﻿Vol. 
  53.] 
  PETALOGRAPTUS 
  AND 
  CEPHALOGRAPTUS. 
  187 
  

  

  Dr. 
  Tornquist 
  was 
  good 
  enough 
  to 
  supply 
  me 
  with 
  material 
  that 
  

   has 
  been 
  invaluable 
  to 
  me 
  in 
  my 
  work. 
  

  

  Owing 
  to 
  the 
  fact 
  that 
  the 
  Swedish 
  specimens 
  are 
  found 
  in 
  almost 
  

   every 
  case 
  in 
  a 
  better 
  state 
  of 
  preservation 
  than 
  the 
  British, 
  I 
  

   have 
  from 
  them 
  been 
  able 
  to 
  make 
  out 
  with 
  comparative 
  ease 
  many 
  

   important 
  structural 
  details 
  which 
  must 
  otherwise 
  have 
  remained 
  

   obscure. 
  This 
  applies 
  especially 
  to 
  the 
  details 
  of 
  structure 
  of 
  the 
  

   proximal 
  end, 
  which 
  in 
  the 
  Swedish 
  specimens 
  is 
  often 
  preserved 
  

   in 
  relief. 
  It 
  will, 
  I 
  hope, 
  be 
  evident 
  from 
  what 
  follows 
  that 
  a 
  

   knowledge 
  of 
  the 
  structure 
  of 
  the 
  proximal 
  end 
  is 
  of 
  the 
  greatest 
  

   importance 
  for 
  accurate 
  specific 
  determination 
  of 
  these 
  forms. 
  

  

  Historical. 
  — 
  As 
  early 
  as 
  1851 
  Suss 
  1 
  used 
  the 
  term 
  Petalolithus 
  

   as 
  a 
  synonym 
  for 
  M'Coy's 
  Diplograpsus, 
  but 
  it 
  should 
  be 
  noted 
  that 
  

   the 
  forms 
  which 
  he 
  considered 
  typical 
  of 
  this 
  genus 
  were 
  : 
  P.palmeus 
  

   (Barr.), 
  P. 
  folium 
  (His.), 
  P. 
  parallelocostatus 
  (sp. 
  nov.), 
  P. 
  ovatus 
  

   (Barr.). 
  

  

  In 
  1868 
  Carruthers 
  2 
  recognized 
  the 
  peculiarity 
  of 
  the 
  form 
  

   Diplograptus 
  cometa 
  (Greinitz, 
  1852), 
  and 
  suggested 
  that 
  perhaps 
  it 
  

   should 
  be 
  made 
  the 
  type 
  of 
  a 
  new 
  genus. 
  Acting 
  on 
  this 
  sugges- 
  

   tion, 
  Hopkinson 
  founded 
  the 
  genus 
  Cephalograptus 
  for 
  this 
  form. 
  3 
  

   He 
  also 
  recognized 
  that 
  the 
  form 
  was 
  distinct 
  from 
  what 
  he 
  erro- 
  

   neously 
  calls 
  D. 
  folium 
  (His.). 
  

  

  In 
  1 
  873 
  Lapworth, 
  in 
  his 
  i 
  Classification 
  of 
  the 
  Rhabdophora,' 
  4 
  

   placed 
  both 
  Petalograptus 
  (Siiss) 
  and 
  Cephalograptus 
  (Hopk.) 
  as 
  

   subgenera 
  of 
  Diplograptus 
  (M'Coy), 
  and 
  considered 
  Petalograptus 
  

   folium 
  (His.) 
  as 
  a 
  type 
  of 
  the 
  subgenus 
  Petalograptus, 
  and 
  Cephalo- 
  

   graptus 
  cometa 
  (Gein.) 
  as 
  a 
  typical 
  Cephalograptus. 
  He 
  stated 
  

   that 
  such 
  subgenera 
  are 
  of 
  value 
  merely 
  ' 
  as 
  enabling 
  us 
  to 
  group 
  

   together, 
  for 
  convenience 
  of 
  reference, 
  species 
  having 
  a 
  certain 
  

   amount 
  of 
  similarity 
  in 
  their 
  external 
  features.' 
  But 
  this 
  ' 
  simi- 
  

   larity 
  in 
  external 
  features 
  ' 
  is 
  accompanied 
  by 
  certain 
  structural 
  

   modifications. 
  

  

  Accepting 
  the 
  definition 
  of 
  Diplograptus 
  as 
  given 
  by 
  Lapworth 
  

   {op. 
  cit.), 
  both 
  Petalograptus 
  and 
  Cephalograptus 
  must 
  be 
  included 
  

   in 
  it. 
  In 
  structure 
  also 
  they 
  follow, 
  broadly 
  speaking, 
  the 
  same 
  

   general 
  plan. 
  Hence 
  they 
  should, 
  I 
  think, 
  be 
  regarded 
  as 
  sub- 
  

   genera 
  of 
  that 
  genus. 
  But 
  because 
  they 
  possess 
  many 
  characters 
  

   in 
  common 
  with 
  all 
  forms 
  of 
  Diplograptus, 
  and 
  yet 
  have 
  some 
  

   peculiar 
  to 
  themselves, 
  I 
  venture 
  to 
  think 
  that 
  these 
  subgenera 
  

   have 
  great 
  value 
  as 
  representing 
  a 
  distinct 
  phase 
  in 
  graptolite 
  

   phylogeny. 
  The 
  Petalograpti 
  are 
  the 
  closest 
  to 
  those 
  forms 
  of 
  

   Diplograptus 
  which 
  we 
  regard 
  as 
  normal 
  ; 
  these 
  are 
  included 
  in 
  

   Lapworth's 
  Orthograptus 
  and 
  Glyptograptus. 
  

  

  It 
  will 
  be 
  shown 
  in 
  the 
  sequel 
  that 
  Petalograptus 
  must 
  have 
  been 
  

   derived 
  from 
  Orthograptus, 
  and 
  Cephalograptus 
  from 
  Petalograptus, 
  

   as 
  the 
  result 
  of 
  development 
  along 
  certain 
  lines. 
  Cephalograptus 
  

   may 
  be 
  regarded 
  as 
  the 
  most 
  extreme 
  Diplograptus-tyipe. 
  

  

  1 
  ' 
  Ueber 
  Bohuiische 
  Graptolitken.' 
  2 
  Geol. 
  Mag. 
  p. 
  131. 
  

  

  3 
  See 
  Journ. 
  Quek. 
  Microsc. 
  Club, 
  1869. 
  4 
  Geol. 
  Mag. 
  pt. 
  ii. 
  p. 
  555. 
  

  

  o2 
  

  

  