Weno and Pawpaw Formations 119 



its dorsal three-fifths and bears a taxodont series of nine teeth of which 

 the two dorsalmost and the one ventralmost are shorter and narrower 

 than the others. The antero-ventral two-fifths of the margin is concave 

 and lies just posterior to the truncately elliptical scar of the post-adductor 

 muscle. The posterior end of this margin is the posterior angle, from 

 which the rounded ventral margin runs to the anterior angle. This margin 

 is more sharply rounded near the posterior angle. The antero-dorsal mar- 

 gin is distinctly bulged, and its dorsal two-thirds bears a taxodont series 

 of twenty teeth, which taper in size at each end. 



Beneath the umbo is a narrow triangular pit directed downward and 

 anteriorly; and posterior to it is a parallel tooth containing along its 

 length, except at the top, a sulcate depression. 



Left valve, exterior: The beak is rounded and forms an angle of about 

 105 degrees. The posterior area is contained between the straight post- 

 erior margin and a curved ridge which is the arc of a circle. In this area 

 the growth lines turn sharply dorsally and converge at the margin. In 

 its center is a radial elevation on whose outer surface are irregular crenu- 

 lations which are oblique to the growth lines and continuous with those on 

 the main portion of the valve. Lateral to the elevation is a concurrent 

 depression and dorsal to it a small plane area. The posterior angle is 

 100 degrees. The ventral margin is curved ; and more sharply so at the 

 posterior end. The anterior area is a triangular space included between 

 the convex anterior margin and a concurrent but divergent ridge which 

 runs from the beak to the anterior angle. It bears numerous^ coarse 

 crenulations which are continuous with those over the main body of the 

 valve. 



The ornamentation of the valve consists of numerous radial simple flat 

 ribs similar to those of the right valve; and concentric depressions and 

 growth lines. The two valves fit together evenly with a slight ventral 

 gape. 



Left valve, interior: This valve is essentially similar to the right valve. 

 Anterior to the beak and ventro-lateral to the taxodont series of teeth 

 is an elongate tooth pointed antero-ventrally and fitting into a socket in 

 the right valve. Its end lies below the tenth tooth anterior to the beak. 



Type individual: This description is compiled from three individual? 

 of which the first, showing the exterior of the right and left valves, is 

 to be considered the type individual. It and the other two, as many of 

 the other individuals, were found in the blue marl of the upper half of 

 the Weno formation, 20 feet below Quarry limestone in cut of Frisco track, 

 one mile north of Union Station, Denison, Texas (604). 



