ia4 PALAEONTOLOGY OF NEW-YORK. 



impressions are well defined, and between "them there is a strong mesial 

 ridge which is extended in a bidentate cardinal process. The lower half 

 of the surface is strongly papillose. 



In general form, this species differs little from C. lineata (compare figures 3 

 o-/and 4 a-g, Plate 20). It is less gibbous, and not flattened on the middle of 

 the ventral valve ; while the interior presents more strongly defined markings. 

 In a collection of more than twenty specimens entirely separated from the lime- 

 stone, which were sent me by the kindness of Dr. James Knapp, of Louisville, 

 Kentucky, the general form and moderate convexity are preserved, and it is 

 rarely that more than three spines are seen on one side of the centre ; but this 

 feature is subject to variation, and the spines are sometimes unequal on the two 

 sides of the same specimen. 



All the difierences indicated between this and the preceding species, I can 

 readily believe may be produced in the sam^ type by the diflferent conditions 

 of sea bottom, geographical distance, and other causes. 



Geological formation and localities. In the hydraulic beds of the Corniferous 

 limestone, at the Falls of the Ohio, Louisville ( Kentucky), and Jeffersonville 

 (Indiana). 



Chonetes mucronata. 



PLATES XX & XXL 



Strophomena mucronata : Hall, Geol. Report 4th District New- York, p. 180, f. 3. 1843. 

 Chonetei laticotta : Hall in Tenth Report on State Cabinet, p. 119. 1857. 



Shell small, semioval, moderately convex, nearly flat (often flattened in 

 the shale and gibbous in the limestone) : cardinal line equalling or a 

 little greater than the width of the shell below ; the extremities 

 sometimes salient. 



In the original specimens of this species from the Marcellus shale, the 

 ventral valve is slightly convex or nearly flat, one-fourth to one-third 

 wider "than long : the hinge-extremities are rarely a little produced, 

 but the spines lying in the direction of the hinge-line often gite it the 

 appearance of extreme extension. 



The dorsal valve is very moderately concave or nearly flat. 

 The surface is marked by twenty to twenty-four or twenty-six nearly 

 simple subangular striae, which are not so wide as the spaces between 

 them. Sometimes one, two or three of these striae are bifurcated towards 



