268 MISSISSIPPIAN BRACHIOPODA 



the beak, sometimes nearly straight towards the anterior margin, trans- 

 versely it is strongly convex with the curvature more abrupt towards the 

 postero-lateral margins; mesial portion of the valve not differentiated 

 from the general curvature of the surface, or elevated in a slight mesial 

 fold near the anterior margin ; beak sharply pointed, incurved beneath the 

 pseudodeltidium of the opposite valve. Internally the crural plates are 

 distinct from the socket plates and diverge anteriorly from the beak, 

 being attached to the inner surface of the valve for nearly one-fourth 

 the length of the valve; the concave muscle bearing plate joins the inner 

 surface of the valve along its median line and extends anteriorly about 

 two-fifths the length of the valve. 



Surface of both valves nearly smooth, marked only by rather incon- 

 spicuous lines of growth. The shell structure finely punctate. 



Remarks. This species differs from D. sinuata in its smaller size, less 

 strongly convex valves, especially the less longitudinal convexity of the 

 brachial valve, and in its more slender form. 



Horizon. Chester group, Paint Creek formation. 



DlELASMA SHUMARDANUM (Miller) 



Plate XXXI, Figs. 25-27 



1863. Terebratula arcuata Swallow, Trans. St. Louis Acad. Sci., vol. 2, 



p. 83. (Not T. arcuata Roemer, 1840.) 

 1883. Terebratula shumardana Miller, Am. Pal. Foss., 2nd ed., p. 299. 



Description. Shell of medium size or a little larger, elongate subovate 

 in outline, with the greatest width well towards the anterior, the anterior 

 margin rounded, the postero-lateral margins gently convex. The dimen- 

 sions of a nearly perfect specimen are : length of pedicle valve 29 mm., 

 length of brachial valve 26.5 mm., greatest width 19.6 mm., thickness 

 14.5 mm. 



Pedicle valve strongly arcuate from beak to front, the curvature in- 

 creasingly convex towards the beak, the greatest convexity posterior to 

 the middle, the surface broadly flattened, gently convex transversely in 

 the posterior portion of the valve to near the postero-lateral margins 

 where it is rather abruptly curved towards the opposite valve and in- 

 flected towards the cardinal extremities, anteriorly the surface is gently 

 convex to the lateral margins ; mesial sinus narrow and shallow, originat- 

 ing at or near the beak, not sharply defined along its lateral margins; 

 the beak truncated, incurved, the foramen large, subovate, encroaching 

 wholly upon the umbonal portion of the valve, the deltidial plates hidden 

 by the incurvature of the beak. Internally the dental lamellae are well 

 developed and of moderate length. 



Brachial valve gently convex from beak to front along the median 

 line with the highest point near the middle, strongly convex transversely 



