LAMELLIBRANCHIATA. 431 



the shell. Interspaces narrower than the radii. All the plications, except 

 those of the anterior end, are arcuate, corresponding essentially with the 

 direction of the umbonal slope. Entire surface marked by fine concentric 

 striae of growth. 



Two specimens measure respectively 45 and 50 mm. in length, and 32 and 

 37 mm. in height. 



In form this species somewhat resembles P. transversa, but it has a much 

 larger shell, the radii are coarser and the umbonal slope not nearly so abrupt. 



Formation and localities. In the Chemung group at Meadville, associated with 

 Spirifera Verneuili, and also near Uniontown, Pa. 



Paraeca venusta, n. sp. 



PLATE XCIV, FIG. 22. 



Shell large, broadly elliptical; length less than one-third greater than the 

 height ; basal margin regularly and gently curving. Posterior extremity 

 broadly rounded. Cardinal line short, slightly arcuate. Anterior end short, 

 regularly rounded below the lunule. 



Valves regularly convex in the lower and posterior part, gibbous in the 

 middle and above. 



Beaks sub-anterior, not prominent. Umbonal slope not defined, the line 

 of greatest convexity arcuate and continuing from the beak to the post-infe- 

 rior extremity. 



Surface marked by numerous fine rounded radii, with narrower interspaces, 

 the radii of the anterior end being stronger as they approach the margin of 

 the shell ; crossed by fine concentric lamellose striae, which, toward the mar- 

 gins, become fasciculate and produce stronger varices of growth. 



Two specimens measure respectively 39 and 43 mm. in length, and 31 and 

 32 mm. in height. 



This species resembles P. Sao, in general form, but is proportionally narrower 

 posteriorly, and marked by much finer radii. 



Formation and localities. In the upper part of the Chemung group, at Warren, 

 Pa. ; and at Panama, N. Y., above the conglomerate. 



