﻿374 
  J. 
  W. 
  HULKE 
  ON 
  THE 
  OS 
  PTTMS 
  AND 
  

  

  surface, 
  plainly 
  acetabular. 
  It 
  is 
  continuous 
  with 
  the 
  adjoining 
  

   smooth 
  acetabular 
  part 
  of 
  the 
  os 
  pubis, 
  than 
  which 
  it 
  is 
  slightly 
  wider. 
  

   The 
  distal 
  end 
  of 
  the 
  ischium 
  is 
  thin, 
  and 
  quite 
  unlike 
  that 
  of 
  the 
  os 
  

   pubis; 
  it 
  is 
  devoid 
  of 
  indications 
  of 
  symphysial 
  union 
  with 
  its 
  fellow 
  

   bone 
  of 
  the 
  other 
  side 
  ; 
  and 
  since 
  it 
  is 
  nearly 
  perfect 
  it 
  may 
  be 
  re- 
  

   garded 
  as 
  certain 
  that 
  no 
  such 
  union 
  was 
  ever 
  present. 
  The 
  posterior 
  

   border, 
  stout 
  and 
  rounded, 
  forms 
  a 
  large 
  simple 
  curve 
  from 
  end 
  to 
  end. 
  

   It 
  is 
  the 
  stoutest 
  part 
  of 
  the 
  bone, 
  attaining 
  its 
  greatest 
  thickness 
  

   iii 
  the 
  iliac 
  process, 
  and 
  decreasing 
  from 
  this 
  towards 
  the 
  distal 
  end. 
  

   The 
  anterior 
  border, 
  much 
  less 
  stout 
  than 
  the 
  posterior, 
  has 
  near 
  

   the 
  acetabular 
  end 
  a 
  flat 
  straight 
  surface 
  by 
  which 
  it 
  was 
  connected 
  

   with 
  the 
  os 
  pubis. 
  In 
  the 
  rest 
  of 
  its 
  extent 
  it 
  is 
  non-articular. 
  

   A 
  change 
  in 
  the 
  direction 
  of 
  its 
  surfaces 
  towards 
  its 
  ends 
  gives 
  the 
  

   ischium 
  the 
  illusive 
  appearance 
  of 
  a 
  twist. 
  At 
  the 
  ripper 
  end 
  a 
  line 
  

   drawn 
  across 
  the 
  bone 
  in 
  the 
  direction 
  this 
  is 
  thought 
  to 
  have 
  had 
  

   in 
  the 
  articulated 
  skeleton 
  would 
  be 
  approximately 
  parallel 
  to 
  tho 
  

   vertebral 
  column, 
  whilst 
  a 
  second 
  line 
  drawn 
  across 
  the 
  surface 
  near 
  

   the 
  lower 
  or 
  distal 
  end 
  of 
  the 
  bone 
  would 
  cut 
  the 
  first 
  line 
  at 
  a 
  small 
  

   angle. 
  

  

  The 
  cortical 
  bony 
  tissue 
  of 
  the 
  os 
  pubis 
  and 
  ischium 
  is 
  compact, 
  

   and 
  its 
  external 
  surface 
  is 
  smooth 
  ; 
  but 
  these 
  characters 
  are 
  much 
  

   less 
  pronounced 
  than 
  in 
  the 
  chambered 
  thoracic 
  and 
  cervical 
  ver- 
  

   tebras, 
  and 
  the 
  cancellous 
  tissue 
  makes 
  no 
  approach 
  to 
  the 
  megacel- 
  

   lular 
  texture 
  so 
  conspicuous 
  in 
  them. 
  

  

  A 
  moment's 
  comparison 
  of 
  the 
  side 
  views 
  of 
  the 
  pelves 
  of 
  Iguauo- 
  

   (fo)i* 
  and 
  Orniihopsis 
  (PI. 
  XIY. 
  fig. 
  1) 
  will 
  suffice 
  to 
  show 
  how 
  widely 
  

   different 
  are 
  the 
  form 
  and 
  arrangement 
  of 
  the 
  constituent 
  bones 
  of 
  the 
  

   os 
  innominatum. 
  In 
  Iguanodon 
  the 
  ischium 
  and 
  the 
  long, 
  slender, 
  

   rod-like 
  part 
  of 
  the 
  os 
  pubis 
  (post-pubis 
  as 
  Prof. 
  0. 
  C. 
  Marsh 
  terms 
  

   it), 
  which 
  is 
  the 
  homologue 
  of 
  the 
  os 
  pubis 
  of 
  OrnitTiapsis, 
  are 
  much 
  

   more 
  slender 
  than 
  the 
  broad, 
  flattened, 
  plate-like 
  form 
  of 
  the 
  same 
  

   bones 
  in 
  this 
  latter. 
  In 
  Iguanodon 
  they 
  are 
  placed 
  parallel 
  to 
  each 
  

   other, 
  whereas 
  in 
  OrnitTiopsis 
  their 
  distal 
  ends 
  are 
  widely 
  separated. 
  

   The 
  ischium 
  of 
  Ornithopsis 
  in 
  those 
  respects 
  in 
  which 
  it 
  differs 
  

   greatly 
  from 
  that 
  of 
  Iguanodon 
  roughly 
  resembles 
  that 
  of 
  Megalo- 
  

   saurus. 
  It 
  was 
  this 
  resemblance 
  which 
  chiefly 
  guided 
  me 
  in 
  1873 
  

   in 
  my 
  determination 
  of 
  the 
  skeletal 
  position 
  of 
  the 
  bone. 
  

  

  It 
  is, 
  however, 
  in 
  the 
  pelvis 
  of 
  Ceteosaurus 
  oxoniensis 
  amongst 
  

   •British 
  Sauropsida 
  that 
  the 
  strongest 
  resemblance 
  to 
  that 
  of 
  Orni- 
  

   thopsis 
  is 
  to 
  be 
  found. 
  The 
  similarity 
  of 
  their 
  os 
  pubis 
  and 
  ischium 
  

   is 
  so 
  evident 
  as 
  to 
  need 
  no 
  comment 
  other 
  than 
  that 
  it 
  is 
  an 
  addi- 
  

   tional 
  evidence 
  of 
  their 
  affinity, 
  to 
  which 
  in 
  1871 
  I 
  called 
  attention 
  

   upon 
  the 
  evidence 
  of 
  their 
  vertebral 
  remains. 
  But 
  a 
  still 
  closer 
  re- 
  

   semblance 
  is 
  to 
  be 
  found 
  in 
  the 
  pelvis 
  of 
  Atlantosaurus 
  immanis. 
  If 
  

   we 
  exclude 
  some 
  very 
  trivial 
  details, 
  the 
  figure 
  of 
  this 
  given 
  by 
  Prof. 
  

   0. 
  C. 
  Marsh, 
  in 
  his 
  ' 
  Principal 
  Characters 
  of 
  American 
  Dinosauria; 
  

   shows, 
  as 
  regards 
  the 
  os 
  pubis 
  and 
  ischium, 
  an 
  extremely 
  close 
  agree- 
  

   ment 
  with 
  those 
  of 
  the 
  Wealden 
  Saurian 
  (PI. 
  XIV. 
  fig. 
  2). 
  

  

  In 
  the 
  similarity 
  of 
  the 
  constituents 
  of 
  its 
  haunch-bone 
  to 
  those 
  

   * 
  Quart. 
  .Toum, 
  Geol, 
  >So<\ 
  vol. 
  xxxii. 
  p, 
  365, 
  fig. 
  1. 
  

  

  