﻿Vol. 
  53.] 
  PETALOGEAPIUS 
  AND 
  CEPHALOGKAPTTJS. 
  205 
  

  

  viewed 
  from 
  the 
  obverse 
  side 
  it 
  is 
  free 
  for 
  the 
  whole 
  of 
  its 
  length 
  

   on 
  the 
  right-hand 
  side, 
  recalling 
  strongly 
  the 
  position 
  of 
  the 
  sicula 
  

   in 
  all 
  forms 
  of 
  Monograptus. 
  

  

  The 
  first 
  theca 
  of 
  the 
  primordial 
  series 
  arises 
  close 
  to 
  the 
  base 
  of 
  

   the 
  sicnla 
  and 
  grows 
  slightly 
  downward, 
  so 
  as 
  to 
  be 
  a 
  little 
  below 
  

   the 
  base 
  of 
  the 
  sicula 
  ; 
  it 
  grows 
  also 
  upward 
  (apparently) 
  simul- 
  

   taneously, 
  making 
  a 
  very 
  gentle 
  outward 
  curve. 
  

  

  The 
  first 
  theca 
  of 
  the 
  second 
  series 
  does 
  not 
  develop 
  from 
  theca 
  l 
  1 
  

   till 
  it 
  has 
  grown 
  some 
  considerable 
  way 
  past 
  the 
  sicula. 
  Hence 
  

   the 
  reason 
  why 
  the 
  sicula 
  is 
  completely 
  visible 
  in 
  both 
  aspects 
  

   of 
  the 
  rhabdosoma. 
  The 
  virgula 
  is 
  seen, 
  as 
  usual, 
  to 
  arise 
  at 
  the 
  

   apex 
  of 
  the 
  sicula, 
  and 
  it 
  is 
  worthy 
  of 
  note 
  that 
  at 
  first 
  it 
  runs 
  

   along 
  the 
  dorsal 
  side 
  of 
  the 
  first 
  theca 
  for 
  some 
  little 
  distance; 
  

   then, 
  however, 
  it 
  is 
  seen 
  to 
  pass 
  into 
  the 
  centre 
  of 
  the 
  rhabdosoma. 
  

   It 
  is 
  very 
  slender 
  in 
  this 
  species, 
  especially 
  at 
  its 
  origin. 
  The 
  

   entire 
  proximal 
  end 
  is 
  slender, 
  being 
  much 
  protracted 
  ; 
  the 
  attenua- 
  

   tion 
  is 
  especially 
  conspicuous 
  at 
  a 
  short 
  distance 
  from 
  the 
  proximal 
  

   end, 
  and 
  it 
  is 
  at 
  this 
  point 
  that 
  the 
  rhabdosoma 
  is 
  so 
  frequently 
  

   broken 
  off. 
  When 
  the 
  rhabdosoma 
  is 
  perfect 
  there 
  is 
  seen 
  to 
  be 
  

   a 
  slight 
  dilatation 
  at 
  the 
  extreme 
  proximal 
  end 
  where 
  the 
  sicula 
  is 
  

   placed. 
  

  

  The 
  whole 
  proximal 
  end 
  would 
  seem 
  to 
  have 
  been 
  flexible, 
  for 
  

   the 
  most 
  slender 
  forms 
  are 
  often 
  seen 
  to 
  be 
  somewhat 
  sinuous. 
  

  

  The 
  two 
  earliest 
  thecae 
  are 
  both 
  very 
  long 
  and 
  narrow. 
  The 
  

   first 
  of 
  the 
  primordial 
  series 
  is 
  ordinarily 
  T 
  7 
  g 
  inch 
  long, 
  but 
  may 
  be 
  

   as 
  much 
  as 
  | 
  inch. 
  All 
  the 
  thecae 
  subsequently 
  deve- 
  

   loped 
  are 
  shorter. 
  

  

  Character 
  of 
  Mature 
  Rhabdosoma. 
  — 
  The 
  whole 
  rhabdosoma, 
  when 
  

   mature 
  and 
  complete, 
  seems 
  always 
  to 
  have 
  exceeded 
  J 
  inch 
  in 
  

   length, 
  and 
  is 
  often 
  nearer 
  1 
  inch. 
  Owing, 
  however, 
  to 
  the 
  

   extremely 
  rare 
  occurrence 
  of 
  a 
  complete 
  specimen, 
  it 
  usually 
  appears 
  

   much 
  less. 
  Specimens 
  from 
  different 
  localities 
  are 
  apt 
  to 
  vary 
  

   slightly 
  in 
  width, 
  but 
  never 
  to 
  any 
  very 
  marked 
  extent. 
  

  

  The 
  maximum 
  width 
  of 
  -^ 
  inch 
  is 
  attained 
  opposite 
  the 
  aperture 
  

   of 
  the 
  first 
  theca 
  of 
  the 
  primordial 
  series. 
  This 
  width 
  is 
  sometimes 
  

   maintained 
  as 
  far 
  as 
  the 
  aperture 
  of 
  the 
  first 
  theca 
  of 
  the 
  second 
  

   series, 
  and 
  then 
  decreases, 
  or 
  a 
  decrease 
  in 
  width 
  may 
  have 
  taken 
  

   place 
  before 
  the 
  aperture 
  of 
  the 
  first 
  theca 
  of 
  the 
  second 
  series 
  is 
  

   reached. 
  

  

  The 
  thecae 
  are 
  always 
  few 
  in 
  number 
  ; 
  I 
  have 
  never 
  seen 
  a 
  

   specimen 
  in 
  which 
  there 
  were 
  more 
  than 
  6 
  on 
  each 
  side. 
  The 
  

   apertures 
  are 
  all 
  within 
  a 
  small 
  fraction 
  of 
  the 
  distal 
  extremity, 
  

   forming 
  a 
  kind 
  of 
  crown; 
  they 
  are 
  arranged 
  alternately. 
  The 
  

   thecae 
  are 
  very 
  long 
  and 
  tubular. 
  They 
  are 
  usually 
  rather 
  less 
  than 
  

   \ 
  inch 
  long 
  and 
  are 
  12 
  times 
  as 
  long 
  as 
  wide. 
  The 
  thecal 
  aper- 
  

   tures 
  are 
  perpendicular 
  to 
  the 
  long 
  axes 
  of 
  the 
  thecae, 
  and 
  often 
  

   nearly 
  perpendicular 
  to 
  the 
  long 
  axis 
  of 
  the 
  rhabdosoma 
  itself. 
  

   Growth-lines 
  are 
  often 
  beautifully 
  seen 
  in 
  specimens 
  preserved 
  in 
  

   relief. 
  The 
  angle 
  at 
  which 
  the 
  thecae 
  are 
  inclined 
  is 
  very 
  steep 
  ; 
  

  

  