No. 109.] 119 



eating and increasing by interstitial addition, so that the margin 



of the shell presents from sixty to seventy (or perhaps a greater 



number in large specimens) : striss distinct, rounded, and the 



depressions between them sharply defined ; area narrow, linear. 



Interior striate papillose. 



The strise extend over the entire surface of 'the shell, and are 



as distinct on the latero-cardinal extremities as elsewhere upon 



the surface. 



This species is about the same size and general form as the Chonetes 

 pusilla ; but the striae are much finer, and, when no exfoliation has taken 

 place, are well defined, extending to the cardino-lateral margins, while in 

 that species a considerable space at the angles is plain. It is a smaller and 

 more rotund species than Chonetes scitula ; resembling C. glabra in form 

 and proportions, but dififering in the character and distribution of the striae. 



This shell has usually been referred to Chonetes nana ; but its surface 

 presents about double the number of striae which that species possesses 

 according to the original description of M. E. de Terneuil, thirty-five 

 to forty-five being the number of striae upon the margin. 



The Chonetes nana has been cited by M. de Verneuil as occurring 

 at the Falls of the Ohio, but I have not seen it among my collections from 

 that locality. 



Geological ■position and locality. In the upper beds of the limestone of 

 the age of the Upper Helderberg, at the Falls of the Ohio. 



Chonetes laticosta ( n. s.). 



Shell small, subhemispherical ; hinge-line a little shorter than the 

 greatest width of the shell : dorsal valve with a narrow linear 

 area and prominent dental process : ventral valve with a wider 

 area ; cardinal margin garnished by four or five tubular spines 

 on each side of the beak, which diverge very abruptly, making 

 a low angle with the hinge-line. Surface marked by about ten 

 or twelve strong rounded striae near the beak, which increase to 

 about twenty or twenty -four near the margin j bifurcations of 

 the stride strongly marked and divergent. Shell concentrically 

 marked by fine close strise and more distant somewhat squamose 

 lamellee. The surface of the shell, when partially exfoliated, 

 presents a peculiar striato-punctate appearance. 



