ADDENDA. 419 



Surface marked only by distant concentric lines of growth. 



This species resembles T. lens more nearly than any other in the New York 

 rocks, but is a little more convex in proportion to its size, and the beak is com- 

 paratively higher and less incurved, while the shell is more nearly circular, aud 

 has its greatest width nearer the front. 



Geological formation and locality. In the Hamilton shales on Canandaigua Lake, 

 N. Y. 



Centronella ovata (n. s.). 



PLATE LXr AT, FIGS. 47-i9. 



Shell small, oval or slightly ovate ; width and height as four to five, great- 

 est width near the middle of the length. 

 DoKSAL valve depressed convex, without perceptible sinus. 

 Ventral valve much deeper than the dorsal, subcarinate along the centre, 

 most ventricose just below the beak, which is strongly incurved ; lateral 

 margins of the beak slightly carinate. 

 Sruface marked only by concentric lines of growth. 



This species resembles O. julia in form, but is a narrower shell, with a less con- 

 vex dorsal valve ; the ventral valve is more ventricose and carinate along the mid- 

 dle, and the beak more strongly incurved. 



Geological formation and locality. In the Upper Helderberg limestones at 

 Cayuga, C. W. 



Centronella julia. 



plate LXI a, figs. 41-46. 

 Centronella julia : Winchell. Proc. Acad. Nat. Science, Phil. Vol. xiv., page 405. 1862. 



Shell small, lenticular. 



Ventral valve a little more ventricose than the opposite, scarcely carinate 



along the middle. 

 Dorsal valve regularly convex, without sinus. 

 Surface unknown. 



This shell closely resembles O. ovata, but differs in several particulars, as indi- 

 cated above. 



Geological formation and locality. In the Marshall sandstones, Point aux 

 Barques, Mich. 



