LOWER HELDERBERG ROCKS. 
293 
the name Platyostoma* : 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 1 
to Littorina), i 
Loxonema, j- 
Murchisonia. J 
These genera occur, for the most part, in the lower portions 
of the group. 
Platyostoma, 
Strophostylis ( n. g.), 
Platyceras. 
] These genera are characteristic of the middle and the upper 
I portions of the group, below the Upper Pentamerus 
limestone. 
Pleurotomaria, 
Euomphaltts, 
Bucania. 
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 Platystoma of Hornes, 1855; nor with Platystoma of Klein, 
1753. 
