﻿194 
  ACTINOPTERYGII. 
  

  

  symphysis, 
  and 
  bearing 
  a 
  row 
  of 
  prehensile 
  teeth. 
  An 
  angular 
  

   plate, 
  though 
  not 
  extending 
  upwards 
  quite 
  to 
  the 
  oral 
  border, 
  

   completes 
  the 
  posterior 
  part 
  of 
  the 
  outer 
  face 
  of 
  the 
  mandibular 
  

   ramus 
  ; 
  and 
  its 
  hinder 
  ascending 
  portion 
  meets 
  a 
  small 
  coronoid 
  

   bone, 
  with 
  which 
  it 
  frequently 
  becomes 
  fused. 
  The 
  hyoid 
  elements 
  

   are 
  not 
  satisfactorily 
  known, 
  though 
  in 
  Mesturus 
  verrucosus 
  a 
  short 
  

   and 
  deep 
  ceratohyal 
  with 
  a 
  small 
  hypohyal 
  is 
  evidently 
  observed 
  

   (PI. 
  XV. 
  fig. 
  1, 
  ch., 
  %.)• 
  

  

  The 
  opercular 
  and 
  branchiostegal 
  apparatus 
  is 
  remarkably 
  

   reduced. 
  The 
  operculum 
  is 
  relatively 
  small, 
  deep, 
  and 
  narrow, 
  

   truncated 
  above 
  where 
  it 
  meets 
  the 
  postero-lateral 
  edge 
  of 
  the 
  

   cranial 
  roof, 
  and 
  tapering 
  below 
  to 
  a 
  point. 
  This 
  bone 
  has 
  hitherto 
  

   been 
  named 
  " 
  supraclavicle," 
  but 
  its 
  true 
  nature 
  is 
  indicated 
  both 
  

   by 
  a 
  specimen 
  of 
  Gyrodus 
  heocagonus 
  (PI. 
  XYI. 
  fig. 
  3) 
  and 
  by 
  one 
  

   of 
  Mesturus 
  leedsi 
  (no. 
  P. 
  6834). 
  The 
  preoperculum 
  (commonly 
  

   termed 
  " 
  operculum 
  ") 
  is 
  very 
  large, 
  triangular 
  in 
  shape 
  and 
  much 
  

   expanded 
  below 
  ; 
  its 
  mode 
  of 
  attachment 
  to 
  the 
  mandibular 
  sus- 
  

   pensorium 
  is 
  indicated 
  in 
  the 
  specimens 
  both 
  of 
  Anomceodus 
  willetti 
  

   and 
  Gyrodus 
  heocagonus 
  already 
  referred 
  to. 
  The 
  suboperculum 
  

   and 
  interoperculum 
  are 
  wanting 
  ; 
  and 
  only 
  two 
  branchiostegal 
  

   rays 
  have 
  been 
  observed 
  immediately 
  below 
  the 
  preoperculum 
  in 
  

   Mesturus 
  leedsi 
  and 
  in 
  Gyrodus 
  frontatus 
  (PL 
  XYI. 
  fig. 
  2). 
  In 
  

   Gyrodus 
  and 
  Mesturus 
  the 
  space 
  between 
  the 
  rami 
  of 
  the 
  mandible 
  

   is 
  completely 
  covered 
  with 
  small 
  polygonal 
  tesserae, 
  there 
  being 
  no 
  

   gular 
  plate. 
  On 
  the 
  branchial 
  arches, 
  long 
  and 
  slender 
  calcified 
  

   gill-filaments, 
  more 
  or 
  less 
  denticulated, 
  are 
  often 
  observed. 
  

  

  Teeth 
  are 
  confined 
  to 
  the 
  vomer, 
  splenial, 
  premaxilla, 
  and 
  dentary. 
  

   They 
  are 
  all 
  hollow, 
  with 
  a 
  short 
  base 
  firmly 
  anchylosed 
  to 
  the 
  

   supporting 
  bone 
  ; 
  and 
  the 
  present 
  writer 
  has 
  never 
  observed 
  any 
  

   provision 
  for 
  replacement. 
  The 
  only 
  suspicion 
  of 
  such 
  an 
  arrange- 
  

   ment 
  is 
  afforded 
  by 
  the 
  Oxfordian 
  specimens 
  of 
  Mesturus 
  leedsi. 
  

   The 
  teeth 
  of 
  the 
  premaxilla 
  and 
  dentary 
  are 
  prehensile 
  and 
  arranged 
  

   in 
  a 
  single 
  series 
  ; 
  those 
  of 
  the 
  vomer 
  and 
  splenial 
  are 
  tritoral 
  and 
  

   form 
  an 
  extended 
  pavement. 
  Anteriorly 
  the 
  teeth 
  of 
  these 
  pave- 
  

   ments 
  are 
  nearly 
  equal 
  in 
  size 
  and 
  do 
  not 
  exhibit 
  any 
  very 
  definite 
  

   arrangement 
  ; 
  but 
  posteriorly 
  as 
  a 
  rule 
  they 
  soon 
  become 
  differ- 
  

   entiated 
  and 
  disposed 
  in 
  regular 
  longitudinal 
  series 
  of 
  more 
  or 
  less 
  

   different 
  sizes. 
  Appearances 
  suggest 
  that 
  the 
  splenial 
  and 
  vomerine 
  

   elements 
  are 
  continually 
  being 
  lengthened 
  by 
  growth 
  behind, 
  the 
  

   only 
  new 
  teeth 
  obtained 
  during 
  the 
  life 
  of 
  the 
  animal 
  being 
  those 
  

   added 
  in 
  this 
  manner. 
  If 
  so, 
  it 
  is 
  much 
  the 
  same 
  kind 
  of 
  growth 
  

   as 
  that 
  which 
  takes 
  place 
  in 
  the 
  dentition 
  of 
  the 
  Cochliodontidse 
  and 
  

   MyliobatidaB 
  among 
  Elasmobranchs 
  : 
  the 
  only 
  differences 
  being 
  due 
  

  

  