466 MISSISSIPPIAN BRACHIOPODA 



Description. Shell of medium size or larger, transversely subelliptical 

 in outline, the valves moderately convex, the length two-thirds or more 

 than two-thirds the width, the greatest width near or a little posterior to 

 the mid-length of the shell, the hinge-line much shorter than the greatest 

 width of the shell, cardinal extremities rounded. The dimensions of a 

 rather large individual are : length 39 mm., width 51 mm., approximate 

 thickness 24 mm. 



Pedicle valve moderately convex, the greatest convexity near or poste- 

 rior to the middle, the surface curving abruptly from the umbonal region 

 to the cardinal margin and gently to the lateral and anterior margins, 

 frequently somewhat compressed towards the cardinal extremities ; the 

 beak contiguous with that of the opposite valve, pierced by a subcircular 

 foramen; cardinal area very narrow and inconspicuous, the lateral mar- 

 gins usually well defined ; delthyrium broadly triangular, the width three 

 or four times the height, nearly filled by the cardinal process and hinge- 

 plate of the brachial valve when the two valves are in articulation, the 

 apex connected with the foramen; lateral slopes convex except towards 

 the cardinal extremities where they sometimes become a little concave; 

 mesial sinus originating at or near the beak, shallow, rounded in the bot- 

 tom, rather narrow and more or less illdefined laterally, often curving 

 rather abruptly towards the opposite valve in adult specimens as it ap- 

 proaches the anterior margin. 



Brachial valve equally or a little more convex than the pedicle, the 

 greatest convexity posterior to the middle, the surface curving rather 

 abruptly to the cardinal margin and more gently to the antero-lateral 

 margins, sometimes a little compressed towards the cardinal extremities; 

 the beak rather strongly incurved beneath that of the opposite valve ; 

 mesial fold low, rounded and ill-defined laterally, sometimes becoming 

 obsolete towards the beak, often rather abruptly elevated in front in 

 adult individuals. 



Surface of both valves marked by strong, subparallel, concentric, lam- 

 elliform extensions 3 to 5 mm. apart, towards the front of the shell they 

 are often much more closely crowded, and on the body of the shell weaker 

 lamelke are often intercalated between the stronger ones. In all cases 

 where the surface is well preserved, subparallel lines of greater or less 

 strength are disposed irregularly between the stonger lamellae. 



Remarks. -This species occurs not infrequently in such condition of 

 preservation that the surface characters are more or less obscure, but in 

 all cases, even in internal casts, the concentric lamellae are indicated by 

 the presence of ridges. In internal casts which are frequently found 

 in the cherts of the Osage formations, the subovate muscular scar of the 

 pedicle valve is more or less strongly defined, dependent upon the age of 

 the individual, the length of the muscular scar is about equal to the dis- 



