LAMELLIBRANCHIATA. 261 



margin abruptly rounded, gently curving into the dorsal margin which is 

 straight above the middle. 



Right valve gently convex in the posterior part, becoming gibbous and 

 angular in the umbonal region. Left valve unknown. 



Beak erect, acute, pointed. 



Test of moderate thickness, marked by fine concentric strise of growth, 

 which at intervals have been raised into lamella 1 which have left strong 

 varices upon the cast. 



Interior unknown. 



A right valve has a length of 65 mm. and height 24.5 mm. 



This species differs from M. Chemungensis in its more elongate, straighter form, 

 and in the angularity of the umbonal slope. From M. carinata it is distin- 

 guished by its elongate form, straight ventral side, and more prominent anterior 

 umbonal slope. 



Formation and locality. In the arenaceous beds associated with fossils of the 

 middle Chemung group, from a loose mass collected at Elmira, N. Y. 



Mytilarca simplex. 



PLATE XXXIII, FIGS. 19, 21. 



Hytilops (Modiola) simplex, Hall. Pal. N. Y.. vol. v. pt. 1. Plates and Explanations: PI. 33, fig^. 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 striae 



Ligamental area and interior unknown. 



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



