Weno and Pawpaw Formations 181 



straight, having a roughened, striate protruding area the rostrum reach- 

 ing to the umbo; and anterior to it is the straight posterior margin of 

 the valve. 



The greatest height of the valve is near its center. Its convexity is 

 greater on the umbonal margin than on the ventral margin; from the 

 center the surface slopes gently to a line, marked in places by a prominent 

 growth line, below which the surface is curved at a sharply rounded angle 

 of 90 degrees to the margins of the valve, giving to the valve a charac- 

 teristic geniculated and basin-like appearance. The rim of the valve 

 thus slopes inward at all points except on the dorsal margin of the ros- 

 trum, so that the margin of the valve has a smaller circumference than 

 the prominent growth line mentioned above. 



The ornamentation consists of radial striae and concentric growth lines. 

 The radial striae are fine, unbranched, wavy, or punctate lines which end 

 near the geniculation. The concentric growth lines are more conspicuous 

 (toward the ventral margin they resemble fine ribs), and pass posteriorly, 

 turning sharply dorsally at the line of junction of the main body of the 

 valve and the slightly developed rostrum. They then pass dorsally in 

 straight lines which give to the elongate triangular rostrum a roughened 

 striated appearance. 



Right valve, interior: As viewed directly from above the interior of 

 the right valve has three prominent contours : the outer one is the margin 

 of the valve ; the middle one is a narrow groove, the junction between the 

 thicker outer (prismatic) shell layer and the thinner inner (porcellanous) 

 shell layer; the innermost contour is the limit of the main body cavity. 

 The first two are the shape of a long ellipse, and are situated a short 

 distance apart ; they begin at the antero-dorsal one-third of the prominent 

 tooth, and run concentrically around the anterior and ventral margins of 

 the valve, making an evenly rounded elliptical curve to the postero-ventral 

 extremity of the rostrum. The innermost contour encloses roughly a sub- 

 quadrate area whose boundary begins at the ventral margin of the tooth 

 and describes a slight curve to the dorsal end of the anterior adductor 

 scar. Here it runs almost straight ventrally with a slight posteriorward 

 convexity formed by the scar, in such a manner that a sharp-edged, cres- 

 cent shaped shelf, elevated above the main body cavity, is formed between 

 it and the middle contour mentioned. The scar is pear-shaped and its 

 inflated ventral end lies at the widest point of the shelf. The shelf con- 

 tinues along the interior of the ventral side of the visceral space to a point 

 one-third the way from the anterior end, and then fuses with the wall of 

 the valve. Just anterior to it lies the pallial line, a narrow groove with 

 a slight and irregular interior elevated crest which descends to the bottom 



