. 



Test marked by fine, closely arranged, concentric stride. 



A specimen has a length of 41 mm., and height 25 mm. 



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 succeeding were referred to Mytilops, but a care- 

 ful comparison shows them to be related to the species here arranged 

 under Mytilarca. 



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

 west of Smethport, Pa. 



Mytilarca gibbosa. 



Mytilarca gibbosa, Hall. Pal. K Y., vol. v, pt. 1. Unpublished. 



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. Eight 

 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 upon the surface of the 

 cast. 



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. 



