222 



MISSISSIPPIAN BRACHIOPODA 



SHUMARDELLA MISSOTJRIENSIS (Shumard) 

 Plate XXV, Figs 43-52 



1855. Ehynchonella Missouriensis Shumard, Geol. Rep. Mo., p. 204, pi. C, 



figs. 5a-c. 



1894. Rhynchonella missouriensis Keyes, Mo. Geol. Surv., vol. 5, p. 100. 

 1910. Shumardella missouriensis Weller, Bull, Geol. Soc. Am., vol. 21, 



p. 512, fig. 14. 



Description. Shell below medium size, triangularly subovate in outline, 

 usually wider than long, the greatest width anterior to the mid-length, 

 the postero-lateral margins nearly straight, meeting at the beak in an 

 angle of about 100 degrees, the antero-lateral margins rounded, the a7i- 

 terior margin nearly straight or slightly emarginate. The dimensions 

 of two nearly perfect specimens are : length 20 mm. and 18 mm., width 

 22.4 mm. and 17.9 mm., thickness 16.5 mm. and 15 mm., width of sinus in 

 front 16 mm. and 14.8 mm. 



Pedicle valve very shallow, the umbo narrowly convex, usually with a 

 small concave region on either side, beyond which a narrow margin, be- 

 coming abruptly wider towards the beak, is sharply inflected, towards the 

 antero-lateral margins the surface is nearly flat or slightly convex, and 

 from the beak to the front along the median line it is strongly but not 

 regularly arched; the mesial sinus originates posterior to the middle of 

 the valve, it is broad and of moderate depth with the surface rather ab- 







FIG. 18. A series of nine cross-sections of the rostral portion of the shell of 

 Shumardella missouriensis (X 2 1 /), showing the dental lamellae of the pedicle 

 valve, the hinge-plate, median septum, crural cavity, and crura of the 

 brachial valve. 



ruptly curved towards the opposite valve anteriorly and produced into a 

 broad, lingual extension at nearly a right angle to the plane of the valve, 

 it is coarsely serrate on the front margin ; beak small, pointed, moderately 

 incurved; the delthyrium nearly filled by the beak of the opposite valve, 



