﻿The 
  Fauna 
  of 
  the 
  Niagara 
  Group. 
  181 
  

  

  ANNELIDA. 
  

  

  SPIRORBIS 
  Lamarck. 
  

  

  Spikorbis 
  INORNATUS. 
  

  

  Plate 
  31, 
  Figs. 
  14, 
  15. 
  

  

  Spirorbis 
  inornatus 
  Hall. 
  Trans. 
  Alb. 
  lust., 
  vol. 
  iv, 
  p. 
  224. 
  Abstract, 
  p. 
  30; 
  

   May, 
  1863; 
  including; 
  Spirorbis 
  ? 
  flexuosus 
  Hall. 
  Trans. 
  Alb. 
  Inst., 
  vol. 
  iv, 
  

   p. 
  224. 
  Abstract, 
  p. 
  30; 
  May, 
  1863. 
  

  

  Convolute, 
  discoid, 
  adhering, 
  deeply 
  depressed 
  or 
  umbili- 
  

   cate, 
  consisting 
  of 
  about 
  two 
  volutions, 
  the 
  outer 
  one 
  robust, 
  

   transverse 
  diameter 
  the 
  greater, 
  margin 
  subangular. 
  

  

  Surface 
  smooth 
  or 
  with 
  lines 
  of 
  growth 
  which 
  on 
  the 
  

   exposed 
  edge 
  of 
  the 
  volution 
  sometimes 
  become 
  slight 
  ridges. 
  

  

  The 
  specimens 
  of 
  this 
  species 
  are 
  comparatively 
  numerous, 
  

   but 
  the 
  greater 
  part 
  are 
  without 
  distinctive 
  surface-markings, 
  

   or 
  incomplete 
  in 
  their 
  volutions. 
  

  

  In 
  a 
  single 
  specimen 
  before 
  me, 
  the 
  tube, 
  after 
  making 
  about 
  

   one 
  volution 
  and 
  a 
  half 
  in 
  contact, 
  becomes 
  free, 
  assuming 
  a 
  

   sinuate 
  or 
  spiral 
  direction, 
  extending 
  about 
  two 
  mm. 
  from 
  

   the 
  point 
  of 
  divergence, 
  and 
  somewhat 
  rapidly 
  enlarging 
  to 
  

   the 
  outer 
  extremity, 
  which 
  is 
  imperfect; 
  in 
  other 
  respects 
  

   this 
  form 
  has 
  the 
  character 
  of 
  Spirorbis 
  inornatus. 
  A 
  simi- 
  

   lar 
  feature 
  has 
  already 
  been 
  illustrated 
  in 
  Spirorbis 
  laxus 
  of 
  

   the 
  Lower 
  Helderberg 
  group 
  (Pal. 
  N. 
  Y., 
  vol. 
  iii, 
  page 
  349, 
  

   plate 
  54, 
  fig. 
  18). 
  

  

  The 
  original 
  specimens 
  described 
  as 
  Spirorbis 
  f 
  flexuosus 
  

   are 
  not 
  now 
  accessible, 
  but 
  it 
  is 
  presumed 
  that 
  they 
  were 
  simi- 
  

   lar 
  to 
  the 
  one 
  referred 
  to, 
  with 
  the 
  outer 
  volution 
  becoming 
  

   free 
  and 
  continuing 
  in 
  a 
  sinuate 
  or 
  direct 
  line, 
  and 
  are 
  prob- 
  

   ably 
  not 
  distinct 
  from 
  S. 
  inornatus. 
  

  

  