114 PALAEONTOLOGY OF KENTUCKY. 



the Coal Measures. The true character of the plications of these shells is not 

 shown exactly in the drawings. 



This shell, when well preserved, is one of the finest Spirifera of our palaeo- 

 zoic fauna ; it is easily distinguished from the others by its form, by its bifur- 

 cating plications, and by its sharp zigzag, concentric striae. 



Its size differs from two to three inches in width, and from one and three- 

 fourths to two inches in length, by a depth of one inch and one-eighth. 



Formation and Locality. Found in the Hamilton group and in the Corniferous limestone of Ken- 

 tucky, around Louisville, and near Lebanon, Kentucky, where W. T. Knott, Esq., found some very fine 

 specimens, which are figured on plate 11. This species is not very rare, though it is very difficult to pro- 

 cure good specimens. 



Spirifera duodenaria. HALL. 



Plate XII., figures 12, 13 and 16. - 



Delthyris duodenaria, Hall. Geol. Rep. 4th Dist. N. Y., p. 171, figure 5 1843. 

 Spirifer duodenaria, Hall. Catalogue in Rep. on State Cabinet. 

 Spirifera duodenaria, Hall. Pal. N. Y., Vol. IV., p. 189, pi. 27 and 28. 



Shell transverse, semi-circular ; hinge-line equal to greatest width of shell ; 

 cardinal extremities obtuse or acute, rarely acuminate ; valves sub-equally con- 

 vex ; area very narrow. . Surface plicated. 



Ventral valve moderately gibbous ; arcuate, compressed towards the cardinal 

 extremities. Mesial sinus of moderate width and depth ; rounded or slightly 

 flattened on the bottom; umbo prominent; beak small, neatly curved over a 

 wide triangular fissure, and reaching to within half a line of the umbo of the 

 opposite valve ; area concave, sub- linear, a little wider on each side near the 

 center. 



Dorsal valve regularly convex, a little gibbous in the middle and flattened 

 or sometimes slightly concave at the cardinal extremities. Mesial fold rather 

 narrow, rounded, prominent and strongly defined, sometimes a little flattened 

 on the middle. The surface is marked by six and rarely seven strongly 

 rounded ribs on each side of the mesial fold or sinus. The ribs gradually 

 decrease in size and prominence from the center ; and the outer ones are often 

 scarcely elevated in young or medium- sized individuals. The entire surface is 

 marked by lamellose, concentric striae, giving a papillose or sub-fimbriate 

 aspect at their junction ; it usually happens, however, that the surface is 

 smooth from partial exfoliation. (Hall.) 



This species has usually a width of one inch to one inch and a quarter, and a 

 length of one-half to three-fourths of an inch. Younger individuals, such as 

 are figured on plate 12, are smaller in size, and generally contain less plications. 



Formation and Locality. This species is found in the Corniferous limestone of Kentucky and 

 Indiana, around the Falls of the Ohio, but is very rare. During ten years of collecting I have found only 

 two or three specimens, and these not even in very good condition, 



