﻿426 
  L. 
  C. 
  MIALL 
  ON 
  THE 
  REMAINS 
  OE 
  LABYRINTH.ODONTA 
  

  

  ternal 
  or 
  splenial 
  plate 
  has 
  suffered 
  injury, 
  and 
  it 
  is 
  impossible 
  to 
  

   trace 
  all 
  the 
  details 
  satisfactorily. 
  The 
  course 
  of 
  the 
  angulari-arti- 
  

   cular 
  suture, 
  in 
  particular, 
  is 
  uncertain. 
  But 
  one 
  interesting 
  fact 
  is 
  

   clear. 
  The 
  strong 
  internal 
  buttress, 
  which 
  in 
  Mastodonsaurus 
  sup- 
  

   ports 
  the 
  inwardly 
  produced 
  part 
  of 
  the 
  condyle, 
  is 
  entirely 
  wanting 
  

   in 
  the 
  present 
  fossil, 
  which 
  presents 
  in 
  its 
  place 
  a 
  smooth 
  vertical 
  

   plate, 
  slightly 
  concave 
  from 
  above 
  downwards, 
  and 
  differing 
  in 
  no 
  

   respect 
  from 
  the 
  adjacent 
  splenial 
  tract. 
  The 
  superior, 
  or 
  dentary, 
  

   surface 
  of 
  the 
  ramus 
  presents 
  equally 
  striking 
  characteristics. 
  There 
  

   is 
  a 
  postarticular 
  process 
  (Pt.Art.) 
  about 
  2 
  in. 
  long 
  when 
  perfect. 
  Its 
  

   upper 
  surface 
  is 
  triangular, 
  the 
  base 
  being 
  adjacent 
  to 
  the 
  condyle, 
  

   and 
  the 
  apex 
  occupying 
  the 
  posterior 
  extremity 
  of 
  the 
  ramus. 
  The 
  

   process 
  does 
  not 
  fall 
  away 
  on 
  its 
  inner 
  side, 
  as 
  in 
  Mastodonsaurus, 
  

   but 
  is 
  bounded, 
  like 
  the 
  external 
  margin, 
  by 
  a 
  ridge 
  of 
  bone 
  ; 
  it 
  

   presents, 
  consequently, 
  upon 
  its 
  upper 
  surface 
  a 
  cup 
  enclosed 
  by 
  

   three 
  ridges, 
  of 
  which 
  the 
  anterior 
  (the 
  ascending 
  condylar 
  process) 
  

   is 
  the 
  highest. 
  The 
  outer 
  ridge 
  is 
  convex 
  upwards 
  ; 
  behind, 
  it 
  de- 
  

   scends 
  upon 
  the 
  external 
  surface 
  of 
  the 
  ramus, 
  and 
  terminates 
  

   without 
  quite 
  reaching 
  the 
  extremity 
  of 
  the 
  postarticular 
  process. 
  

   The 
  inner 
  ridge 
  is 
  concave 
  from 
  before 
  backwards, 
  and 
  extends 
  

   further 
  back 
  than 
  the 
  other. 
  The 
  cavity 
  enclosed 
  by 
  these 
  ridges 
  

   bears 
  much 
  resemblance 
  to 
  a 
  true 
  articular 
  cavity; 
  and 
  I 
  have 
  

   hesitated 
  as 
  to 
  whether 
  such 
  might 
  not 
  be 
  its 
  true 
  character. 
  Two 
  

   considerations, 
  both 
  influential, 
  oppose 
  this 
  view. 
  First, 
  displace- 
  

   ment 
  of 
  the 
  quadrate 
  either 
  backwards, 
  forwards, 
  or 
  inwards 
  would 
  

   be 
  easy 
  ; 
  secondly, 
  the 
  homologous 
  part 
  of 
  the 
  mandible 
  in 
  Masto- 
  

   donsaurus, 
  Trematosaur-us, 
  and 
  several 
  other 
  Labyrinthodont 
  genera 
  

   is 
  not 
  articular 
  but 
  postarticular. 
  "What 
  I 
  take 
  for 
  the 
  true 
  articular 
  

   cavity 
  of 
  Diadetognathus 
  is 
  the 
  space 
  immediately 
  in 
  front 
  of 
  the 
  as- 
  

   cending 
  condylar 
  process 
  (Art.). 
  The 
  posterior 
  wall 
  of 
  this 
  space 
  is 
  

   formed 
  by 
  the 
  anterior 
  face 
  of 
  the 
  ascending 
  condylar 
  process, 
  which 
  

   is 
  about 
  \ 
  in. 
  high 
  ; 
  it 
  is 
  concave 
  forwards 
  at 
  its 
  base, 
  while 
  above 
  it 
  

   inclines 
  very 
  slightly 
  forwards. 
  Transversely 
  it 
  is 
  nearly 
  straight, 
  

   so 
  that 
  on 
  the 
  whole 
  the 
  cavity 
  has 
  the 
  form 
  of 
  a 
  segment 
  of 
  a 
  

   cylinder, 
  or 
  more 
  exactly, 
  of 
  the 
  impression 
  of 
  such 
  a 
  solid 
  figure. 
  

   The 
  surface 
  is 
  so 
  placed 
  as 
  to 
  resist 
  backward 
  and 
  nearly 
  horizontal 
  

   pressure. 
  In 
  front 
  of 
  the 
  articular 
  surface 
  is 
  seen 
  the 
  commence- 
  

   ment 
  of 
  two 
  ridges 
  of 
  bone, 
  both 
  of 
  which 
  are 
  furnished 
  by 
  the 
  arti- 
  

   cular 
  element. 
  The 
  outer 
  ridge 
  passes 
  straight 
  forwards, 
  gradually 
  

   expanding 
  in 
  width 
  until, 
  at 
  a 
  distance 
  of 
  4 
  in. 
  from 
  the 
  ascending 
  

   condylar 
  process, 
  it 
  is 
  f 
  in. 
  broad. 
  It 
  is 
  at 
  first 
  convex 
  upwards 
  ; 
  

   but 
  | 
  in. 
  from 
  the 
  ascending 
  condylar 
  process 
  a 
  groove 
  appears 
  upon 
  

   its 
  upper 
  surface. 
  The 
  groove 
  increases 
  in 
  width 
  with 
  the 
  ridge, 
  

   and 
  ultimately 
  occupies 
  nearly 
  the 
  whole 
  of 
  its 
  transverse 
  extent. 
  

   It 
  may 
  have 
  received 
  the 
  posterior 
  and 
  unopposed 
  part 
  of 
  the 
  maxil- 
  

   lary 
  series 
  of 
  the 
  teeth. 
  The 
  inner 
  of 
  the 
  two 
  ridges 
  diverges 
  in- 
  

   wards, 
  and 
  ends 
  by 
  fracture 
  almost 
  immediately. 
  It 
  was 
  originally 
  

   continuous 
  with 
  a 
  similar 
  ridge 
  which 
  now 
  projects 
  inwards 
  from 
  

   the 
  upper 
  edge 
  of 
  the 
  articular 
  bone, 
  above 
  the 
  internal 
  mandibular 
  

   foramen. 
  It 
  is 
  better 
  seen 
  in 
  the 
  following 
  fossil 
  (No. 
  9). 
  These 
  

   two 
  ridges 
  are 
  the 
  upper 
  edges 
  of 
  the 
  outer 
  and 
  inner 
  wall 
  of 
  the 
  

  

  