LOWER HELDERBERG ROCKS. 303 



subsequently obtained, show that the fossils having the character just 

 noticed possess a peculiar form of columella, which is more or less dis- 

 tinctly twisted or folded, or with a broad spiral groove within the outer 

 edge of the columella, and between that and a parallel ridge or callosity. 

 For these shells I propose the generic name Strophostylus. 



Genus Stropiiostilis ( n. g.). 



[ Gr. gTfSifu, verto ; tfruXo?, columella^] 



Generic chabactek. Shells subglobose or ovoid-globose. Spire small, with 

 a large ventricose body-whorl ; outer lip thin, not reflected (sometimes 

 slightly expanded ) ; columella twisted or spirally grooved within, not 

 reflected; umbilicus none : aperture somewhat round-ovate or trans- 

 versely broad oval. 



When the attention has been once called to these forms, there is usually little 

 difficulty in distinguishing them from all the other genera by external characters 

 alone. The columella is rarely seen, though I have been so fortunate as to discover 

 it in three species from the Oriskany sandstone; while it is partially exposed in two 

 other species, one of which is from the Oriskany sandstone, and the other from the 

 limestone of the Lower Helderberg group. 



( 



The figures 1 and 2 are illustrations of a well-marked species from the Oriskany sandstone. 



Fig. 3 is of another species from the same rock, showing the same general character of the 

 columella. 



