CABBONIFEROUS SPECIES. 97 



beak pointed, convex, nearly central, projecting slightly beyond the hinge- 

 margin, and somewhat incurved; ears nearly equal, compressed; the anterior 

 one, which is nearly rectangular at the extremity, generally separated from 

 the swell of the umbo by a broad, rather deeper concavity, which is continued 

 to the m^argin, to which it imparts a slight sinuosity under the ear ; posterior 

 ear a little larger than the other, but not extending back as far as the margin 

 of the valve below, in some examples apparently less than rectangular at the 

 extremity, and in others more, in consequence of the more or less sinuous 

 character of the margin. Surface ornamented by small, raised, simple, radi- 

 ating cbstse, or strise, about every fourth to every seventh one of which 

 (especially on the body-part) is a little larger and more prominent than the 

 others; whilethose on the ears, particularly on the anterior one, are all smaller, 

 though they are generally well defined on both ears ; a few very small, irregu- 

 lar, concentric marks of growth may also sometimes be seen crossing the 

 surface of the body-part of the valve. Right valve not certainly known. 



Height of one of the medium-sized specimens of a left valve, 0.44 inch; 

 breadth, 0.37 inch; convexity, 0.10 inch. 



I am not sure that I have seen any right valves of this species. There 

 are among the specimens some imperfect examples that would seem, from 

 the direction of the very slight obliquity, to be right valves. But owing to 

 the fact that they are generally quite as convex as others that are certainly 

 left valves, and have exactly the same surface-markings, while the ears, as 

 nearly as their form can be made out, would also indicate that they are 

 left valves somewhat distorted so as to change their slight obliquity, I 

 am led to regard them as such. The most characteristic features of the 

 species are its rather small size, nearly equal ears, and small radiating 

 costse, always increasing by intercalation, with every fourth, fifth, sixth, or 

 seventh one on the body of the valves (at least the left one) a little larger 

 and more prominent than the others. 



The reason why this and some other Carboniferous and possibly some 

 Jurassic forms are figured together on the lower half of plate 12 is else- 

 where explained. (See note on explanations of plate 12.) 



Locality and position. — Weber Canon, Wasatch Range; Upper Coal- 

 Measure limestone. 

 V P R 



