No. 136.] 95 



depth of the cavity : depth of cavity, directly below the ante- 

 rior margin of the septum, about equal to half the width of 

 the septum ; septum sharply carinate on the lower side, with a 

 slight continuation of the same feature on the upper side, 

 which is concave ; posterior lip reflected, often somewhat 

 abruptly truncated in the middle behind, and extending over 

 the back of the volution. Dorsum angular, not distinctly cari- 

 nated behind, and gradually flattening towards the anterior 

 margin, which is deeply sinuate. Surface finely striate in the 

 direction of the lines of growth. 



This species differs from the preceding in having the first volution and 

 the first part of the second volution proportionally larger, the lip more 

 abruptly deflected; while the carination of the lower side of the sep- 

 tum marks the anterior edge, and dies out upon the upper surface. The 

 exterior strise are less lamellose, and the cavity of the outer volution 

 somewhat deeper. 



In some specimens of this species, the aperture has been from three- 

 fourths of an inch to an inch in diameter. 



Geological formation and locality. In strata of the age of the Hudson- 

 river group, near Nashville, Ten. From Prof. Safford. 



Related apparently to the forms just noticed, but still farther 

 removed from the true Bellerophon, are those shells which have 

 sometimes been referred to Clio or Cleodora. These forms, so 

 far as I know, are all from the Lower Silurian rocks, occurring 

 in the Trenton limestone and associated strata in New-York, 

 Canada and the Western States. These shells have usually a 

 somewhat triangular form, with a rounded sinuate base, or more 

 rarely a subcircular outline. From their external form and 

 appearance, as they usually adhere to the stone by the inner or 

 aperture side, one might fancy them to be expanded carinate 

 Bellerophontides with small apices, which had been vertically com- 

 pressed. There is usually a strong sharp carination on the back, 

 extending from the apex to the aperture. The apex is incurved 

 and pointed, but I have not been able to discover any volutions. 



The shell is composed of two plates ; the inner one a plane 

 concave expansion extending the entire width of the shell, and, 

 joining the lateral margins of the outer or carinate portion, 



