﻿342 
  

  

  G. 
  W. 
  SHEEBSOLE 
  ON 
  THAMXISCES. 
  

  

  admit 
  of 
  specific 
  description, 
  and 
  a 
  determination 
  of 
  its 
  affinities, 
  

   which 
  in 
  outline 
  are 
  clearly 
  with 
  the 
  Thamniscidae. 
  

  

  This 
  Silurian 
  Polyzoan 
  has 
  had 
  a 
  somewhat 
  chequered 
  palaeonto- 
  

   logical 
  history. 
  It 
  was 
  known 
  to 
  Lonsdale 
  as 
  Hornera 
  crassa 
  ; 
  in 
  

   ' 
  Sihiria 
  ' 
  it 
  is 
  referred 
  to 
  Pohjpora 
  : 
  Salter 
  in 
  his 
  ' 
  Catalogue 
  ' 
  refers 
  

   to 
  it 
  as 
  a 
  doubtful 
  Ceriop>ora 
  ; 
  in 
  the 
  TVoodwardian 
  Museum 
  it 
  is 
  

   labelled 
  Ceriopora 
  : 
  by 
  other 
  authorities 
  it 
  is 
  regarded 
  as 
  a 
  

   Poh/pora. 
  

  

  In 
  determining 
  its 
  true 
  affinities 
  it 
  will 
  be 
  sufficient 
  to 
  state 
  that, 
  

   as 
  only 
  one 
  half 
  of 
  the 
  surface 
  is 
  poriferous, 
  it 
  clearly 
  does 
  not 
  belong 
  

   to 
  the 
  Cerioporidae, 
  which 
  are 
  poriferous 
  on 
  all 
  sides. 
  The 
  absence 
  of 
  

   dissepiments, 
  which 
  are 
  a 
  distinguishing 
  mark 
  of 
  Pohjpora, 
  further 
  

   excludes 
  it. 
  Its 
  relation 
  to 
  Hornera 
  is 
  the 
  more 
  feasible 
  on 
  account 
  

   of 
  a 
  certain 
  amount 
  of 
  outward 
  resemblance, 
  but 
  equally 
  fallacious, 
  

   since 
  the 
  cell-arrangements 
  of 
  Hornera 
  are 
  widely 
  distinct. 
  

  

  There 
  remains, 
  then, 
  the 
  Permian 
  Thamniscus 
  with 
  which 
  to 
  com- 
  

   pare 
  it. 
  With 
  this 
  genus, 
  in 
  habit, 
  growth, 
  and 
  cell-arrangement, 
  

   there 
  is 
  a 
  very 
  close 
  agreement, 
  with 
  the 
  one 
  exception 
  of 
  its 
  wanting 
  

   the 
  peculiar 
  adjuncts 
  to 
  the 
  cell 
  which 
  relegate 
  the 
  Permian 
  species 
  

   to 
  the 
  Chilostomata, 
  the 
  Silurian 
  form 
  being 
  clearly 
  Cyclostoma- 
  

   tous. 
  In 
  this 
  respect 
  it 
  agrees 
  with 
  the 
  Carboniferous 
  species 
  of 
  

   Thamniscus 
  described 
  by 
  Messrs. 
  Etheridge 
  and 
  Young. 
  At 
  this 
  

   point 
  an 
  interesting 
  inquiry 
  arises 
  as 
  to 
  the 
  generic 
  identity 
  of 
  these 
  

   three 
  species 
  ; 
  and 
  also 
  does 
  the 
  later, 
  Permian 
  species, 
  to 
  take 
  an 
  

   extreme 
  view, 
  owe 
  its 
  peculiarities 
  in 
  any 
  way 
  to 
  further 
  develop- 
  

   ment 
  ? 
  To 
  set 
  these 
  questions 
  at 
  rest, 
  I 
  decided 
  upon 
  investigating 
  

   the 
  claim 
  of 
  the 
  Permian 
  species 
  to 
  the 
  possession 
  of 
  these 
  exceptional 
  

   characters 
  : 
  for 
  it 
  must 
  be 
  confessed 
  that 
  they 
  are 
  so 
  anomalous, 
  and 
  

   among 
  the 
  Palaeozoic 
  Polyzoa 
  so 
  unique, 
  that 
  it 
  is 
  highly 
  important 
  

   that 
  there 
  should 
  be 
  no 
  doubt 
  upon 
  the 
  point. 
  In 
  elucidating 
  the 
  

   question 
  as 
  to 
  the 
  existence 
  or 
  otherwise 
  of 
  these 
  cell-adjuncts, 
  the 
  

   resources 
  of 
  the 
  Newcastle 
  Museum 
  have 
  been 
  placed 
  at 
  my 
  disposal. 
  

   To 
  Mr. 
  Howse 
  of 
  Newcastle 
  I 
  am 
  especially 
  indebted 
  for 
  much 
  as- 
  

   sistance. 
  

  

  The 
  result 
  of 
  a 
  careful 
  microscopic 
  examination 
  of 
  the 
  poriferous 
  

   face 
  of 
  several 
  specimens 
  of 
  Thamniscus 
  is, 
  that 
  I 
  find 
  no 
  confirma- 
  

   tion 
  of 
  Prof. 
  King's 
  claim 
  for 
  his 
  species 
  of 
  having 
  affinity 
  with 
  recent 
  

   forms 
  and 
  possessing 
  appendages 
  homologous 
  with 
  theirs. 
  I 
  refer 
  

   now 
  more 
  especially 
  to 
  the 
  secondary 
  cell-pore. 
  On 
  the 
  contrary, 
  I 
  

   find, 
  as 
  to 
  cell-character, 
  a 
  perfect 
  agreement 
  between 
  Permian, 
  Car- 
  

   boniferous, 
  and 
  Silurian 
  species. 
  I 
  do 
  notice, 
  however, 
  on 
  the 
  cell- 
  

   face 
  of 
  Thamniscus 
  dubins, 
  a 
  feature 
  not 
  recorded 
  by 
  King, 
  and 
  one 
  

   which 
  is 
  very 
  common 
  in 
  the 
  Palaeozoic 
  Polyzoa 
  — 
  a 
  strong 
  defensive 
  

   spine 
  which 
  is 
  hollow 
  at 
  the 
  base 
  ; 
  it 
  occurs 
  here 
  and 
  there 
  in 
  an 
  

   irregular 
  line 
  between 
  the 
  cells 
  ; 
  more 
  often 
  than 
  not 
  it 
  is 
  worn 
  

   down, 
  showing 
  the 
  hollow 
  centre 
  and 
  projecting 
  circular 
  walls. 
  I 
  

   have 
  little 
  doubt 
  that 
  the 
  t; 
  hollow 
  cavities 
  " 
  and 
  the 
  secondary 
  cells 
  

   of 
  King 
  arc 
  nothing 
  else 
  than 
  the 
  hollow 
  centres 
  of 
  these 
  spines. 
  

   Spines, 
  when 
  worn 
  away, 
  which 
  is 
  more 
  often 
  the 
  case 
  than 
  other- 
  

   wise, 
  have 
  often 
  been 
  mistaken, 
  in 
  the 
  Palaeozoic 
  Polyzoa, 
  for 
  minute 
  

  

  