PTEROPODA. 213 



across the otherwise smooth intermediate space. Usually the crests of 

 the ridges appear to be smooth and flat, and the pustules or tubercles are 

 seen only under favorable conditions. This ornamentation of the surfaces 

 is not visible to the naked eye. 



The surface characters, as given above, are from portions of the very thin 

 shell preserved in many specimens, and from distinct impressions of the 

 exterior in the softer shales from which the shell has been dissolved; aho 

 from the moulds of the interior, preserving in some degree the external 

 ornamentation. 



The specimens are all imperfect ; the largest fragment (making allowance 

 for the absent portion) indicates an original length of about 100 millimetres, 

 while the ordinary specimens are scarcely more than sixty-five or seventy 

 millimetres long; the width, measuring the flattened faces, may have been 

 about twenty to twenty-five millimetres. 



This species differs from all the others in our rocks in the interlocking of 

 the stria) along the median line of each face. The striae are proportionally 

 more distant than in C. undulata and C. crebristriata, and are very similar in 

 this respect to C. Cayuga, in which they are stronger and much more coarsely 

 tuberculate, while the intermediate spaces are more strongly striate. In well 

 preserved specimens, the transverse striae are sharply elevated and distinctly 

 pustulose, and where partially exfoliated the crest is sometimes punctate from 

 the breaking of the pustules. In its flattened condition, in the shales, the 

 general appearance of this fossil is similar to C. congregata, in which the striae 

 are more closely arranged, and the tubercles upon their summits much stronger. 

 The pyramid is not so robust as in C. undulata, and has more nearly the degree 

 of attenuation of C. crebristriata. Compared with C. Newberryi, in which the 

 striae are interrupted on the face of the pyramid, it is distinguished by its finer 

 and more closely arranged striae and broader form. When occurring in the 

 softer shales this species is flattened, the surface ornamentation often obscure, 

 and the details difficult of determination. From the coarser shales, we have 

 a few fragments in which the original form is pretty well preserved, and the 

 surface-markings more distinct. 



