CHONETES OF THE HAMILTON GROUP. 
133 
which is constricted along its upper and lateral margins, -and is marked 
by a longitudinal depression indicating the place of the median ridge 
and muscular impressions, while they are depressed towards the lateral 
and basal margins. 
The original specimens designated as C. lepida are very small shells of almost 
hemispheric form, and one of the characteristics is the mesial depression of the 
ventral valve. The strise are rather strong, angular, and, from dividing below, 
have the appearance of being fasciculate. More extensive collections have brought 
together a large number of individuals; and while the characteristic features are 
preserved in most of the specimens, there are others of the same size which seem 
like the young of C. scitula; but the well-marked specimens'of this species have 
a convexity which precludes them from acquiring by growth the foi’m and con¬ 
vexity of C. scitula in its characteristic phases. 
Geological formation and localities . In the Marcellus shale near Darien in Erie 
count}’', and in the Hamilton group at Ludlowville and Ogden’s ferry on the shore 
of Cayuga lake : it is often abundant at the outlet of Crooked lake, and at 
Hamburgh on the shore of Lake Erie, and occurs also in numerous other localities. 
Chonetes coronata. 
PLATE XXI. 
Strophomena carinata (Scrive coronata ) : Conrad, Jour. Acad. Na. Sci.Phila., Vol. viii, p. 257, 1842. 
Not Strophomena carinata , Conrad, Annual Report on the Palaeontology of New-York, p. 64. 1839. 
Strophomena syrtalis : Conrad, Jour. Acad. Nat. Sci. Philadelphia, pa. 254, pi. 14, f. 1. 1842. 
Compare Chonetes littoni, C. maclurea , C. tuomeyi and C. martini, Norwood and Pratten on the Genus 
Chonetes, pp. 25, 28 and 29, pi. 2, f. 4, 8, 9, and 10. 1855. Journal Academy of Nat. Sciences, 
Philadelphia, Vol. iii. 1854-5. 
Shell transverse, somewhat broadly elliptical, the hinge-line being 
sometimes shorter than the width of the shell and the cardinal angles 
rounded : in others it is often equal to the greatest width of the shell, 
and its form is semioval, with the lateral margins nearly rectangular 
to the hinge-line, the width being about once and a half as great as 
the length. The cardinal angles are sometimes produced in short acute 
auriculate extensions. 
Ventral valve varying from moderately convex in the younger shells, 
to very gibbous in the older ones; often a little flattened below the 
umbo, and this plane space gradually widening to the front. Sometimes 
there is a shallow undefined depression along the middle bf the valve. 
