LAMELLIBRANCHIA TA. 
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. 
Pararca venusta, n. sp. 
'PLATE XCIV, PIG. 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. 
