CHONETES OP 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 striae 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 form and con- 

 vexity of C. scitula in its characteristic phases. 



Geological formation and localities. In the Marcellus shale near Darien in Erie 

 county, 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. 



Clionetes coronata. 



PLATE XXL 



Strophomena carinata (Scrive coronata) : Conead, Jour. Acad. Na. Sci. Phila., Vol. viii, p. 257, 1842. 

 Not Strophomena carinata, Conead, Annual Report on the Palaeontology of New- York, p. 64. 1839. 

 Strophomena tyrtalU : Conbad, Jour. Acad. Nat. Sci. Philadelphia, pa. 254, pi. 14, f. 1. 1842. 

 Compare Chonetet littoni, C. maclurea, C. tuomeyi and C. martini, Noewood and Pkatten on the Geniu 



Chorete.s, pp. 25, 28 and 29, pi. 2, f. 4, 8, 9, and 10. 1855. Journal Academy of Nat. Sciences, 



Philadelphia, Vol. iii. 1864-5. 



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 of the valve. 



