CARBONIFEROUS PERIOD. 147 



than the other ear by the auricular furrow; obtuse at the extremity, inferior 

 border concave ; its surface marked by distinct radiating costse, which are 

 a little coarser than those upon the body of the valve, at the same distance 

 from the beak ; posterior ear clearly defined from the body of the valve by 

 a shallow auricular furrow, sharply angular at the outer extremity ; outer 

 margin concave ; surface marked by concentric lines, all radiate markings 

 being absent or obsolete. Surface of the body of the valve marked by 

 depressed flattened or very slightly convex, radiating costas, which very 

 gradually increase in size toward the fi-ee margins, and increase in number 

 by implantation at different distances from the beak, only about a dozen of 

 them reaching it. The implanted costal, beginning as mere strise between 

 the others, are of unequal size on all parts of the valve ; costse crossed by 

 numerous rather distinct concentric striae. 



Right valve flat or very slightly convex ; beak flattened and not dis- 

 tinct at the cardinal border ; costse similar in character to those of the 

 other valve, but they are not nearly so distinct ; outline corresponding with 

 that of the left valve, except that the anterior ear is narrower and defined 

 by a deeper and sharply angular sinus. 



Length from base to cardinal border, of an example somewhat above 

 average size, forty-two millimeters ; breadth, thirty-seven millimeters. 



This shell is one of the most common Conchifers found in the Carbon- 

 iferous rocks of Kentucky, Illinois, Missou-ri, Iowa, Nebraska, and Kan- 

 sas. It is most abundant in the Upper Coal-Measures ; but, according to 

 Mr. Meek, it is found in the Lower Coal-Measures of Illinois, and also in 

 the true Permian strata of Kansas. 



Position and locality. — Strata of the Carboniferous period ; Camp Apache, 



Arizona. 



Aviculopecten Coreyanus White. 



Plate XI, fig. 1 a and i. 



Aviculopecten Coreyanus White, 1874, Expl. & Surv. west 100th Merid., Prelim. Eep. 

 Invert. Foss., 21. 



Shell moderately large ; breadth not exceeding the height, and is usually 

 a little less ; margin of the basal half forming almost a true semicircle in 

 some examples, but is less regular in others; posterior margin continued 



