PTEROPODA. 209 



truncated by a convex septum in the best preserved specimens. Shell 

 extremely thin ; in most of the specimens entirely dissolved. 

 Surface, as determined from the best preserved specimens, and from external 

 moulds, marked by fine transverse striae, which, upon the sides, are gently 

 curving towards the aperture, and slightly recurved in crossing the median 

 groove ; the striae are interrupted by minute pustulose elevations, which 

 give the surface (as seen under a strong lens, in its usual condition of pre- 

 servation) a minutely crenulate or pustulose aspect. These elevations do 

 not extend to the narrow interstriate spaces, which are apparently quite 

 Smooth, and about twice as wide as the elevated striae, but vary with the 

 growth and age of the shell. 



In the harder and more arenaceous shales the fossil is often preserved in 

 its natural proportions; but it is more frequently compressed, and, where 

 occurring in the softer shales, is always flattened. The greater proportion of 

 the specimens have the shell entirely dissolved (in none is there any portion 

 fully preserved); and it is often difficult or impossible to discover the crenulate 

 markings on the surface of the cast. Impressions of the exterior often show 

 the punctate impressions of the slender stria?, while the interspaces appear as 

 flattened, narrow, elevated bands, crossing the face of the fossil. 



This species is distinguished from C. conlinens and C. congregata by the 

 usually closer arrangement of the transverse ridges, and by the absence of striae 

 crossing the interspaces. The larger examples attain a length of about six 

 inches, and the one figured on plate 33 has a length of 140 millimetres, with 

 a width of about 40 millimetres at the base of the widest face exposed. 



The specimen illustrated in figure 1, plate XXXIV A, is distinguished from 

 the ordinary specimens of C. undulata by its more slender form, and more 

 closely ai Tanged transverse striae, which are nodulose on their crests, with the 

 interstriate spaces smooth, as in typical individuals of the species. 



Formation and localities. In the coarser shales of the Hamilton group of New 

 York, at Cazenovia, Hamilton, Schoharie ; at Plainfield in Otsego county, and 

 at Pratt's Falls in Onondaga county. 

 27 



