CHONETES OF THE HAMILTON GROUP. 1<M 



StTBFACE marked by somewhat regular even rounded striae, which, are 

 little elevated, and become obsolete on the cardinal margins : from 

 twenty to thirty striae may be counted on the margin of the shell, and 

 in most of the individuals they are apparently simple throughout the 

 greater part of their extent. In some specimens the striae are conspicu- 

 ously bifurcated, and also increased by interstitial additions. In well- 

 preserved surfaces, fine crowded concentric striae cross the radiating 

 striae. The cardinal margin exhibits bases of two or three and rarely 

 four spines on each side, their extent or direction unknown. 

 Interior of the valves unknown. 

 This small Chonetes was referred by Messrs. Norwood and Peatten to C. 



armata, but it does not correspond with the figures and descriptions of De 



KoNINCK. 



In general aspect of form, and convexity of the ventral valve, this species 

 resembles C. deflecta; but all the specimens I possess are smaller, and the strias 

 more depressed. I am disposed to believe, however, that a larger collection of 

 specimens from western localities may prove that this form is but another phase 

 of the same species, induced by geographical and physical conditions, and 

 that in reality the eastern and western forms are but varieties of one and the 

 same type. 



Geological formation and locality. In limestone of the age of the Hamilton 

 group, at the Bake-oven, Illinois. 



Chonetes setigera. 



PLATE XXL 



Slrophomtna ttiigera : Hau, Geol. Keport 4th District New York, p. 180. 1843. 

 ChoneUt aetigera .- Hau, Tenth Report on the State Cabinet, p. 150. 



Shell semielliptical ; the cardinal line equalling the greatest width of 

 the shell, and rarely extending beyond. 



Ventral valve moderately convex, rarely a little gibbous in the middle ; 

 umbo scarcely rising above the hinge-line ; the greatest elevation is 

 above the middle of the shell, and gradually sloping to the front and 

 baso-lateral margins ; while it is a little more abruptly depressed in 

 the upper lateral portions, and flattened on the cardinal angles. The 

 cardinal margin is furnished with three slender tubular spines on each 



[PAL.BONT0L00Y IV.] 17 



