﻿^Ol. 
  53.] 
  PETALOGKAPTTJS 
  AND 
  CEPHALOGBAPTUS. 
  189 
  

  

  This 
  species, 
  though 
  very 
  well 
  known 
  in 
  Scandinavia, 
  seems 
  

   never 
  to 
  have 
  been 
  clearly 
  recognized 
  in 
  this 
  country. 
  It 
  has 
  most 
  

   frequently 
  been 
  confounded 
  with 
  Petalograptus 
  palmeus 
  (Barr.). 
  It 
  

   may 
  be 
  said 
  to 
  show 
  some 
  general 
  relationships 
  to 
  that 
  species, 
  but 
  

   it 
  has 
  peculiar 
  characteristics 
  which 
  mark 
  it 
  out 
  as 
  a 
  perfectly 
  

   •distinct 
  form. 
  

  

  Tullberg, 
  in 
  his 
  paper 
  on 
  the 
  ' 
  Graptolites 
  described 
  by 
  Hisinger/ 
  

   has 
  grouped 
  it 
  with 
  Cephalograptus 
  cometa 
  (Gein.) 
  in 
  the 
  sub- 
  

   genus 
  Cephalograptus 
  (Hopk.). 
  To 
  this 
  I 
  cannot 
  agree. 
  In 
  the 
  

   development 
  of 
  the 
  earliest 
  thecae, 
  in 
  the 
  position 
  of 
  the 
  sicula, 
  and 
  

   in 
  the 
  relation 
  of 
  the 
  thecae 
  to 
  the 
  sicula, 
  Petalograptus 
  folium 
  is 
  

   far 
  more 
  closely 
  allied 
  to 
  Petalograptus 
  palmeus 
  (Barr.) 
  than 
  to 
  

   Cephalograptus 
  cometa 
  (Gein.). 
  

  

  Hisinger's 
  original 
  figures 
  were 
  generalized 
  from 
  many 
  imperfect 
  

   specimens, 
  but 
  during 
  my 
  recent 
  visit 
  to 
  Sweden 
  I 
  had 
  the 
  oppor- 
  

   tunity 
  of 
  studying 
  more 
  perfect 
  specimens 
  of 
  the 
  species, 
  collected 
  

   at 
  the 
  same 
  time 
  as 
  Hisinger's 
  types. 
  

  

  The 
  forms 
  from 
  different 
  localities 
  vary 
  within 
  certain 
  narrow 
  

   limits 
  as 
  regards 
  width, 
  but 
  the 
  difference 
  is 
  so 
  slight 
  that 
  the 
  forms 
  

   -can 
  hardly 
  be 
  considered 
  worthy 
  of 
  being 
  ranked 
  as 
  distinct 
  varieties. 
  

   The 
  circumstance 
  of 
  variation 
  in 
  width 
  depends 
  merely 
  on 
  the 
  

   amount 
  of 
  curvature 
  of 
  the 
  thecae. 
  If 
  the 
  curvature 
  is 
  rather 
  more 
  

   pronounced 
  the 
  width 
  is 
  increased. 
  

  

  Some 
  of 
  my 
  specimens 
  are 
  in 
  half-relief, 
  and 
  exhibit 
  details 
  of 
  

   structure 
  which 
  have 
  not 
  hitherto 
  been 
  noticed. 
  

  

  Structure 
  of 
  the 
  Proximal 
  End. 
  — 
  The 
  sicula 
  is 
  somewhat 
  slender, 
  

   but 
  is 
  short 
  compared 
  with 
  the 
  great 
  length 
  attained 
  by 
  the 
  thecae. 
  

   It 
  never 
  exceeds 
  y 
  1 
  ^ 
  inch 
  in 
  length, 
  and 
  is 
  usually 
  rather 
  more 
  than 
  

   y 
  1 
  ^ 
  inch 
  long 
  (about 
  2 
  mm.). 
  

  

  It 
  certainly 
  possessed 
  an 
  apertural 
  spine, 
  though 
  this 
  is 
  not 
  seen 
  

   in 
  all 
  specimens, 
  since 
  it 
  would 
  seem 
  to 
  have 
  been 
  easily 
  broken 
  off. 
  

   From 
  a 
  short 
  distance 
  above 
  the 
  base 
  of 
  the 
  sicula 
  and 
  on 
  the 
  same 
  

   .side 
  as 
  the 
  apertural 
  spine 
  the 
  ' 
  primordial 
  bud 
  ' 
  is 
  produced. 
  This 
  

   gives 
  rise 
  to 
  the 
  first 
  theca 
  of 
  the 
  primordial 
  series, 
  and 
  also 
  to 
  the 
  

   connecting-canal, 
  from 
  which 
  the 
  first 
  theca 
  of 
  the 
  second 
  series 
  is 
  

   •developed. 
  

  

  Here 
  I 
  must 
  explain 
  a 
  few 
  of 
  the 
  terms 
  of 
  which 
  I 
  shall 
  make 
  use 
  

   in 
  the 
  following 
  pages. 
  The 
  term 
  ' 
  primordial 
  series 
  ' 
  is 
  used 
  in 
  

   Tornquist's 
  sense 
  to 
  denote 
  all 
  the 
  thecae 
  lying 
  on 
  the 
  same 
  side 
  of 
  

   the 
  sicula 
  as 
  the 
  first. 
  For 
  the 
  sake 
  of 
  brevity 
  these 
  are 
  repre- 
  

   sented 
  in 
  the 
  tables 
  by 
  the 
  following 
  symbols 
  : 
  l 
  1 
  , 
  2 
  1 
  , 
  3 
  1 
  , 
  etc. 
  ; 
  

   'while 
  those 
  which 
  lie 
  on 
  the 
  opposite 
  side 
  are 
  termed 
  the 
  ' 
  second 
  

   series,' 
  since 
  they 
  contain 
  the 
  second 
  theca. 
  These 
  are 
  denoted 
  by 
  

   the 
  symbols 
  l 
  2 
  , 
  2 
  2 
  , 
  3 
  2 
  , 
  etc. 
  That 
  side 
  of 
  the 
  rhabdosoma 
  on 
  which 
  

   the 
  sicula 
  is 
  completely 
  visible 
  is 
  termed 
  the 
  ' 
  obverse 
  ' 
  side. 
  In 
  such 
  

   an 
  aspect 
  the 
  first 
  theca 
  of 
  the 
  primordial 
  series 
  (l 
  1 
  ) 
  is 
  always 
  on 
  

   the 
  left 
  side. 
  The 
  other 
  aspect 
  of 
  the 
  rhabdosoma 
  is 
  termed 
  the 
  

   * 
  reverse 
  ' 
  side. 
  The 
  ' 
  connecting-canal 
  ' 
  is 
  a 
  convenient 
  term 
  to 
  use 
  

   for 
  the 
  canal, 
  at 
  or 
  near 
  the 
  base 
  of 
  the 
  sicula, 
  which 
  connects 
  the 
  

  

  