LAMELLIBRANCHIATA. 421 



Valves moderately convex below, gibbous in the middle and above. 



Beaks prominent, attenuate. 



Surface marked by fine, undulating concentric striae and by about forty- 

 five rounded radii, with narrow interspaces. Some of the radii become less 

 conspicuous and duplicate upon the posterior slope. 



The specimen described has a length, in its compressed condition, of about 

 28 mm. and a height of 31 mm. 



In form this species is similar to P. retusa, but the shell is less oblique, the 

 posterior slope longer and the radii are more direct. 



Formation and locality. In the Marcellus shale ; precise locality unknown. 



Panenka retusa, n. sp. 



PLATE LXIX, FIG. 6. 



In part Cardiolaf radians (Cosrad), Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 69, 



figs. 5-11. 1883. 



Shell of medium size, obovate, oblique ; length nearly equal to the height. 

 Pallial margin regularly curving. 



Valves moderately convex below and in the middle, becoming gibbous in 

 the umbonal region. 



Hinge-line short, straight. Ligamental area distinct. 



Beaks sub-central, small, curving forward, not prominent. 



Surface concentrically striated, and marked by about thirty-five moderately 

 rounded, curving plications, with somewhat narrower interspaces. 



The specimen described has a length of 30 mm. and a height of 31 mm. 



This species is distinguished by its broad obovate form, its sub-equal length 

 and height, and by its regular curving simple plications. 



Formation and locality. In the shales of the Hamilton group on the shore of 

 Cayuga lake, N. Y. 



54 



