LAMELLIBRANCHIATA. 339 



centric stri«. The intermediate striae become obscure or obsolete, according 

 to the degree of weathering and nature of the matrix. 



Five specimens measure respectively 22, 24, 26, 27, 28 and 40 mm. in 

 length, and 12, 13.5, 16, 12 and 16 mm. in height. 



This species is distinguished by its regular, strong, lamelliform, concentric 

 striae, and deeply emarginate, post-inferior extremity. 



Formations and localities. In the arenaceous layers of the Hamilton group, at 

 Jefferson, Summit and Fultonham, Schoharie county ; in the soft shales on the 

 shores of Skaneateles, Cayuga, Seneca and Canandaigua lakes, and at other 

 localities in Western N. Y. ; also in beds of the same age near Cumberland, 

 Md. It likewise occurs in the lower beds of the Chemung group, at Ithaca, N. Y. 



Pal^oneilo perplana. 



PLATE L, FIGS. 15-22; AND PLATE XCHI, FIG. 12. 



Pakeoneiln f perplana. Hall. Prelim. Notice Lamellibranchiata, 2, p. 12. 1870. 



" " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations : PI. ,50, figs. 15-22. 1883. 



Shell large, ovate, sub-cuneate ; length less than twice the height ; basal 

 margin gently curving, sometimes a little flattened or slightly sinuate in the 

 posterior portion ; post-inferior margin doubly truncate. Cardinal line gently 

 arcuate. Anterior end broadly and regularly rounded. 



Valves convex in the lower part of the anterior half, gibbous above and 

 in the umbonal region. 



Beaks usually at about the anterior third, sometimes just anterior to the 

 centre, broad, prominent, rising but little above the hinge-line, gently 

 incurved. Umbonal slope marked by two broad ridges, separated by a 

 broad, undefined sulcus, producing sinuations in the post-inferior and post- 

 cardinal margins; the shell, at the termination of the upper ridge, and 

 sometimes between the two ridges, is produced into a short, linguiform exten- 

 sion. Post-cardinal slope flattened or concave. 



Entire surface marked by extremely fine, crowded striae, and at intervals 

 by strong, elevated lamellae, which are usually obsolete, except on the poste- 

 rior portion of the shell. 



