OAEBONIFEROUS PERIOD. 123 



the Missouri River in Missouri, Iowa, and Nebraska, and is represented 

 lower in the Carboniferous series in the two first-named States and in 

 IJhnois by shells of somewhat smaller size, but otherwise apparently identi- 

 cal in species. It is probable also that this last-named variety is identical 

 with C. SmitJiii Norwood and Pratten, and if so it becomes another synonym 

 of C. granulifera. 



Position and locality. — Strata of the Carboniferous period (Upper 

 Aubrey limestone) ; Kanab Canon, Arizona. 



Chonetes mesoloba Norwood and Pratten. 



Plate IX, fig. 7 a. 



Chonetes mesoloba Norwood and Pratten ? 1854, Jour. Acad. Nat. Sci. Phila., n. s., iii, 27. 



Shell rather small, transverse, somewhat distinctly quadrilateral, seldom 

 wider at the hinge than it is in front of it. Ventral valve having a greater 

 or less general convexity ; mesial sinus comparatively large, and bearing 

 along its middle a small mesial fold or lobe ; between the sinus and the 

 postero-lateral portions of the valve there is at each side a comparatively 

 large, more or less prominent, rounded fold, extending from near the beak 

 to the antero-lateral margin. Dorsal valve concave, having at the front two 

 folds, with a mesial depression between them, the depression corresponding 

 with the mesial lobe within the sinus of the venti'al valve. Surface marked 

 by fine radiating strise. 



Length of the specimen figured, six millimeters ; breadth, nine milli- 

 meters ; but examples are often found that are one-third larger than this. 



The constant presence of a lobe or fold in the mesial sinus of the ven- 

 tral valve of this shell clearly distinguishes it from all other known species 

 of the genus. Its range is through the whole series of strata of the Carbon- 

 iferous period in Illinois, Missouri, and Iowa. In the latter State, it lias 

 been found almost wholly confined to the Middle Coal-Measures. 



Position and locality. — The collections contain only a single specimen 

 of this interesting species, which was obtained from strata of the Carbon- 

 iferous period, at the confluence of White Mountain and Black Eivers, 

 Arizona. 



