No. 186.J 21 



STROPHOSTYLUS. 



There are, besides the true Platyostom^, some other shells of not 

 very dissimilar character in the rocks of the Lower Helderberg and 

 Oriskany sandstone periods. Some of these, in the casts and in their 

 exterior conformation, are remarkable for their oblique form and the 

 wide-spreading of the last volution, which is also often exaggerated by 

 pressure in the same direction. 



These fossils have sometimes the globose form of Platyostoma ; 

 but in these instances they are usually more symmetrical, and may be 

 recognized by the smoothly rounded outline and the extreme posterior 

 extension of the peristome on the adjacent volution. The surface is 

 evenly striated by fine elevated threadlike striae parallel to the lines of 

 growth. 



Although difi'ering in external characters from any other gasteropods 

 of the group, I have not until recently had the means of determining 

 the generic relations of these forms. During the past year ( 1857 ) I 

 obtained from Cumberland (Maryland), some Gasteropoda of the Ori- 

 skany sandstone, among which was a single specimen of one of these 

 shells entire and without adhering stone. This specimen, and some others 

 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 have proposed the generic name Strophostylus.* 



Genus Strophostylus (Hall). 



[Gt. orpEfu. verto ; CTv?iog^ columella.'] 



Generic Character. 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 transversely broad oval. 



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 Ori- 



• Palaeontology of New-Tork, Vol. iii. p. 303. 



