﻿212 
  PETALOGRAPTUS 
  AND 
  CEPHALOGRAPTUS. 
  [May 
  1 
  89 
  7, 
  

  

  Figs. 
  17-21. 
  Petalograptus 
  minor 
  y 
  sp. 
  nov. 
  

  

  Fig. 
  17. 
  Obverse 
  aspect 
  of 
  a 
  typical 
  form, 
  showing 
  the 
  long 
  sicula 
  and 
  the 
  

   general 
  chai'acters 
  of 
  the 
  proximal 
  end. 
  Dobb's 
  Linn. 
  

  

  18. 
  Obverse 
  aspect 
  of 
  a 
  mature 
  specimen 
  ; 
  the 
  thecse 
  on 
  the 
  right 
  side 
  are 
  

  

  not 
  perfectly 
  preserved. 
  Tommarp. 
  

  

  19. 
  Reverse 
  aspect, 
  in 
  relief, 
  of 
  a 
  typical 
  form. 
  Note 
  the 
  absence 
  of 
  the 
  

  

  septum. 
  Skelgill. 
  

  

  20. 
  Specimen 
  preserved 
  so 
  as 
  to 
  show 
  some 
  internal 
  structure. 
  Note 
  the 
  

  

  long 
  sicula 
  and 
  the 
  origin 
  of 
  tbe 
  lowest 
  theca 
  on 
  the 
  right. 
  

  

  Dobb's 
  Linn. 
  

  

  21. 
  Reverse 
  aspect 
  of 
  a 
  young 
  specimen. 
  The 
  sicula 
  is 
  seen 
  through 
  

  

  from 
  the 
  obverse 
  side. 
  Dobb's 
  Linn. 
  

  

  Discussion-. 
  

  

  Mr. 
  Hopklnson 
  said 
  that 
  this 
  paper 
  was 
  of 
  much 
  interest, 
  and 
  he 
  

   considered 
  it 
  to 
  be 
  an 
  important 
  contribution 
  to 
  the 
  history 
  of 
  the 
  

   small 
  section 
  of 
  the 
  Diplograptidse 
  of 
  which 
  it 
  treated. 
  The 
  details 
  

   of 
  the 
  structure 
  had 
  been 
  well 
  worked 
  out, 
  evidently 
  from 
  better 
  

   specimens 
  than 
  he 
  had 
  had 
  an 
  opportunity 
  of 
  examining. 
  He 
  

   agreed 
  with 
  the 
  Author 
  as 
  to 
  the 
  derivation 
  of 
  Cephalograptus 
  

   from 
  Ortliograptus 
  through 
  Petalograptus 
  ; 
  but 
  he 
  thought 
  that 
  she 
  

   did 
  not 
  attach 
  sufficient 
  importance 
  to 
  the 
  absence 
  of 
  a 
  longitudinal 
  

   septum 
  in 
  Cephalograptus, 
  which 
  was 
  one 
  of 
  the 
  main 
  points 
  upon 
  

   which 
  he 
  relied 
  in 
  founding 
  the 
  genus. 
  There 
  appeared 
  to 
  be 
  such 
  

   a 
  septum 
  in 
  all 
  other 
  Diplograptidee, 
  and 
  on 
  this 
  account 
  Cephalo- 
  

   graptus 
  was 
  farther 
  removed 
  from 
  Petalograptus 
  than 
  was 
  that 
  

   subgenus 
  from 
  Ortliograptus. 
  In 
  some 
  cases 
  there 
  could 
  be 
  no 
  

   doubt 
  about 
  the 
  phylogeny 
  of 
  the 
  Graptolites. 
  For 
  instance, 
  

   Dicranograptus 
  was 
  evidently 
  derived 
  from 
  Climacograptus 
  by 
  the 
  

   distal 
  portion 
  of 
  the 
  hydrosoma 
  dividing 
  into 
  two 
  branches, 
  and 
  the 
  

   hydrothecse 
  becoming 
  slightly 
  modified; 
  while 
  Dicellograptus 
  was 
  

   derived 
  from 
  Dicranograptus 
  by 
  the 
  further 
  division 
  of 
  the 
  hydro- 
  

   soma 
  into 
  two 
  entirely 
  separate 
  branches, 
  and 
  the 
  further 
  modifi- 
  

   cation 
  of 
  the 
  hydrothecse 
  in 
  the 
  same 
  direction 
  (from 
  an 
  angular 
  to 
  

   a 
  rounded 
  outline) 
  resulting 
  in 
  a 
  two-branched 
  graptplite 
  which 
  has 
  

   no 
  relation 
  to 
  Diclymograptus. 
  The 
  Author's 
  final 
  suggestions 
  had 
  

   no 
  such 
  clear 
  foundations 
  as 
  this 
  ; 
  and 
  he 
  did 
  not 
  think 
  it 
  at 
  all 
  

   likely 
  that 
  Petalograptus 
  was 
  derived 
  from 
  Phyllograptus, 
  nor 
  

   could 
  he 
  see 
  how 
  Cephalograptus 
  could 
  be 
  the 
  ancestor 
  of 
  

   Dim 
  orphograptus. 
  

  

  Mr. 
  Marr 
  bore 
  testimony 
  to 
  the 
  scrupulous 
  care 
  with 
  which 
  the 
  

   Author 
  had 
  conducted 
  her 
  researches. 
  He 
  was 
  not 
  interested 
  

   in 
  the 
  separation 
  of 
  Cephalograptus 
  from 
  Petalograptus, 
  for 
  he 
  

   believed 
  that 
  most 
  of 
  these 
  terms 
  were 
  temporary 
  in 
  the 
  case 
  

   of 
  the 
  Graptolites 
  ; 
  but 
  he 
  felt 
  that 
  the 
  Author, 
  by 
  her 
  careful 
  

   examination 
  of 
  the 
  whole 
  structure 
  of 
  one 
  group 
  of 
  the 
  Diplo- 
  

   graptidse, 
  had 
  given 
  geologists 
  a 
  detailed 
  study 
  in 
  phylogeny 
  of 
  

   great 
  importance. 
  

  

  