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 striae, while the interspaces appear as 
flattened, narrow, elevated bands, crossing the face of the fossil. 
This species is distinguished from C. continens 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 XXXIY A, is distinguished from 
the ordinary specimens of C. undulata by its more slender form, and more 
closely arranged 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 
