﻿178 
  MR. 
  C. 
  W. 
  ANDREWS 
  ON 
  THE 
  STRUCTURE 
  [May 
  1897,. 
  

  

  being 
  circular 
  in 
  section 
  instead 
  of 
  trihedral. 
  They, 
  however, 
  show 
  

   a 
  distinct 
  tendency 
  towards 
  this 
  typical 
  form 
  in 
  the 
  absence, 
  or 
  at 
  

   least 
  scanty 
  development, 
  of 
  ridges 
  on 
  their 
  outer 
  surface 
  ; 
  and 
  since 
  

   the 
  skull 
  and 
  skeleton 
  of 
  the 
  Oxfordian 
  and 
  Kimeridgian 
  forms, 
  

   so 
  far 
  as 
  known, 
  are 
  closely 
  similar, 
  I 
  prefer, 
  for 
  the 
  present 
  at 
  

   least, 
  to 
  follow 
  the 
  British 
  Museum 
  Catalogue 
  in 
  referring 
  both 
  ta 
  

   one 
  genus 
  — 
  Pliosaurus. 
  

  

  Description 
  of 
  the 
  Skull. 
  

  

  Most 
  unfortunately, 
  this 
  skull 
  was 
  not 
  found 
  associated 
  with 
  any 
  

   other 
  portions 
  of 
  the 
  skeleton, 
  even 
  the 
  mandible 
  being 
  entirely 
  

   absent. 
  As 
  already 
  mentioned, 
  it 
  is 
  much 
  crushed, 
  nevertheless 
  

   all 
  the 
  more 
  important 
  points 
  in 
  the 
  structure 
  can 
  be 
  made 
  outr 
  

   A 
  number 
  of 
  the 
  characteristic 
  teeth, 
  beautifully 
  preserved, 
  occurred 
  

   in 
  close 
  proximity 
  to 
  the 
  skull, 
  but 
  only 
  one 
  or 
  two 
  broken 
  bases- 
  

   remain 
  actually 
  in 
  the 
  alveoli. 
  

  

  The 
  basi-occipital 
  (PI. 
  XII. 
  fig. 
  1, 
  oc.c.) 
  is 
  very 
  similar 
  to 
  

   that 
  of 
  Peloneustes, 
  but 
  the 
  condyle 
  is 
  relatively 
  rather 
  larger, 
  and 
  

   is 
  also 
  rounder 
  and 
  more 
  prominent 
  ; 
  it 
  has 
  a 
  w 
  r 
  ell-marked 
  dimple- 
  

   like 
  depression 
  marking 
  the 
  former 
  position 
  of 
  the 
  notochord. 
  The 
  

   arrangement 
  of 
  the 
  very 
  massive 
  ventro-lateral 
  processes 
  is 
  the 
  same 
  

   as 
  in 
  the 
  smaller 
  form 
  ; 
  in 
  the 
  present 
  specimen 
  the 
  pterygoids, 
  

   which 
  terminate 
  posteriorly 
  in 
  a 
  strong 
  transverse 
  ridge, 
  have 
  been 
  

   crushed 
  up 
  upon 
  them. 
  The 
  surfaces 
  for 
  union 
  with 
  the 
  exoccipital 
  

   look 
  upward 
  and 
  somewhat 
  outward 
  ; 
  they 
  are 
  separated 
  by 
  a 
  

   narrow 
  interval 
  only. 
  

  

  The 
  exoccipitals 
  (fig. 
  2, 
  p. 
  181, 
  ex.oc), 
  which 
  form 
  the 
  sides 
  

   of 
  the 
  foramen 
  magnum, 
  are 
  stout 
  solid 
  bones, 
  the 
  upper 
  ends 
  of 
  

   which 
  have 
  been 
  broken 
  away 
  by 
  the 
  crushing 
  down 
  of 
  the 
  dorsal 
  

   surface 
  of 
  the 
  skull. 
  Laterally 
  they, 
  or, 
  more 
  correctly 
  speaking, 
  the 
  

   opisthotic 
  elements 
  fused 
  with 
  them, 
  bear 
  extremely 
  long 
  (15 
  cm.) 
  

   paroccipital 
  processes 
  (fig. 
  2,p.oc). 
  The 
  distal 
  extremity 
  of 
  these 
  

   is 
  somewhat 
  expanded 
  into 
  a 
  spathulate 
  form, 
  the 
  outer 
  surface 
  of 
  

   the 
  expanded 
  portion 
  being 
  flattened 
  and 
  rugose. 
  This 
  surface 
  

   appears 
  to 
  have 
  formed 
  a 
  close 
  union 
  with 
  the 
  inner 
  surface 
  of 
  the 
  

   anteriorly-directed 
  process 
  of 
  the 
  quadrate, 
  by 
  which 
  that 
  bone 
  

   unites 
  with 
  the 
  posterior 
  pterygoid 
  ramus 
  ; 
  possibly, 
  however, 
  a 
  

   portion 
  of 
  the 
  overlapping 
  ' 
  squamosal 
  ' 
  may 
  intervene 
  to 
  some 
  

   extent 
  between 
  the 
  outer 
  end 
  of 
  the 
  paroccipital 
  process 
  and 
  the 
  

   quadrate. 
  

  

  The 
  supra-occipital 
  is 
  too 
  much 
  crushed 
  for 
  description, 
  and 
  

   the 
  basisphenoid 
  is 
  completely 
  concealed 
  above 
  by 
  the 
  depressed 
  

   roof 
  of 
  the 
  skull 
  and 
  ventrally 
  by 
  the 
  pterygoids. 
  

  

  In 
  front 
  of 
  the 
  basisphenoid, 
  the 
  ventral 
  axis 
  of 
  the 
  skull 
  is 
  

   prolonged 
  forwards 
  by 
  the 
  bone 
  which, 
  for 
  reasons 
  given 
  elsewhere, 
  

   I 
  regard 
  as 
  the 
  parasphenoid 
  (fig. 
  1, 
  p. 
  179, 
  pas.). 
  The 
  hinder 
  

   portion 
  of 
  this 
  is 
  partly 
  concealed 
  by 
  the 
  pterygoids, 
  and 
  indeed 
  the 
  

   exact 
  line 
  of 
  division 
  between 
  the 
  median 
  expansion 
  of 
  these 
  bones 
  

   and 
  the 
  parasphenoid 
  is 
  not 
  in 
  this 
  case 
  to 
  be 
  made 
  out. 
  Anteriorly 
  

  

  