﻿190 
  ACTINOPTERYGII. 
  

  

  the 
  teeth 
  apparently 
  without 
  vertical 
  successors. 
  Notochord 
  per- 
  

   sistent, 
  destitute 
  of 
  ossifications 
  in 
  its 
  sheath, 
  Fin-rays 
  robust, 
  

   the 
  majority 
  well-spaced 
  and 
  articulated 
  ; 
  fulcra 
  absent, 
  except 
  

   perhaps 
  quite 
  at 
  the 
  base 
  of 
  the 
  caudal 
  fin 
  ; 
  dorsal 
  and 
  anal 
  fins 
  

   more 
  or 
  less 
  extended. 
  Scales 
  rhombic 
  when 
  present, 
  frequently 
  

   wanting 
  on 
  the 
  whole 
  or 
  part 
  of 
  the 
  caudal 
  region 
  ; 
  almost 
  in- 
  

   variably 
  strengthened 
  by 
  the 
  inner 
  rib 
  on 
  their 
  anterior 
  margin 
  

   and 
  united 
  by 
  a 
  peg-a.nd-socket 
  articulation 
  in 
  connection 
  there- 
  

   with. 
  

  

  The 
  cranial 
  osteology 
  of 
  the 
  Pycnodont 
  fishes 
  is 
  as 
  yet 
  very 
  

   imperfectly 
  known. 
  The 
  little 
  information 
  available 
  suggests 
  that 
  

   there 
  was 
  as 
  much 
  variation 
  in 
  this 
  group 
  as 
  in 
  certain 
  aberrant 
  

   tribes 
  of 
  modern 
  fishes, 
  such 
  as 
  the 
  Plectognaths 
  and 
  Gymnodonts 
  ; 
  

   and 
  in 
  any 
  general 
  description 
  it 
  is 
  therefore 
  necessary 
  to 
  state 
  

   precisely 
  in 
  which 
  genus 
  or 
  genera 
  several 
  of 
  the 
  characters 
  have 
  

   been 
  observed. 
  When 
  this 
  statement 
  is 
  not 
  made 
  in 
  the 
  following 
  

   account, 
  the 
  character 
  in 
  question 
  has 
  been 
  met 
  with 
  very 
  widely 
  

   and 
  may 
  be 
  regarded 
  as 
  normal 
  in 
  the 
  family. 
  The 
  best-preserved 
  

   specimens 
  hitherto 
  obtained 
  are 
  those 
  of 
  Mesturus 
  leedsi 
  from 
  the 
  

   Oxford 
  Clay 
  of 
  Peterborough. 
  The 
  laterally-compressed 
  examples 
  

   of 
  Gyrodus 
  and 
  Mesodon, 
  from 
  the 
  Lithographic 
  Stone 
  of 
  Bavaria 
  and 
  

   Prance, 
  are 
  also 
  important 
  ; 
  while 
  the 
  unique 
  specimen 
  of 
  Ano- 
  

   moeodus 
  tuilletti 
  from 
  the 
  Sussex 
  Chalk 
  (see 
  p. 
  263) 
  reveals 
  for 
  the 
  

   first 
  time 
  the 
  pterygo-palatine 
  arcade. 
  

  

  The 
  chondrocranium 
  is 
  well 
  ossified 
  and 
  always 
  completely 
  

   covered 
  with 
  membrane-bones, 
  there 
  being 
  no 
  vacuities 
  in 
  the 
  

   cranial 
  roof. 
  The 
  basicranial 
  axis 
  is 
  more 
  or 
  less 
  parallel 
  with 
  the 
  

   roof 
  behind 
  the 
  angulation 
  of 
  the 
  frontal 
  profile, 
  and 
  is 
  then 
  bent 
  

   sharply 
  downwards 
  to 
  the 
  den 
  tiger 
  ous 
  oral 
  face 
  of 
  the 
  vomer 
  ; 
  while 
  

   the 
  latter 
  is 
  finally 
  directed 
  in 
  a 
  plane 
  almost 
  parallel 
  with 
  the 
  

   first. 
  The 
  axis 
  of 
  the 
  skull 
  as 
  a 
  whole 
  is 
  directed 
  downwards 
  

   and 
  forwards, 
  making 
  a 
  considerable 
  angle 
  with 
  the 
  axis 
  of 
  the 
  

   trunk. 
  

  

  Of 
  the 
  chondrocranial 
  elements, 
  only 
  the 
  postfrontal 
  (sphenotic), 
  

   mesethmoidal, 
  and 
  supposed 
  basisphenoidal 
  bones 
  have 
  hitherto 
  

   been 
  clearly 
  observed. 
  The 
  postfrontal 
  in 
  Mesturus 
  is 
  small 
  and 
  

   does 
  not 
  appear 
  on 
  the 
  cranial 
  roof; 
  nor 
  is 
  there 
  any 
  external 
  

   ornamented 
  plate 
  fused 
  with 
  its 
  outer 
  face. 
  Its 
  lower 
  extremity 
  

   meets 
  the 
  basipterygoid 
  process 
  of 
  the 
  parasphenoid 
  in 
  a 
  deeply 
  

   interdigitating 
  suture 
  ; 
  while 
  its 
  posterior 
  outer 
  face 
  takes 
  part 
  in 
  

   the 
  facette 
  for 
  the 
  upper 
  end 
  of 
  the 
  hyomandibular. 
  The 
  meseth- 
  

   moid 
  is 
  always 
  conspicuous 
  as 
  a 
  deep 
  thin 
  lamina 
  extending 
  forwards 
  

   from 
  the 
  anterior 
  border 
  of 
  the 
  orbital 
  cavity 
  and 
  articulating 
  below 
  

  

  