FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 161 



istome sinuous, and on the posterior side spreading partially over the preceding 

 volution. 



Surface marked by fine, closely arranged concentric striae, which are undu- 

 lated towards the margin of the aperture, and sometimes over the greater part 

 of the surface, the irregularity having commenced during the earlier stage of 

 growth. In well preserved specimens there are revolving striae or fascicles, 

 rising in little bands of obsolescent striae, giving a waved aspect to the sur- 

 face. 



This species resembles somewhat P. ventricosum, of the lower Helderberg 

 group, but the first volution is more slender, the spire less closely enrolled, 

 while the pouch-like expansion and the revolving bands are distinctive fea- 

 tures. In size it differs greatly in different specimens, as may be noticed in the 

 different figures. 



Formation and Locality Occurs in the Corniferous limestone of the Devonian formation in 

 Jefferson county, Jy,, and in Clark county, Ind. 



Platyceras conicum. HALL. 



Plate XXV., figures 2 and 11. 



Platyceras conicum, Hall. Descript. of New Foss., p. 3 1861. 

 Platyceras conicum, Hall. 15th Keg. Report, p. 31 1862. 

 Platyceraa conicum, Hall. Illust. Devon. Fossils 1876. 

 Platyceras conicum, Hall. Pal. N. Y., Vol. 5, pt. 2, p. 31885. 



Shell above medium size, erect conical, the minute apex closely incurved 

 but mostly missing, at least in the specimens found in our rocks. Body- 

 chamber entirely straight, with broad, undefined, longitudinal ridges and 

 depressions, which are faint or obsolete at or near the apex, but become more 

 distinct approaching the aperture. The height of the shell is usually greater 

 than the width of the aperture, which is somewhat elliptical in shape ; its 

 length exceeding its width. 



The surface is covered by concentric undulating striae, which become sub- 

 lamellose towards the aperture, and are sometimes closely crowded and 

 wrinkled with numerous knots or nodes. Peristome deeply sinuous. The 

 length of the shell varies in different specimens, and can not be exactly as- 

 certained on account of the missing apex; but it is certain that the shell 

 illustrated in figure 11, plate 25, if complete, would certainly measure not less 

 than two inches and a quarter. 



This species resembles P. pyramidatum of the Lower Helderberg group, but 

 is less elongate ; the peristome more sinuous, and the longitudinal ridges and 

 depressions are more distinct; besides, the crowded, wrinkled and nodose striae 

 of our shell are missing in the Silurian species. 



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

 Falls of the Ohio, on the Kentucky side of the river, and in different other places in Jefferson county, Ky., 

 and Clark county, Ind. It is not very rare, but fair specimens are not abundant. 

 OEOL. SUR. 21 



