﻿pycnodontid^:. 
  193 
  

  

  and 
  these 
  tesserae 
  frequently 
  fuse 
  together. 
  One 
  median 
  plate 
  thus 
  

   formed 
  extends 
  backwards 
  between 
  the 
  anterior 
  ends 
  of 
  the 
  frontals, 
  

   and 
  these 
  elements 
  are 
  sometimes 
  entirely 
  separated 
  by 
  it 
  ; 
  moreover, 
  

   this 
  median 
  series 
  is 
  even 
  continued 
  further 
  back 
  by 
  another 
  azygous 
  

   plate 
  (m.eth.) 
  which 
  meets 
  (or 
  is 
  fused 
  with) 
  the 
  supraoccipital. 
  The 
  

   arrangement 
  of 
  azygous 
  median 
  plates 
  in 
  the 
  cranial 
  roof 
  of 
  Mesturus 
  

   leedsi 
  is 
  thus 
  suggestive 
  of 
  the 
  corresponding 
  plates 
  in 
  Acipenseroid 
  

   fishes. 
  In 
  Coccodus 
  all 
  the 
  elements 
  of 
  the 
  cranial 
  roof 
  are 
  fused 
  

   into 
  one 
  continuous 
  shield. 
  

  

  The 
  cheek-plates 
  are 
  small, 
  thin, 
  and 
  apparently 
  without 
  any 
  

   very 
  definite 
  arrangement. 
  In 
  Mesturus 
  and 
  Gyrodus 
  one 
  plate 
  

   larger 
  than 
  the 
  remainder, 
  bounding 
  the 
  upper 
  jaw, 
  may 
  be 
  the 
  

   maxilla 
  ; 
  but 
  it 
  is 
  delicate 
  and 
  toothless. 
  The 
  premaxillse 
  are 
  a 
  

   pair 
  of 
  small, 
  deep 
  and 
  narrow 
  bones, 
  moderately 
  thick, 
  resting 
  

   upon 
  the 
  anterior 
  extremity 
  of 
  the 
  vomer, 
  and 
  covered 
  with 
  the 
  

   ethmoidal 
  dermal 
  plates. 
  The 
  postero-external 
  border 
  of 
  each 
  is 
  

   notched, 
  as 
  if 
  for 
  the 
  opening 
  of 
  the 
  nasal 
  capsule. 
  The 
  sclerotic 
  

   capsule 
  is 
  often 
  ossified. 
  

  

  The 
  hyomandibular 
  is 
  a 
  long 
  narrow 
  lamina 
  about 
  twice 
  as 
  

   broad 
  above 
  as 
  in 
  its 
  lower 
  half, 
  with 
  a 
  posterior 
  process 
  near 
  its 
  

   upper 
  end 
  for 
  the 
  support 
  of 
  the 
  operculum 
  (PI. 
  XVI. 
  fig. 
  3, 
  Tim.) 
  ; 
  

   but 
  nothing 
  is 
  known 
  of 
  its 
  connections 
  below. 
  The 
  pterygo-palatine 
  

   arcade 
  is 
  delicate 
  and 
  toothless, 
  and 
  is 
  shown 
  both 
  in 
  Anomoeodus 
  

   willetti 
  and 
  Mesturus 
  leedsi 
  to 
  have 
  been 
  directly 
  fused 
  for 
  a 
  

   considerable 
  distance 
  with 
  the 
  edge 
  of 
  the 
  basicranial 
  axis. 
  The 
  

   thickened 
  articular 
  end 
  of 
  the 
  quadrate 
  is 
  turned 
  forwards 
  so 
  that 
  

   the 
  facette 
  for 
  the 
  mandible 
  is 
  in 
  a 
  vertical 
  plane 
  ; 
  and 
  this 
  

   facette 
  is 
  irregularly 
  oval, 
  deeper 
  than 
  broad, 
  and 
  slightly 
  concave. 
  

   The 
  free 
  border 
  of 
  the 
  ectopterygoid 
  is 
  concavely 
  arched; 
  and 
  

   there 
  is 
  not 
  much 
  doubt 
  that 
  a 
  distinct 
  symplectic 
  element 
  was 
  

   apposed 
  to 
  the 
  hinder 
  margin 
  of 
  the 
  quadrate, 
  though 
  this 
  still 
  

   remains 
  to 
  be 
  definitely 
  proved. 
  The 
  two 
  rami 
  of 
  the 
  mandible 
  are 
  

   separate 
  at 
  the 
  symphysis, 
  and 
  in 
  Mesturus 
  leedsi 
  each 
  is 
  observed 
  

   to 
  consist 
  of 
  five 
  elements. 
  The 
  proximal 
  end 
  of 
  the 
  meckelian 
  

   cartilage 
  is 
  distinctly 
  ossified 
  as 
  an 
  articular 
  bone 
  ; 
  and 
  there 
  is 
  a 
  

   very 
  large 
  splenial 
  element 
  within, 
  while 
  three 
  more 
  delicate 
  splints 
  

   appear 
  without. 
  The 
  splenial 
  is 
  much 
  the 
  largest 
  and 
  stoutest 
  

   bone 
  in 
  the 
  jaw, 
  meeting 
  its 
  fellow 
  of 
  the 
  opposite 
  side 
  at 
  the 
  

   symphysis 
  and 
  well 
  covered 
  with 
  a 
  pavement 
  of 
  teeth 
  on 
  its 
  oral 
  

   face. 
  It 
  is 
  exposed 
  on 
  the 
  outer 
  aspect 
  of 
  the 
  entire 
  margin 
  of 
  the 
  

   ramus 
  behind 
  the 
  short 
  oral 
  border 
  of 
  the 
  dentary, 
  and 
  rises 
  pos- 
  

   teriorly 
  into 
  a 
  large 
  coronoid 
  process. 
  The 
  dentary 
  is 
  comparatively 
  

   insignificant, 
  thin, 
  deep 
  and 
  narrow, 
  meeting 
  its 
  fellow 
  at 
  the 
  

  

  PAET 
  III. 
  o 
  

  

  