270 MISSISSIPPIAN BRACHIOPODA 



Brachial valve less convex than the pedicle, its greatest convexity near 

 or posterior to the middle, the surface curving rather abruptly from the 

 median line to the postero-lateral margins and more gently to the antero- 

 lateral and anterior margins ; mesial portion of the valve not differentiated 

 from the general convexity of the valve; the beak acutely pointed and 

 incurved beneath the pseudodeltidium of the opposite valve. Internally 

 the socket plates and crural plates are distinct, the attachment of the 

 crural plates to the inner surface of the valve diverging from the apex 

 and extending anteriorly for about one-fourth the length of the valve; 

 the concave muscle bearing plate lying between the crural plates is 

 elongate, rounded in front, and extends anteriorly about two-fifths the 

 length of the valve; it is marked by strong concentric wrinkles parallel 

 with the anterior margin which are discontinuous along a narrow median 

 line. 



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

 generally obscure, concentric lines of growth. Shell structure finely and 

 closely punctate. 



Remarks. In general form this species resembles D. fvrmosa, but it is 

 proportionally a broader and thinner shell with the greatest width situated 

 more anteriorly, it also usually differs from that species in having a slight 

 median sinus in the pedicle valve which is absent in D. formosa. The form 

 of the shell perhaps more closely resembles D. ckouteauensis, but that species 

 lacks the median sinus of the pedicle valve and does not have its greatest 

 width anterior to the middle. 



Locality. Washington County, Arkansas; Webb City, Missouri (Carter- 

 ville formation?). 



DlELASMA SUBSPATULATUM n. Sp. 



Plate XXXIII, Figs. 6-11 



Description. Shell below medium size, elongate-subovate in outline, 

 the width much greater than the thickness, the greatest width towards 

 the front of the shell, the postero-lateral margins elongate and gently 

 convex. The dimensions of a nearly complete specimen are: length of 

 pedicle valve 19 mm., length of brachial valve 17.5 mm., greatest width 

 14.1 mm., thickness 8.2 mm. 



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

 terior to the middle, arched from beak to front along the median line 

 with the curvature a little more convex posteriorly, the surface gently 

 convex transversely, but as it approaches the postero-lateral margins it 

 is abruptly curved towards the opposite valve and becomes inflected 

 towards the cardinal extremities; the mesial sinus obsolete; the beak 

 incurved, perforated by a large foramen which encroaches upon the 

 umbonal portion of the valve. Internally the dental plates are well de- 

 veloped and are of moderate length. 



