FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 167 



extending on the left side, and often reaching to the front of the shell, giving 

 a peculiar obliquity to the form. Aperture expanded, obliquely ovate ; peris- 

 tome entire, or with a slight sinuosity on the left posterior side, and sometimes 

 showing the effect of the undefined folds of the posterior margin usually con- 

 tiguous to the preceding volution. 



Surface marked by undulating concentric striae, and a few broad undulations 

 longitudinally. Aperture a little greater in the diameter from front to rear 

 than in the lateral one. Height reaching sometimes to an inch and a quarter. 



Formation and Locality. Occurs in the Coruiferous limestone of the Devonian formation in Jeffer- 

 son county, Ky., and in ClurU county, Ind. 



Platyceras symmetricum. HALL. 



Plate XXIII., figure 10. 



Platyceras symmetricum, Hall. Desc. of New Sp. of Foss., p. 61861. 

 Platyceras symmetricum, Hall. 15th Eeg. Kep., p. 34 1862. t 

 Platyceras symmetricum, Hall. Illust. of Dev. Fossils, pi. 3 1876. 

 Platyceras symmetricum, Hall. Pal. N. Y., Vol. 5, pt. 2, p. 91885. 



Shell elongate, sub-ovoid, arcuate, incurved nearly in the same plane ; apex 

 minute, making about one or one and a half volution before the body-whorl 

 becomes free and rapidly or somewhat abruptly expanded ; spreading about 

 equally on the two sides of the dorsum, which is more prominent and some- 

 times marked by a ridge. Aperture oblique, sub-quadrate or rhomboidal ; 

 margin of the peristome sinuate, and on the posterior side distant from the 

 spire. 



Surface marked by concentric undulating striae, and longitudinally by 

 obscure interrupted ridges, which, on some parts of the older shells, become 

 regular and uniform, with a narrow groove between. 



This species is well marked by the equilateral expansion on each side of the 

 dorsum, and by the volution of the apex being nearly in the same plane. The 

 posterior margin of the aperture is widely separated from the preceding volu- 

 tion. The longitudinal ridges are strongly marked, and of a different char- 

 acter from those of characteristic specimens of Plat, bucculentum ; it is not, 

 however, improbable that we may find intermediate forms uniting the two 

 species. It is also possible that a comparison of a larger number of specimens 

 may prove that the forms included under P. thetis, symmetricum and buccu- 

 lentum, are only varieties of one and the same species. 



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

 Falls of the Ohio, and in different other places of Jefferson county, Ky., and of Clark county, Ind., where 

 it is usually found already weathered put and entirely silicified. 



