BEEDE: UPPER PERMIAN RED BEDS. 161 



margin, which may be nearly straight and nearly parallel to 

 the hinge or a little sinuate. Posterior extremity rounded 

 and meeting the hinge at a very obtuse angle. Valves about 

 equally convex. The semielliptical adductor scars are very 

 deeply impressed, with the strong ridge of shell behind 

 fading out toward the base of the shell. The pedal scar, sit- 

 uated above and back of the adductor, is very small. Cardinal 

 teeth two, one nearly parallel to the hinge, while the other 

 rests more obliquely over the adductor impression. A faint 

 line indicates a slight forking of the upper tooth in some casts. 

 The posterior teeth are shown in the casts as being nearly 

 parallel to the hinge, but diverging downward from it at a 

 slight angle. The tooth on the left valve is more strongly 

 developed than the one on the right, the latter not being pre- 

 served in the great majority of casts. The matrix is coarse 

 material and poorly adapted to preserving the finer marks or 

 ridges. The surface is marked by growth lines which are 

 strongest on the anterior part of the shell, two to five or six 

 faint radiating lines extending from the beak to the posterior 

 end of the shell and "granulations" on the depression below 

 the beak. The ligament is external, situated in grooves. 

 Length, 14 mm. ; height, 5.5 mm. 



Whitehorse spring, Oklahoma; very abundant. Dozier, 

 Tex. ; common. 



This species differs from P. occidentalis Meek and Hayden 

 in being larger, having an arcuate hinge, and in having the 

 pedal scar farther back, judging from his description. It 

 differs from P. oblorigus Meek in being proportionally much 

 longer and in possessing radiating ridges. Compared with 

 P. sabciineatus Meek and Hayden, our species has the ridge of 

 shell back of the adductor scar extending somewhat back- 

 ward rather than vertical, as well as much narrower and more 

 sharply defined. The umbonal ridge is more poorly defined 

 on our specimens, which also show two to six radiating 

 ridges. There also appears to be some difference in the 

 dentition. 



Our species seems to be closely related in general appear- 

 ance, at least, with P. meeki Walcott, but does not widen 



4-Uni^r. Sci. Bull.. Vol. IV, No. 3. 



