DIELASMA 269 



with the curvature more abrupt to the postero-lateral margins ; the mesial 

 portion of the valve either not differentiated from the general convexity, 

 or elevated in a slight, obscure, mesial fold near the front; beak sharply 

 pointed, incurved beneath the deltidial plates of the opposite valve. 

 Internally the socket plates and crural plates are distinct, the lines of at- 

 tachment of the crural plates to the inner surface of the valve diverge 

 from the beak and extend anteriorly about one-fourth the length of the 

 valve; the concave muscle bearing plate between the crural lamellae is 

 emarginate in front and extends anteriorly from the beak about two- 

 fifths the total length of the valve, it is marked by fine concentric 

 wrinkles parallel with the anterior margin of the plate and discontinuous 

 along a narrow median line. 



Surface of both valves nearly smooth, marked only by more or less 

 obscure concentric lines of growth. Shell structure finely punctate. 



Remarks. This species has not been commonly recognized Siince its 

 original definition by Swallow as Terebratula arcuata, and it seems to be 

 a rather rare species. In its arcuate form the shell resembles the Pennsyl- 

 vanian species D. bovid\ens, and it has sometimes been so identified, but it is 

 somewhat more elongate than that species, less strongly arcuate, and the 

 median sinus of the pedicle valve is much narrower. 



Horizon. Chester Group, Okaw formation. 



DIELASMA ARKANSANUM n. sp. 

 Plate XXXI, Figs. 35-44 



Description. Shell about medium size, subovate in outline, longer than 

 wide, the greatest width a little in front of the middle. The dimensions 

 of a nearly complete specimen are: length of pedicle valve 30.5 mm., 

 length of brachial valve 28.2 mm., greatest width 21 mm., thickness 

 14 mm. 



Pedicle valve rather strongly convex, the greatest convexity posterior 

 to the middle, the surface arched longitudinally from beak to front with 

 the curvature becoming progressively more convex posteriorly, from the 

 umbonal region the surface is moderately convex to near the postero- 

 lateral margins, where it is more abruptly curved and inflected towards 

 the cardinal extremities, towards the antero-lateral and anterior margins 

 the convexity is more gentle ; mesial sinus narrow and very shallow, 

 ill-defined laterally, originating at or near the mid-length of the valve, 

 frequently nearly or quite obsolete ; beak strongly incurved, pierced by a 

 large, oblique, subovate foramen which encroaches wholly upon the 

 umbonal portion of the valve, the deltidial plates hidden by the in- 

 curvature of the beak. Internally the dental plates are well developed 

 and of moderate length. 



