LOWER HELDERBERG ROCKS. 



299 



the name Plattostoma* : the latter, in some of its forms, approaches the 

 Pleurotomarise. 



The forms illustrated in this volume, from the Lower Helderberg group 

 and from the Oriskany sandstone, may afford the means towards a better 

 arrangement of these very difficult subjects which have been referred to 

 the genera cited. 



The arrangement of the subjects upon the plates illustrating the Gasteropoda is 

 not quite as coherent and systematic as I could have wished; but this has arisen 

 from causes beyond my control. Some of the plates were engraved in 1847, when 

 it was expected that the materials of the Clinton, Niagara and Onondaga-salt groups, 

 together with the Lower Helderberg group and Oriskany sandstone, would consti- 

 tute a single volume. It was at this time that Plates 56, 57, 60, 61, 62 & 64 were 

 engraved, and the drawings for several other plates completed for engraving. The 

 additional species subsequently obtained could not, therefore, be so well disposed as 

 if all the materials had been before me at one time. The same remarks are true of 

 some other portions of the volume. 



The genera of Gasteropoda recognized in the Lower Helderberg group are the 

 following, in the order in which they are described : 



HoLOPEA ( heretofore referred ' 



to Littoeina), 

 loxonema, 



mubghisonia. 



Plattostoma, 



Stbophostylis ( n. g.), 



Platycebas. 



Pleueotomabia, 



EtrOMPHALUS, 

 BUCANIA. 



These genera occur, for the most part, in the lower portions 

 of the group. 



These genera are characteristic of the middle and the upper 

 portions of the group, below the Upper Pentamerus 

 limestone. 



These genera characterize the upper portion of the group, 

 or the Upper Pentamerus limestone, and rarely occur 

 below that rock. 



' This genus is not synonymous with Flattstoma of Hobnes, 1855; nor with PLATraTOMA of Klein, 



1763. 



