FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 223 



1. In N. niotica the umbo is inflated, in .Nucula neda it is very ventricose. 



2. In N. niotica the depth is only one-half of the depth of neda, measured 

 in specimens of the same size, at the point of their greatest convexity. 



3. In !N. niotica both slopes of the beaks are concave, while those in N. neda 

 are almost straight. 



4. In N. niotica the basal margin is broadly curved, while that of N. neda is 

 straight or even inflected. 



Formation and Locality. Irrthe chert beds over the hydraulic limestone of the Devonian forma- 

 mation, in Jefferson county, Ky., and in Clark county, Ind. Pound in silicified, fairly preserved speci- 

 mens; not very rare. 



Yoldia? ValvulUB. HALL AND WHITFIBLD. 



.Plate IV., figures 4 and 5. 



Yoldia ? valvulus, H. and W. 24th Regent's Rep., p. 1901872. 

 Yoldia ? valvulus, H. and W. 27th Regent's Rep., pi. 111875. 



Shell elongate, narrow, sub-elliptical, more than twice as long as high, the 

 depth a little more than half the height ; anterior end nearly one-fourth wider 

 than the posterior. Beaks situated at three-fifths the length from the anterior 

 end ; an obsolete post-umbonal ridge extending from near the beak to the 

 post-basal margin ; posterior extremity not recurved. 



Surface marked by somewhat coarse, wavy, concentric lines to the post- 

 umbonal ridge, above which they are even and much finer. 



i 



Formation and Locality. Occurs above the hydraulic limestone in the cherty layers of the De- 

 vonian formation, in Jefferson county, Ky., and in Clark county, Ind., where it is found rather abundantly 

 in silicified specimens. It is represented by fair individuals in several collections in the Falls Cities. 



Genus Aviculopecten. McCoy. 



Etymology: resembling the shells of the genus Avicula. 

 Aviculopecten CraSSlcOStatllS. HALL AND WHITF^ELD. 







Aviciilopectfn crassicasta/Ms,H. and W. 24th and 27th Reg. Reps., p. 188, and pi. 111872 and 1875. 



Shell below medium size, its body broadly ovate or sub-elliptical. The left 

 valve is only known to me, and shown in the specimen before me ; it is de- 

 pressed convex ; body of the shell oblique ; hinge-line straight, equal to three- 

 fourths the length of the shell ; anterior wing very small, separated from the 

 body of the shell by an abrupt deep sinus; posterior wing narrow, obtusely 

 pointed, and extending nearly as far as the posterior extremity. 



Surface marked by strong, coarse, angular ribs, of which there are about 



