106 [Assembly 



aperture transversely^ elliptical. Surface marked by strong 

 revolving stri^ and finer transverse striae, which are strongly 

 bent backward on the carina of the outer volution. 



This species is larger and more rotund than P. sulcomarginata of the 

 Hamilton group. The specimens examined are all imperfect, and the 

 shell crystalline, so that the surface markings are indistinctly seen. 



Geological formation and locality. In limestone of the Upper Helder- 

 berg group : Western New- York. 



PLATYOSTOMA TURBINATA ( n. s.). 



Shell depressed subturbinate, sometimes approaching to globose. 

 Spire depressed, little elevated above the outer volution, and 

 sometimes nearly on a plane with it. Volutions three or four, 

 very rapidly expanding ; the last one extremely ventricose, and 

 the lower part projected downwards in the direction of the 

 columella, which is unusually extended : aperture ovate, 

 broader above. Surface marked by five somewhat unequal 

 striae, parallel to the aperture, and crossed by finer and less 

 conspicuous revolving striae. 



This species, which seems clearly referable to the Genus Platyostoma, 

 is .remarkable for the attenuation of the last volution and the contrac- 

 tion of the aperture below. 



Geological formation and locality. In the Upper Helderberg limestone : 

 Helderberg mountains, and places in Western New- York. 

 / 



PLATYOSTOMA LICHAS ( n. s.). 



Shell obliquely subglobose. Spire moderately elevated : volu- 

 tions about four, rounded, the earlier ones very gradually ex- 

 panding, and the last one becoming extremely ventricose ; 

 aperture nearly circular. Surface marked by fine striae of 

 growth. 



This species resembles P. ventricosa of the Oriskany sandstone; but 

 the spire is more elevated, and the specimens examined do not attain a 



