146 PALAEONTOLOGY OF NEW YORK. 



basal margin is broadly rounded ; posterior extremity recurved at a little less 

 than a right angle. 



Left valve more convex than the right and gibbous above, obscurely 

 angular along the post-cardinal slope. This diff('r(Mio(' in the valves is also 

 shown in the direction of the strioe of growth. 



Hinge-line straight, about three-fourths as long as the length of the shell. 



Beaks near the anterior fourth of the hinge, prominent in the left valve, 

 acute, arching over the cardinal line ; in the right valve depressed, rising only 

 as high as the hinge. Umbonal region of the left valve moderately gibbous, 

 sul)tending an acute angle. 



Anterior end short, small, limited by an oblique byssal depression which 

 extends along the base about one-third the length of the shell ; extremity 

 acute. Wing large, extending along the shell to near the posterior extremity ; 

 margin obliquely trnncate, concave just below the hinge-line, which is 

 al)ruptly produced. 



Test thin, marked by line, interrupted or undulated radii, which are more 

 strongly marked on the body and wing, and obsolete on the anterior of the 

 valve ; the surface is also marked by fine concentric striae, which are crowded 

 into fascicles at unequal distances, undulating the surface; tlu'v turn aliruptly 

 outwiird. just Itclow the hinge, and are lamellose on the anterior part of the 

 valve. 



Interior unknown. 



A specimen preserving both valves has a length of ("i iiiiii.. height 2S nmi., 

 hinge-line ?>'~i mm. The right valve is somewhat smaller. Aimtlirr 

 specimen lias a length of 42 mm., height 28 min., hinge-line of) mm. 



This species resembles in form P. Sao, but is larger, the wing furrow nioie 

 strongly defined on the left valve, the concentric lines more acutely recurved 

 at the ba.sal angle, and the radii are finei- and more numerous. 



Formation and locality. In the rppei- Clieunuig group, the niiddU' lieds of 

 the series, as seen at Warren, Pa. 



