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  ACTINOPTERYGII. 
  

  

  a 
  long 
  curved 
  maxilla 
  (PL 
  I. 
  figs. 
  2, 
  3, 
  mx.\ 
  deep 
  behind 
  and 
  much 
  

   contracted 
  anteriorly, 
  meeting 
  its 
  fellow 
  of 
  the 
  opposite 
  side 
  in 
  

   front 
  in 
  a 
  distinct 
  symphysis. 
  Another 
  small 
  membrane-bone, 
  

   abutting 
  upon 
  the 
  postero-superior 
  margin 
  of 
  the 
  maxilla, 
  may 
  be 
  

   regarded 
  as 
  the 
  jugal 
  (PL 
  I. 
  fig. 
  3, 
  ju.). 
  The 
  mandible 
  exhibits 
  

   three 
  bones, 
  the 
  dentary 
  being 
  much 
  the 
  largest, 
  the 
  articular 
  and 
  

   angular 
  insigDificant. 
  The 
  dentary 
  is 
  somewhat 
  arched, 
  the 
  inferior 
  

   margin 
  being 
  concave 
  ; 
  but 
  there 
  is 
  scarcely 
  any 
  tapering 
  at 
  the 
  

   symphysis, 
  and 
  the 
  coronoid 
  region 
  is 
  not 
  elevated. 
  The 
  ossified, 
  

   hour-glass-shaped 
  ceratohyal 
  is 
  frequently 
  shown 
  (PL 
  I. 
  fig. 
  2, 
  ch.), 
  

   but 
  nothing 
  further 
  is 
  known 
  of 
  the 
  hyoid 
  apparatus. 
  

  

  The 
  operculum 
  is 
  relatively 
  small 
  and 
  shaped 
  almost 
  like 
  an 
  

   inverted 
  comma 
  with 
  the 
  tail 
  pointing 
  forwards 
  and 
  upwards 
  

  

  Pig. 
  4. 
  

  

  Cranial 
  roof 
  of 
  Chondrosteus 
  acipenseroidcs 
  (after 
  Traquair). 
  From 
  specimen 
  

   in 
  Museum 
  of 
  Practical 
  Geology. 
  

   /., 
  frontal; 
  op., 
  operculum; 
  p., 
  parietal; 
  p.f., 
  post-frontal; 
  p.t, 
  post- 
  

   temporal 
  ; 
  s.t., 
  supra-temporal 
  ; 
  sg., 
  squamosal. 
  

  

  (figs. 
  3, 
  4, 
  6, 
  op.). 
  The 
  suboperculum 
  (s.op.) 
  is 
  much 
  broader 
  and 
  

   almost 
  quadrate, 
  with 
  produced 
  antero-superior 
  angle. 
  About 
  ten 
  

   branchiostegal 
  rays 
  (br.) 
  can 
  be 
  counted, 
  the 
  uppermost 
  broad 
  and 
  

   the 
  lowermost 
  narrower 
  ; 
  but 
  there 
  are 
  no 
  indications 
  of 
  a 
  gular 
  

   plate. 
  

  

  The 
  notochord 
  must 
  have 
  been 
  persistent, 
  with 
  no 
  calcifications 
  

  

  