﻿"^"°L 
  53*] 
  PETALOGRAPTUS 
  AND 
  CEP.KALOGRAPTUS. 
  195 
  

  

  various 
  points 
  along 
  its 
  length, 
  both 
  inside 
  and 
  outside 
  the 
  rhabdo- 
  

   soma 
  (see 
  PL 
  XIV. 
  fig. 
  3). 
  In 
  this 
  case 
  as 
  in 
  all 
  others, 
  the 
  sicula 
  is 
  

   completely 
  exposed 
  on 
  the 
  obverse 
  side 
  ; 
  the 
  virgula, 
  for 
  the 
  earlier 
  

   part 
  of 
  its 
  course, 
  must 
  run 
  very 
  near 
  the 
  obverse 
  surface. 
  Barrande 
  

   suggested 
  that 
  the 
  appearance 
  of 
  splitting 
  might 
  be 
  due 
  to 
  a 
  process 
  

   of 
  exfoliation 
  of 
  the 
  component 
  layers, 
  which 
  could 
  come 
  about 
  only 
  

   after 
  the 
  death 
  of 
  the 
  animal. 
  

  

  General 
  Characters. 
  This 
  species 
  is 
  characterized 
  by 
  — 
  

  

  (1) 
  The 
  short 
  and 
  abruptly-terminated 
  character 
  of 
  the 
  proximal 
  

  

  end. 
  

  

  (2) 
  Length 
  of 
  the 
  sicula 
  relatively 
  to 
  the 
  thecse. 
  

  

  (3) 
  Short 
  thecse 
  and 
  the 
  ratio 
  between 
  their 
  length 
  and 
  breadth. 
  

  

  (4) 
  Relative 
  length 
  and 
  width 
  of 
  rhabdosoma 
  as 
  a 
  whole. 
  

  

  Horizon. 
  — 
  The 
  species 
  seems 
  to 
  have 
  had 
  a 
  long 
  range. 
  It 
  ranges 
  

   in 
  Britain 
  from 
  the 
  bottom 
  of 
  the 
  Middle 
  Birkhill 
  Shales 
  up 
  into 
  the 
  

   Lower 
  Gala-Tarannon 
  beds. 
  In 
  Scandinavia 
  it 
  occurs 
  immediately 
  

   above 
  the 
  zone 
  corresponding 
  to 
  that 
  of 
  Diplograptus 
  vesiculosus 
  

   (Nich.) 
  at 
  Rostanga, 
  and 
  is 
  common 
  in 
  the 
  Rastrites- 
  Shales 
  of 
  

   Tommarp, 
  Bollerup 
  (Skane), 
  and 
  Kongslena 
  (Vestergotland). 
  It 
  

   occurs 
  withilf. 
  turriculatus 
  (Barr.) 
  at 
  Klubudden 
  (Vestergotland), 
  and 
  

   with 
  M. 
  turriculatus 
  and 
  M. 
  ecciguus 
  at 
  Osmundsberg 
  (Dalarne). 
  

  

  It 
  occurs 
  also 
  at 
  similar 
  horizons 
  at 
  Berown 
  and 
  Zelkovice 
  in 
  

   Bohemia, 
  and 
  in 
  the 
  Rastrites- 
  Shales 
  of 
  Heinrichsruhe 
  in 
  Thuringia. 
  

  

  British 
  Localities. 
  — 
  Moffat 
  District 
  : 
  Dobb's 
  Linn, 
  Belcraig 
  Burn, 
  

   Garple 
  Linn, 
  etc., 
  Lundhope-on- 
  Yarrow. 
  Lake 
  District 
  : 
  Skelgill, 
  

   Browgill, 
  Pull 
  Beck, 
  Kentmere, 
  Ashgill. 
  Wales 
  : 
  in 
  Birkhill 
  and 
  

   Tarannon 
  Shales 
  at 
  Conway. 
  

  

  Petaiograptus 
  palmeus, 
  var. 
  lattjs 
  (Barr.). 
  (PL 
  XIV. 
  figs. 
  5-8.) 
  

  

  Structure 
  of 
  the 
  Proximal 
  End. 
  — 
  In 
  nearly 
  all 
  the 
  details 
  of 
  

   structure 
  of 
  the 
  proximal 
  end 
  P. 
  palmeus, 
  var. 
  latus, 
  agrees 
  with 
  

   P. 
  palmeus 
  (restricted). 
  The 
  sicula 
  is 
  about 
  the 
  same 
  length, 
  and 
  

   reaches 
  up 
  to 
  the 
  third 
  theca 
  of 
  the 
  primordial 
  series, 
  as 
  in 
  that 
  form. 
  

   It 
  has 
  an 
  apertural 
  spine, 
  although 
  this 
  is 
  but 
  rarely 
  preserved. 
  

  

  It 
  differs 
  in 
  the 
  fact 
  that 
  the 
  earliest 
  thecse 
  make 
  a 
  greater 
  

   curve 
  from 
  their 
  origin 
  and 
  are 
  rather 
  longer, 
  so 
  that 
  the 
  width 
  of 
  

   the 
  rhabdosoma 
  is 
  increased. 
  

  

  Character 
  of 
  Mature 
  Rhabdosoma. 
  — 
  This 
  variety 
  very 
  seldom 
  

   attains 
  a 
  length 
  as 
  great 
  as 
  that 
  of 
  the 
  longest 
  forms 
  of 
  P. 
  palmeus 
  

   (restr.) 
  ; 
  it 
  is 
  most 
  commonly 
  found 
  with 
  a 
  length 
  of 
  about 
  | 
  inch. 
  

   The 
  maximum 
  width, 
  generally 
  nearly 
  i 
  inch, 
  is 
  attained 
  at 
  once 
  

   and 
  continues 
  quite 
  up 
  to 
  the 
  distal 
  end, 
  which 
  has 
  a 
  broadly 
  trun- 
  

   cate 
  form 
  in 
  most 
  specimens, 
  but 
  occasionally 
  is 
  somewhat 
  rounded. 
  

  

  The 
  thecse 
  are 
  alternate 
  ; 
  they 
  are 
  about 
  three 
  times 
  as 
  long 
  as 
  

   wide, 
  and 
  are 
  fully 
  -^ 
  inch 
  long, 
  except 
  at 
  the 
  distal 
  end. 
  The 
  aper- 
  

   tures 
  are 
  slightly 
  concave 
  ; 
  they 
  are 
  perpendicular 
  to 
  the 
  thecal 
  length, 
  

  

  