CHONBTES OF THE HAMILTON GROUP. 
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Surface 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 Chonet.es was referred by Messrs. Norwood and Pratten 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 strise 
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 XXI. 
Strophomena setigera : Hall, Geol. Report 4th District New York, p.180. 1843. 
Chonetes setigera Hall, 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 
[Palaeontology IY.] ' 17 
