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CHONETES OP THE HAMILTON GROUP. 131 



Aeea of the ventral valve narrow and distinctly wider in the middle, 

 the exterior margin gently curving : foramen partially closed by a 

 convex deltidium, and the aperture filled by the cardinal process of 

 the opposite valve. Dorsal area linear, half as wide as that of the 

 opposite valve. 

 SuKFACE marked by fine subequal striae, which are often alternated in 

 size towards the margin. Of these, from 15 to 20 may be counted near 

 the beak, while from bifurcation and intercalation there are from 50 

 to 60 on the margin, becoming obsolete towards the cardinal extre- 

 mities. The striae are sometimes sharp and angular, and sometimes 

 rounded : fine concentric striae are visible on well-preserved specimens. 

 The cardinal margin of the ventral valve is furnished with two or 

 three spines on each side towards the extremities, and as many more 

 towards the beak, so that the bases of five can be readily counted, and 

 sometimes six ; while there is a minute point on each side of the apex, 

 which may be another spine ; and this, if preserved, would give twelve 

 or fourteen spines on the cardinal line. 



The interior of the dorsal valve is strongly pustulose, with a somewhat 

 broad depression along the centre, in the middle of which there is a 

 slender mesial ridge. The muscular impressions are not strongly defined, 

 but the limits of the vascular impressions are clearly seen. The cardinal 

 process is short, and the inner margin of the hinge-line is thickened and 

 rounded. The ventral valve shows a slender median ridge and two strong 

 dental lamallae. The muscular impressions are not strongly defined in 

 specimens seen. 



This species is closely related on the one side to C. lineata and C. yandellana, 

 and on the other to C. lepida. With the latter it is frequently associated ; differing 

 from it, however, in its large size and the uniformly convex surface of the ventral 

 valve, while that one has a depression along the middle. The striae are more pro- 

 minent than in C. lineata, and there is no flattening of the umbo and central por- 

 tions of the ventral valve. It is nevertheless desirable to make more extensive 

 comparisons of the interior structure than I have been able to do with my 

 materials. 



Geological formation and localities. In the Hamilton group, on the shores of 

 Cayuga and Seneca lakes ; at Ludlowville, on the shore of Canandaigua lake, at 

 Moscow, at Hamburgh on the shore of Lake Erie ; abundant near Oneonta, and 

 other- localities. 



