78 THE DEVONIAN OF MISSOURI. 



1880. Strophodonta demissa White, Second Ann. Rep. Indiana Bureau of Statistics 

 and Geol., p. 500, pi. 4, figs. 6, 7. 



1881. Strophodonta demissa White, Tenth Rep. Indiana State Geol., p. 132, pi. 4, 

 figs. 6, 7. 



1882. Strophodonta demissa Whitfield, Geol. Wisconsin, IV, p. 327, pi. 25, fig. 18. 

 1889. Strophodonta demissa Nettleroth, Kentucky Fossil Shells, Mem. Kentucky 



Geol. Survey, p. 143, pi. 18, figs. 10-16; pi. 33, fig. 22. 

 1892. Stropheodonta demissa Hall and Clarke, Pal. N. Y., VIII, pt. 1, pi. 14, figs. 7-12. 

 1895. Strophodonta demissa Keyes, Geol. Survey Missouri, V, p. 70, pi. 39, fig. 7. 

 1897. Stropheodonta demissa Schuchert, Bull. U. S. Geol. Surv., 87, p. 421. 

 1907. Stropheodonta demissa Grabau and Shinier, North American Index Fossils, I, 



p. 217, fig. 263. 

 1913. Stropheodonta demissa Prosser and Kindle, Maryland Geol. Survey, Middle 

 and Upper Devonian, pp. 136-137, pi. 10, fig. 1; Clarke and Swartz, p. 551, pi. 

 47, fig. 14. 



Hall's description — Somewhat modified in accordance with the specimens from the 

 Missouri Devonian- — Shell commonly semielliptical, usually wider than long, length 

 and breadth often nearly equal, hingeline usually greater than the width of the shell in 

 front, extremities often auriculate; in some specimens the sides are nearly straight, and 

 parallel for more than half the length of the shell. 



Pedicle valve nearly flat to regularly convex, greatest elevation nearly central; 

 umbo small, with the apex slightly incurved and extending beyond the area; surface 

 a little concave towards the cardinal angles, which are slightly deflected. Dorsal valve 

 moderately concave, closely following the convexity of the opposite valve; in some 

 specimens an ill-defined median depression extends from beneath the apex to the front 

 of the shell. 



Area of pedicle valve variable, usually of moderate width, varying from 1 mm. 

 to 2 mm. wide in the centre in average specimens, and narrowing toward the extremities. 

 Inner margin crenulated for nearly its entire length. There is no open delthyrium but 

 a triangular space below the beak of many specimens marks its location. Dorsal area 

 narrow and usually linear, sometimes wider and sometimes narrower in the middle, and 

 the margin for a short space free from crenulations. The planes of the two areas are 

 inclined so as sometimes to give less than a right angle between them, but generally a 

 a greater angle, and along the middle the two are often nearly in the same 

 plaDe. 



Surface marked by numerous crowded striae, about nine or ten of which are much 

 stronger and more elevated on the umbo of the pedicle valve, with finer ones coming 

 in between and on either side; striae frequently increasing by intercalation and bifurca- 

 tion until they become very numerous. 



The interior of the ventral valve, and casts of the same, show a large flabelliform 

 divaricator muscular impression, which is somewhat widely separated in front, and each 

 division distinctly lobed. The occlusor muscular impressions occupy a semielliptical 

 space on each side of the narrow central depression, the marking on either side being 

 double in well-preserved specimens. The upper extremities of this impression are 

 close under the arch of the umbo, and separated by a smooth space from the divaricator 

 impressions. 



Beyond the muscular impressions, the interior surface is minutely pustulose, the 

 points being more prominent just without their limits; beyond which, the course of the 

 vascular impressions can be distinctly traced. 



In the brachial valve, the anterior and posterior occlusor muscular impressions are 

 very conspicuous and deeply marked, and often limited by an elevated ridge, a narrow 

 longitudinal ridge dividing the two parts. On each side, and below the muscular im- 

 pressions, the surface is minutely pustulose, the vascular impressions becoming distinct 

 towards the margin. The cardinal process is divided from the base, the divisions strongly 

 diverging. 



