﻿The 
  Fauna 
  of 
  the 
  Niagaea 
  Group. 
  183 
  

  

  till 
  they 
  have 
  acquired 
  a 
  large 
  size 
  (depending 
  in 
  some 
  degree 
  

   upon 
  the 
  nature 
  and 
  extent 
  of 
  the 
  surface 
  to 
  which 
  they 
  are 
  

   attached), 
  as 
  shown 
  in 
  fig. 
  5 
  of 
  plate 
  31 
  ; 
  the 
  lower 
  extremity 
  

   has 
  here 
  become 
  thickened, 
  and 
  the 
  annulated 
  character 
  

   obsolete. 
  

  

  The 
  annulations, 
  so 
  well 
  defined 
  in 
  the 
  younger 
  stages 
  of 
  

   growth, 
  affect 
  the 
  interior 
  of 
  the 
  tube, 
  which 
  is 
  enlarged 
  at 
  

   these 
  points 
  ; 
  and 
  from 
  the 
  interannular 
  spaces, 
  there 
  are 
  thin 
  

   lamellae 
  projecting 
  into 
  the 
  interior, 
  and 
  for 
  some 
  distance 
  

   across 
  the 
  cavity 
  of 
  the 
  tube. 
  In 
  some 
  phases 
  of 
  wearing 
  or 
  

   decortication, 
  these 
  lamellae 
  give 
  a 
  partially 
  septate 
  character 
  

   to 
  the 
  fossil 
  ; 
  and 
  it 
  is 
  due 
  to 
  the 
  same 
  cause 
  that 
  we 
  have 
  the 
  

   annulated 
  casts 
  of 
  the 
  interiors, 
  where 
  the 
  sharp 
  constrictions 
  

   mark 
  the 
  place 
  of 
  these 
  projecting 
  lamellae. 
  * 
  The 
  same 
  fea- 
  

   ture 
  occurs 
  in 
  all 
  the 
  species, 
  and 
  where 
  the 
  small 
  parasitic 
  

   tubes 
  become 
  worn 
  down 
  they 
  present 
  a 
  septate 
  appearance. 
  

  

  In 
  the 
  young 
  state, 
  the 
  annulations 
  and 
  the 
  interior 
  pro- 
  

   jecting 
  lamellae 
  are 
  alternate 
  and 
  corresponding 
  in 
  number 
  ; 
  

   but 
  as 
  the 
  exterior 
  becomes 
  thickened 
  and 
  its 
  growth 
  irregu- 
  

   lar, 
  the 
  interior 
  continues 
  essentially 
  uniform, 
  and 
  the 
  pro- 
  

   jecting 
  lamellar 
  septa 
  maintain 
  much 
  regularity 
  in 
  their 
  

   occurrence. 
  The 
  thickening 
  of 
  the 
  walls, 
  and 
  the 
  irregularity 
  

   of 
  growth, 
  is 
  confined 
  to 
  the 
  exterior, 
  while 
  the 
  cavity 
  of 
  

   habitation 
  maintains 
  a 
  regularly 
  increasing 
  size, 
  very 
  rarely 
  

   encroached 
  upon 
  by 
  the 
  cellular 
  tissue. 
  

  

  The 
  interior 
  structure 
  of 
  the 
  walls 
  is 
  vesicular, 
  but 
  these 
  

   walls 
  vary 
  greatly 
  in 
  thickness 
  in 
  specimens 
  externally 
  of 
  

   the 
  same 
  size, 
  as 
  maybe 
  seen 
  by 
  comparing 
  figures 
  9 
  and 
  10 
  of 
  

   plate 
  31. 
  Even 
  the 
  smaller 
  parasitic 
  tubes, 
  like 
  those 
  of 
  fig- 
  

   ures 
  1, 
  2 
  and 
  3, 
  show 
  a 
  vesicular 
  wall 
  structure, 
  although 
  this 
  

   is 
  often 
  limited 
  to 
  one 
  or 
  two 
  ranges 
  of 
  vesicles 
  ; 
  and 
  the 
  

   same 
  is 
  sometimes 
  true 
  of 
  the 
  larger 
  specimens, 
  as 
  shown 
  in 
  

   figure 
  10, 
  and 
  in 
  figure 
  11, 
  where 
  the 
  wall 
  on 
  one 
  side 
  is 
  thin 
  

   and 
  nearly 
  or 
  entirely 
  destitute 
  of 
  cellular 
  tissue. 
  This 
  tex- 
  

   ture 
  is 
  not 
  only 
  variable 
  in 
  extent, 
  but 
  also 
  in 
  the 
  form 
  of 
  the 
  

   vesicles 
  (as 
  shown 
  in 
  figures 
  12 
  and 
  13), 
  and 
  it 
  sometimes 
  so 
  

   far 
  invades 
  the 
  interior 
  wall 
  as 
  partially 
  to 
  obliterate 
  the 
  evi- 
  

   dence 
  of 
  annulations, 
  as 
  in 
  the 
  specimen 
  figure 
  13, 
  where 
  this 
  

   cellular 
  tissue 
  has 
  formed 
  upon 
  the 
  inner 
  face 
  of 
  the 
  tube. 
  

  

  Since 
  this 
  fossil 
  occurs 
  in 
  such 
  different 
  phases 
  (the 
  prevail- 
  

   ing 
  one 
  of 
  which 
  is 
  shown 
  in 
  figures 
  1-3, 
  and 
  another 
  in 
  fig- 
  

   ures 
  4-7,) 
  the 
  identity 
  is 
  not 
  readily 
  observed 
  ; 
  but 
  a 
  study 
  and 
  

   comparison 
  of 
  a 
  large 
  number 
  of 
  specimens 
  has 
  left 
  no 
  doubt 
  

   of 
  their 
  identity 
  ; 
  the 
  different 
  forms 
  being 
  simply 
  the 
  dif- 
  

  

  * 
  Illustrations 
  of 
  this 
  feature 
  are 
  given 
  in 
  Murchison's 
  Silurian 
  System, 
  plate 
  2G, 
  

   where 
  both 
  the 
  casts 
  and 
  the 
  interior 
  of 
  the 
  tube 
  are 
  shown. 
  Also 
  in 
  Pal. 
  N. 
  Y., 
  

   vol. 
  ii, 
  plates 
  28 
  and 
  85 
  (this 
  latter 
  supplementary 
  plate 
  is 
  erroneously 
  numbered 
  

   83 
  in 
  some 
  of 
  the 
  copies), 
  where 
  casts 
  and 
  partial 
  casts 
  of 
  a 
  species 
  are 
  illustrated. 
  

   In 
  Pal. 
  N. 
  Y., 
  vol. 
  i, 
  page 
  92, 
  I 
  have 
  mentioned 
  the 
  septate 
  character 
  of 
  the 
  tubes 
  

   of 
  a 
  species 
  which 
  I 
  had 
  referred 
  with 
  doubt 
  to 
  Tentaculites. 
  

  

  