38 PALAEONTOLOGY OF KENTUCKY. 



regard to size, it is impossible to give any dimensions, inasmuch as we find 

 the shells from the size of a small pea to that of an average hazelnut. 



Formation and Locality. This elegant shell is found abundantly and well preserved in the Niagara 

 rocks east of the city of Louisville, where it, however, occurs of very small size; the specimen represented 

 on plate XXXII. is about the largest individual ever found. At Waldron, Indiana, it has attained its 

 largest size. Average specimens from there measure about five-eighths of an inch in width, and some- 

 what more in length. 



Orthis flabellum. SOWERBY. 



Plate XXXIV., figure 30. 



Orthis flabellum, Hall. Kep. 4th Geol. Dist. 1843. 

 Orthis flabellum, Sowerby. Murch. Sil. Syst., p. 639 1839. 

 Orthis flabellum, Var ? Hall. Pal. N. Y., Vol. II., p. 2541852. 



Shell of medium size ; semi-oval or semi-elliptical ; hinge-line equal to 

 greatest width of shell ; cardinal angles rectangular or slightly acute ; lateral 

 margins sometimes slightly contracted below extremities ; balance of the 

 lateral margins and base form a regular curve ; shell plano-convex. Ventral 

 valve depressed convex, almost flat. The shell before me has, in its central 

 line, a gentle depression, beginning at about one- third of whole length of 

 shell from base, and extending to front, forming a kind of mesial sinus, but 

 I am not able to find out with certainty whether this sinus is really a natural 

 character of shell, or whether it is the result of distortion. I am inclined to 

 take it for a real sinus, inasmuch as balance of shell does not show any signs 

 of having been subjected to any violence. The dorsal valve is moderately 

 convex in its marginal portion, but almost flat in its central part or in the 

 nmbonal region. 



The cardinal extremities are slightly deflected, incurving the surface between 

 them and the umbo somewhat. The cardinal area of the ventral valve is of 

 moderate size, forming a low triangle ; it is divided by a triangular fissure, 

 partly closed ; the area of the dorsal valve is narrow, almost linear ; the dorsal 

 beak incurves into ventral foramen. 



The surface of the shell is ornamented by about twenty-four to thirty simple, 

 prominent, sub-angular radii, which increase in number by interpolation. 

 These radii are crossed by several marked, concentric lines of growth, which 

 divide the surface in several concentric zones, and give the shell, wherever they 

 become crowded, which is mostly the case at the margins, a rugose appear- 

 ance. 



Formation and Locality. The only specimen which I ever have seen, and which belongs to my 

 own collection, I found in the strippings of one of the quarries east of the city of Louisville; it was 

 surrounded by Niagara clay, and belongs undoubtedly to the Upper Silurian formation. 



