20 Bulletin 31 20 



nearly equal ; substance of the shell rather thick ; ribs about twentj-'One, 

 large, alternating with as many small ones, all imbricate ; beaks pointed. 

 Diam. . . Length 1.3, Breadth 1.3 of an inch 



Lea' s original description of lyelli. — vShell longitudinal, rounded be- 

 low, angular above, compressed ; ears very unequal, having a channel on 

 the larger one ; substance of the shell thin ; ribs about twenty-jwo, round- 

 ed, slightly imbricate ; beaks acutelj- angular. 



Diam. . . Length .8, Breadth .7 of an inch. 



There seeiiLS to be considerable uncertainty as to what, and 

 how many species of Peden really come from the Claiborne sands. 

 Dall says regarding f/^^/z^jr^// (Trans. Wag., Ill, p. 738): 



"This species is positively known to occur in the Jacksonian at Clai- 

 borne and elsewhere, but I have obtained no specimens from the vast 

 amount of marl belonging to the true Claiborne sands horizon which has 

 come under m}' notice. ' ' 



Cossmann finds this .species "excessivement rare" in his bar- 

 rel-full of .sand. 



Our various collection at the same horizon and place, when- 

 ever at all exten.sive, have never failed to produce several good 

 specimens of Peden of this and perhaps other nearly related 

 forms. 



The specimens of this .so-called species found in famous 

 "sands" are ver}- variable in outline, size and markings. What 

 one usually finds is just what Lea described : one or more large 

 left valves, quite highh' ornamented, along with smaller right 

 valve with simpler ribbing. The right valves seem more elong- 

 ate, just as Lea remarks under his /jW//. Along with these, one 

 generally .secures fragments, at least, of large right valves with 

 flat, smooth, broad ribs, regarded by some as having been worn. 

 (See pi. 13, fig. 8.) 



The right valve while j-oung possesses a very prominent an- 

 terior bys.sal ear with a deep notch, and a broad shallov.^ groove 

 extending to the beak, also about 5 imbricate rays ; posterior 

 ear much smaller, forming an angle of approximateh' 90 de- 

 grees with the hinge line and having about 5, often double, 

 rays ; general shell surface with ribs equalling interspaces in 

 width, about 18 well defined ribs on the face of the shell, with 

 half a dozen or .so fine radiating lines or subdivided riblets on 



