﻿V°l' 
  53'] 
  PETALOGRAPTUS 
  AND 
  CEPHALOGRAPTUS. 
  207 
  

  

  cometa 
  (Gein.). 
  Under 
  such 
  circumstances 
  it 
  seems 
  best 
  to 
  adopt 
  a 
  

   new 
  name 
  for 
  the 
  species 
  represented 
  by 
  figs. 
  12 
  & 
  13, 
  specimens 
  

   of 
  which 
  I 
  have 
  also 
  procured 
  for 
  myself 
  from 
  Dobb's 
  Linn 
  and 
  

   Belcraig 
  Burn. 
  The 
  species 
  in 
  question 
  is 
  undeniably 
  a 
  link 
  

   between 
  the 
  Petalograpti 
  and 
  the 
  Cephalograpti, 
  as 
  represented 
  

   on 
  the 
  one 
  hand 
  by 
  Petalograptus 
  folium 
  (His.), 
  and 
  on 
  the 
  other 
  

   by 
  Cephalograptus 
  cometa 
  (Gein.). 
  

  

  Speaking 
  generally, 
  we 
  might 
  say 
  that 
  this 
  species 
  is 
  distally 
  a 
  

   Petalograptus 
  and 
  proximally 
  a 
  Cephalograptus, 
  since 
  the 
  characters 
  

   of 
  the 
  thecae 
  show 
  that 
  it 
  is 
  closely 
  related 
  to 
  P. 
  folium 
  (His.), 
  but 
  

   the 
  character 
  of 
  the 
  proximal 
  end 
  proclaims 
  it 
  a 
  true 
  Cephalo- 
  

   graptus. 
  

  

  Structure 
  of 
  the 
  Proximal 
  End. 
  — 
  The 
  position 
  of 
  the 
  sicula 
  

   again 
  strongly 
  recalls 
  the 
  Monograptus-tyipe. 
  It 
  is 
  entirely 
  free 
  on 
  

   one 
  side 
  in 
  either 
  aspect 
  of 
  the 
  rhabdosoma. 
  It 
  is 
  furnished 
  with 
  

   an 
  apertural 
  spine. 
  The 
  whole 
  proximal 
  end 
  is 
  slender 
  and 
  pro- 
  

   tracted 
  as 
  in 
  C. 
  cometa 
  (Gein.), 
  but 
  it 
  is 
  not 
  so 
  long 
  as 
  in 
  that 
  

   species. 
  The 
  first 
  theca 
  of 
  the 
  primordial 
  series 
  appears 
  to 
  arise 
  

   slightly 
  above 
  the 
  base 
  of 
  the 
  sicula; 
  it 
  describes 
  a 
  decided 
  outward 
  

   curve, 
  and 
  attains 
  a 
  length 
  of 
  ^ 
  inch. 
  

  

  The 
  first 
  theca 
  of 
  the 
  second 
  series 
  arises 
  from 
  l 
  1 
  just 
  after 
  the 
  

   first 
  of 
  the 
  primordial 
  series 
  has 
  grown 
  past 
  the 
  sicula. 
  

  

  The 
  proximal 
  end 
  is 
  straight, 
  as 
  in 
  P. 
  folium 
  (His.), 
  not 
  sinuous 
  

   as 
  in 
  C. 
  cometa 
  (Gein.). 
  

  

  Character 
  of 
  Mature 
  Rhabdosoma. 
  — 
  The 
  forms 
  of 
  the 
  mature 
  

   rhabdosoma 
  are 
  very 
  varied. 
  Some 
  approach 
  more 
  nearly 
  to 
  

   P. 
  folium 
  (His.), 
  others 
  are 
  more 
  like 
  C. 
  cometa 
  (Gein.). 
  The 
  

   greatest 
  length 
  attained 
  by 
  any 
  specimen 
  that 
  I 
  have 
  seen 
  was 
  

   1 
  inch 
  ; 
  this 
  is 
  exceptionally 
  long 
  : 
  the 
  specimens 
  are 
  more 
  commonly 
  

   about 
  \ 
  inch 
  in 
  length. 
  

  

  The 
  maximum 
  width 
  of 
  -l- 
  inch 
  is 
  not 
  attained 
  opposite 
  tho 
  

   aperture 
  of 
  the 
  first 
  theca, 
  but 
  the 
  rhabdosoma 
  gradually 
  increases 
  

   in 
  width 
  as 
  far 
  as 
  the 
  third 
  theca 
  of 
  either 
  series 
  ; 
  a 
  constant 
  width 
  

   is 
  then 
  maintained 
  up 
  to 
  the 
  distal 
  end. 
  

  

  The 
  thecae 
  are 
  alternate 
  ; 
  they 
  are 
  tubular 
  ; 
  the 
  earliest 
  ones 
  are 
  

   5 
  inch 
  long, 
  but 
  those 
  in 
  the 
  middle 
  of 
  the 
  rhabdosoma 
  have 
  a 
  

   constant 
  length 
  of 
  \ 
  inch; 
  there 
  is 
  a 
  further 
  diminution 
  in 
  length 
  

   at 
  the 
  distal 
  extremity. 
  The 
  thecae 
  on 
  an 
  average 
  are 
  8 
  times 
  as 
  

   long 
  as 
  they 
  are 
  wide. 
  The 
  apertures 
  of 
  the 
  earliest 
  thecae 
  are 
  

   oblique 
  to 
  the 
  general 
  direction 
  of 
  the 
  rhabdosoma, 
  but 
  near 
  the 
  

   distal 
  end 
  they 
  become 
  more 
  nearly 
  perpendicular 
  ; 
  they 
  are 
  always 
  

   perpendicular 
  to 
  the 
  long 
  axis 
  of 
  the 
  theca. 
  The 
  apertures 
  of 
  the 
  

   earliest 
  thecae 
  overlap 
  \, 
  but 
  this 
  decreases 
  distally. 
  

  

  There 
  are 
  24 
  apertures 
  in 
  the 
  space 
  of 
  an 
  inch, 
  and 
  they 
  are 
  not 
  

   all 
  near 
  the 
  distal 
  end 
  as 
  in 
  C. 
  cometa, 
  but 
  the 
  rhabdosoma 
  has 
  

   apertures 
  for 
  fully 
  half 
  its 
  length 
  on 
  either 
  side. 
  The 
  angle 
  of 
  

   inclination 
  is 
  about 
  10° 
  in 
  the 
  middle 
  of 
  the 
  rhabdosoma, 
  but 
  it 
  is 
  

   less 
  proximally 
  and 
  distally. 
  The 
  thecae 
  are 
  almost 
  straight. 
  

  

  