﻿G. 
  W. 
  SHRTTBSOLE 
  ON 
  THAMNISCUS. 
  345 
  

  

  Sjo. 
  char. 
  Zoarium 
  a 
  flattened 
  expansion. 
  Branches 
  round, 
  some- 
  

   what 
  flattened, 
  regularly 
  dichotomizing 
  at 
  intervals 
  of 
  two 
  or 
  three 
  

   lines. 
  Diameter 
  of 
  branch 
  -±j 
  to 
  -^ 
  of 
  an 
  inch, 
  lieverse 
  striated 
  

   longitudinally. 
  Zooecia 
  long 
  and 
  cylindrical, 
  tapering 
  towards 
  the 
  

   mouth, 
  raised 
  margins 
  more 
  than 
  their 
  own 
  diameter 
  apart 
  when 
  

   not 
  eroded 
  ; 
  spirally 
  arranged 
  one 
  side 
  of 
  the 
  branch, 
  three 
  or 
  four 
  

   cells 
  across, 
  and 
  five 
  or 
  six 
  in 
  oblique 
  lines. 
  Spines 
  on 
  the 
  inter- 
  

   spaces 
  between 
  the 
  cells. 
  

  

  Locality. 
  Wenlock 
  Limestone, 
  near 
  Dudley. 
  

  

  Large 
  specimens 
  somewhat 
  rare, 
  small 
  fragments 
  more 
  common. 
  

  

  Ohs. 
  The 
  specimen 
  of 
  Thamniscus 
  in 
  the 
  AVoodwardian 
  Museum, 
  

   while 
  it 
  is 
  the 
  largest 
  and 
  best-preserved 
  fragment 
  that 
  I 
  have 
  

   seen, 
  bears 
  evidence 
  of 
  having 
  suffered 
  from 
  lateral 
  crushing 
  and 
  

   breakage, 
  which 
  interferes 
  somewhat 
  with 
  the 
  characteristic 
  display 
  

   of 
  its 
  growth. 
  The 
  seeming 
  coalescence 
  of 
  the 
  branches 
  is 
  the 
  re- 
  

   sult 
  of 
  this 
  lateral 
  pressure. 
  

  

  It 
  is 
  often 
  seen 
  in 
  this 
  class 
  of 
  organisms 
  that 
  the 
  best 
  -preserved 
  

   portion 
  of 
  the 
  structure 
  is 
  near 
  the 
  base. 
  This 
  is 
  the 
  case 
  here. 
  On 
  the 
  

   basal 
  branches 
  will 
  be 
  seen 
  the 
  remains 
  of 
  large 
  spines 
  half 
  the 
  diameter 
  

   of 
  the 
  cells 
  in 
  size. 
  The 
  cell-openings, 
  it 
  will 
  be 
  observed, 
  are 
  circular, 
  

   with 
  considerable 
  interspaces 
  between 
  them 
  ; 
  comparing 
  these 
  with 
  

   other 
  and 
  more 
  eroded 
  parts 
  of 
  the 
  branch, 
  it 
  will 
  be 
  seen 
  that 
  in 
  the 
  

   latter 
  the 
  interspaces 
  have 
  decreased, 
  and 
  will, 
  with 
  further 
  erosion, 
  

   altogether 
  disappear, 
  leaving 
  the 
  cells 
  with 
  only 
  the 
  division-walls 
  

   between 
  them. 
  The 
  explanation 
  is 
  that 
  while 
  the 
  cells 
  of 
  Thamniscus 
  

   are 
  cylindrical, 
  they 
  are 
  also 
  tapering, 
  the 
  base 
  of 
  the 
  cell 
  being 
  the 
  

   wider 
  part, 
  and 
  twice 
  the 
  diameter 
  of 
  the 
  aperture. 
  The 
  cell- 
  aperture 
  

   was 
  an 
  important 
  as 
  well 
  as 
  a 
  peculiar 
  feature. 
  All 
  that 
  is 
  now 
  to 
  be 
  

   seen 
  of 
  it 
  are 
  the 
  remains 
  of 
  the 
  cell-walls, 
  which 
  appear 
  springing 
  

   from 
  the 
  body 
  of 
  the 
  cell. 
  It 
  might 
  be 
  compared, 
  as 
  far 
  as 
  the 
  outline 
  

   of 
  the 
  aperture 
  is 
  concerned, 
  with 
  a 
  somewhat 
  eroded 
  cell 
  of 
  Glauco- 
  

   nome 
  stdlipora, 
  Young. 
  As 
  to 
  the 
  existence 
  of 
  denticles 
  there 
  is 
  no 
  

   evidence 
  forthcoming. 
  

  

  I 
  may 
  here 
  notice, 
  as 
  showing 
  the 
  agreement 
  between 
  the 
  Car- 
  

   boniferous 
  and 
  Silurian 
  Thamnisci, 
  that 
  the 
  Messrs. 
  Young, 
  when 
  

   describing 
  the 
  former, 
  speak 
  of 
  the 
  cell-aperture 
  as 
  tuberculate 
  ; 
  

   this 
  quite 
  describes 
  the 
  appearance 
  of 
  the 
  Silurian 
  species 
  in 
  parts 
  

   where 
  not 
  eroded. 
  Indeed 
  the 
  cell-neck 
  might 
  be 
  said 
  to 
  be 
  built 
  

   up 
  of 
  a 
  series 
  of 
  pillars, 
  arranged 
  in 
  a 
  circle, 
  judging 
  from 
  their 
  

   present 
  tuberculate 
  appearance. 
  

  

  Fragments 
  of 
  this 
  Polyzoan 
  are 
  not 
  uncommon 
  in 
  exposures 
  of 
  

   Dudley 
  Limestone 
  ; 
  and 
  Mr. 
  Yine 
  informs 
  me 
  that 
  he 
  has 
  found 
  

   it 
  in 
  the 
  Wenlock 
  shale, 
  with 
  characters 
  identical 
  with 
  those 
  above 
  

   described. 
  From 
  the 
  robust 
  character 
  of 
  the 
  fragments 
  it 
  may 
  be 
  

   inferred 
  that 
  it 
  attained 
  considerable 
  size 
  ; 
  its 
  expansion 
  was 
  pro- 
  

   bably 
  fan-shaped, 
  although 
  the 
  fragments 
  I 
  have 
  seen 
  would 
  not 
  be 
  

   inconsistent 
  with 
  an 
  open 
  cup-shaped 
  zoarium. 
  It 
  may 
  readily 
  be 
  

   distinguished 
  by 
  its 
  branches 
  of 
  equal 
  width, 
  regularly 
  dichotomizing 
  

   and 
  not 
  anastomosing. 
  It 
  closely 
  resembles 
  a 
  Polypora 
  without 
  the 
  

   connecting 
  bars; 
  indeed 
  it 
  might 
  be 
  so 
  described. 
  Most 
  of 
  the 
  

  

  