﻿The 
  Fauna 
  of 
  the 
  Niagara 
  Group. 
  175 
  

  

  GASTEROPODA. 
  

  

  PLATYOSTOMA 
  Conrad. 
  

   Platyostoma 
  Niagarense. 
  

  

  Plate 
  28, 
  Figs. 
  1-12 
  ; 
  Plate 
  29, 
  Figs. 
  1-15. 
  

   Platyostoma 
  Niagarensis 
  Hall. 
  Pal. 
  N. 
  Y., 
  vol. 
  ii, 
  p. 
  287, 
  pi. 
  60, 
  figs, 
  la-lv. 
  1852. 
  

  

  Shell 
  ovoid 
  or 
  subglobose, 
  volutions 
  three 
  to 
  four, 
  the 
  last 
  

   one 
  very 
  ventricose 
  , 
  spire 
  varying 
  from 
  the 
  plane 
  of 
  the 
  

   outer 
  volution 
  to 
  an 
  elevation 
  of 
  one-fifth 
  or 
  one- 
  fourth 
  the 
  

   height 
  of 
  the 
  shell 
  above. 
  

  

  Apex 
  minute, 
  somewhat 
  rapidly 
  expanding, 
  the 
  first 
  two 
  

   volutions 
  usually 
  symmetrical; 
  the 
  outer 
  volution 
  often 
  unsym- 
  

   metrical, 
  very 
  ventricose 
  and 
  regularly 
  rounded 
  upon 
  the 
  

   back, 
  but 
  not 
  unfrequently 
  extended 
  and 
  becoming 
  free 
  to- 
  

   ward 
  the 
  aperture, 
  and 
  marked 
  on 
  the 
  upper 
  or 
  lower 
  side, 
  or 
  

   upon 
  both, 
  by 
  a 
  groove, 
  along 
  which 
  the 
  striae 
  are 
  abruptly 
  

   bent, 
  indicating 
  a 
  sinus 
  in 
  the 
  peristome 
  during 
  some 
  period 
  

   of 
  its 
  growth 
  ; 
  peristome 
  entire 
  or 
  undulated, 
  sometimes 
  dis- 
  

   tinctly 
  notched 
  in 
  the 
  margin, 
  free 
  or 
  adhering 
  on 
  the 
  colum- 
  

   ellar 
  side 
  and 
  sometimes 
  expanded 
  and 
  presenting 
  a 
  thick- 
  

   ened 
  callosity 
  or 
  columellar 
  lip. 
  

  

  Surface 
  marked 
  by 
  fine 
  undulating 
  striae 
  of 
  growth, 
  which 
  

   sometimes 
  become 
  lamellose. 
  In 
  well-preserved 
  specimens, 
  

   finer 
  revolving 
  striae 
  cancellate 
  the 
  striae 
  of 
  growth, 
  and 
  some- 
  

   times 
  the 
  surface 
  is 
  marked 
  by 
  revolving 
  ridges. 
  

  

  The 
  principal 
  varieties 
  of 
  form 
  and 
  mode 
  of 
  growth 
  are 
  

   illustrated 
  on 
  plates 
  28 
  and 
  29. 
  The 
  large 
  collections 
  present 
  

   numerous 
  intermediate 
  forms 
  and 
  varieties 
  of 
  surface-mark- 
  

   ing. 
  

  

  Platyostoma 
  plebeium. 
  

  

  Plate 
  28, 
  Figs. 
  14, 
  15. 
  

  

  Platyostoma 
  pleMa 
  Hall. 
  Doo. 
  Edit. 
  28th 
  Rep. 
  St. 
  Mus. 
  Nat.;Hist. 
  Explanation 
  

   of 
  pi. 
  28, 
  figs. 
  14, 
  15. 
  1876. 
  

  

  Shell 
  dextral, 
  conical 
  ; 
  volutions 
  four, 
  very 
  convex, 
  gradu- 
  

  

  