FOSSILS OF THE SILURIAN ANL> DEVONIAN ROCKS. 181 



Surface smooth ; figures 16 and 17 are of natural size, while figures 18 and 19 

 are enlarged to two diameters. Figures and description made from specimens 

 belonging to the collection of the late Dr. James Knapp. 



Formation and Locality. Occurs in the hydraulic limestone of the Devonian formation at the 

 Falls of the Ohio, in Kentucky and Indiana. 



Genus Euomphalus. 



Euomphalus, Sowerby. Minn. Conch., Vol. 1 1814. 

 Etymology: eu, wide; omphalos, umbilicus, 



The type of this genus is Euomphalus pentagonalis. Shell depressed or 

 discoidal ; whorls angular or coronated ; aperture polygonal ; umbilicus very 

 large ; operculum shelly, round, multispiral. The genus Euomphalus is nearly 

 related to Straparollus of Montfort, and to Phanerotinus of Sowerby. It is dif- 

 ficult to mark out any features of importance in which these three genera differ. 

 Prof. Hall uses the name Straparollus in a sub-generic sense for those shells 

 with close rounded volutions, where the spire rises moderately above the plane 

 of the outer volution, while those shells with disjointed volutions form the 

 genus Phanerotinus. 



Euomphalus decewi. BILLINGS. 



Plate XXI. figures 1 and 2. 



Euomphalus decewi, Billings. Can. Jour., p. 358 1861. 

 Euomphalus conradi, Hall. 14th Reg. Rep., p. 107 1861. 

 Euomphalus decewi, Meek. Geol. Rep. Ohio Pal., Vol. 1 1873. 

 Euomphdlus decewi, Hall. Illust. of Dev. Poss., pi. 15 1876. 

 Euomphalus decewi, Hall. Pal. N. Y., Vol. 5, pt. 2, p. 651879. 



Shell discoid, upper side moderately concave or sometimes nearly flat, the 

 lower side broadly and deeply concave. Periphery moderately convex or 

 nearly flat, and slightly oblique to the plane of the shell ; sometimes, in the 

 casts of young shells, gently rounded from the upper margin to the edge 

 of the umbilical depression. Volutions three or four (there are rarely more 

 than two or three preserved in the casts), inner ones rounded, gradually be- 

 coming depressed on the upper and lower sides. The periphery, at first 

 rounded and undefined, becomes more flattened and distinctly limited by a 

 defined angularity above and below, becoming more flattened towards the 

 aperture ; the upper side being gently depressed, while the lower side grad- 

 ually assumes a more abruptly concave aspect, forming a broad umbilicus. 

 Aperture unknown; section of the outer volution sub-quadrilateral or tri- 

 angular, with the inner angle truncated. 



