﻿A 
  PLESIOSATTRIAN 
  FROM 
  THE 
  OXFORD 
  CLAY. 
  203 
  

  

  mina 
  are 
  very 
  much 
  smaller 
  than 
  in 
  the 
  early 
  part 
  of 
  the 
  neck, 
  in 
  

   which 
  there 
  is 
  only 
  one 
  pair. 
  In 
  length 
  from 
  back 
  to 
  front 
  the 
  

   centrum 
  is 
  diminished 
  to 
  rather 
  less 
  than 
  2 
  inches 
  ; 
  its 
  width 
  from 
  

   side 
  to 
  side 
  is 
  increased 
  to 
  2% 
  inches, 
  while 
  from 
  above 
  downward 
  

   the 
  centrum 
  measures 
  2\ 
  inches. 
  

  

  Pectoral 
  Vertebrae. 
  

  

  In 
  these 
  vertebrae 
  the 
  face 
  of 
  the 
  centrum 
  is 
  somewhat 
  larger 
  

   than 
  in 
  the 
  terminal 
  neck-vertebrae, 
  and 
  has 
  a 
  small 
  central 
  pit 
  ; 
  

   the 
  articulation 
  for 
  the 
  rib 
  is 
  long 
  and 
  oblique, 
  formed 
  partly 
  by 
  the 
  

   neural 
  arch, 
  partly 
  by 
  the 
  centrum 
  ; 
  it 
  is 
  longest 
  in 
  the 
  middle 
  

   pectoral, 
  broadest 
  in 
  the 
  third. 
  The 
  under 
  surface 
  of 
  the 
  centrum 
  

   is 
  rounded 
  from 
  side 
  to 
  side 
  in 
  the 
  first, 
  flatter 
  in 
  the 
  second, 
  

   while 
  in 
  the 
  third 
  it 
  is 
  so 
  flat 
  on 
  the 
  under 
  surface 
  as 
  almost 
  to 
  

   make 
  an 
  angle 
  with 
  the 
  sides, 
  a 
  character 
  which 
  is 
  slightly 
  exag- 
  

   gerated 
  by 
  compression. 
  The 
  neural 
  arches 
  are 
  imperfectly 
  pre- 
  

   served, 
  but 
  appear 
  to 
  be 
  inclined 
  a 
  little 
  backward 
  as 
  in 
  the 
  last 
  

   cervical. 
  

  

  Dorsal 
  Vertebras. 
  

  

  The 
  neural 
  arch 
  is 
  not 
  well 
  preserved 
  in 
  the 
  dorsal 
  vertebrae 
  till 
  

   the 
  seventh 
  ; 
  and 
  none 
  of 
  the 
  neural 
  arches 
  of 
  the 
  dorsal 
  vertebrae 
  

   retain 
  their 
  transverse 
  processes 
  for 
  the 
  attachment 
  of 
  the 
  ribs. 
  The 
  

   centrum 
  is 
  modified 
  in 
  form, 
  owing 
  to 
  the 
  rib 
  being 
  raised 
  on 
  to 
  the 
  

   neural 
  arch 
  ; 
  it 
  is 
  flat 
  on 
  the 
  underside 
  from 
  back 
  to 
  front, 
  very 
  

   slightly 
  concave 
  there 
  from 
  side 
  to 
  side, 
  and 
  has 
  its 
  sides 
  con- 
  

   siderably 
  compressed, 
  and 
  shows 
  below 
  the 
  middle 
  on 
  each 
  side 
  a 
  

   large 
  nutritive 
  foramen. 
  What 
  was 
  in 
  the 
  cervical 
  vertebrae 
  the 
  

   pit 
  in 
  the 
  centre 
  of 
  the 
  articular 
  surface 
  of 
  the 
  centrum 
  now 
  becomes 
  

   slightly 
  elevated, 
  with 
  a 
  central 
  puncture, 
  in 
  some 
  respects 
  recalling 
  

   a 
  Pliosaurian 
  character. 
  The 
  zygapophyses 
  retain 
  the 
  singular 
  

   method 
  of 
  articulation 
  already 
  described 
  in 
  the 
  later 
  cervicals. 
  

   The 
  neural 
  spines 
  now, 
  however, 
  widen 
  in 
  antero-posterior 
  exten- 
  

   sion, 
  while 
  from 
  below 
  upward 
  they 
  are 
  extremely 
  compressed 
  from 
  

   side 
  to 
  side. 
  In 
  all 
  the 
  vertebrae 
  both 
  of 
  neck 
  and 
  back 
  the 
  neura- 
  

   pophyses 
  join 
  the 
  centrum 
  after 
  the 
  manner 
  characteristic 
  of 
  Plesio- 
  

   saurus, 
  and 
  show 
  no 
  approach 
  to 
  the 
  circular 
  pedicles 
  of 
  Pliosaurus. 
  

   The 
  transverse 
  processes 
  of 
  the 
  seventh 
  and 
  succeeding 
  vertebrae 
  

   are 
  directed 
  upward 
  and 
  outward 
  from 
  the 
  part 
  of 
  the 
  neural 
  arch 
  

   in 
  front 
  of 
  the 
  posterior 
  zygapophyses 
  ; 
  these 
  transverse 
  processes 
  are 
  

   compressed 
  from 
  above 
  downward 
  in 
  a 
  sigmoid 
  fold, 
  being 
  higher 
  

   in 
  front 
  than 
  behind, 
  and 
  convex 
  above 
  in 
  front, 
  and 
  concave 
  

   behind, 
  with 
  corresponding 
  folds 
  below. 
  

  

  The 
  tenth 
  dorsal 
  vertebra 
  has 
  the 
  centrum 
  much 
  as 
  in 
  the 
  ver- 
  

   tebrae 
  already 
  described, 
  except 
  that 
  the 
  circumference 
  of 
  the 
  arti- 
  

   cular 
  margin 
  of 
  the 
  centrum 
  is 
  now 
  becoming 
  much 
  sharper 
  and 
  

   losing 
  the 
  rounded 
  edge 
  seen 
  in 
  the 
  neck 
  ; 
  the 
  base 
  of 
  the 
  transverse 
  

   process 
  is 
  getting 
  shorter 
  from 
  back 
  to 
  front 
  ; 
  the 
  sides 
  of 
  the 
  

   neural 
  spine 
  are 
  subparallel 
  ; 
  it 
  is 
  L| 
  inch 
  wide, 
  truncated 
  as 
  usual 
  at 
  

   the 
  uppermost 
  end, 
  which 
  is 
  7 
  inches 
  from 
  the 
  base 
  of 
  the 
  centrum 
  ; 
  

  

  