﻿80 
  ACTIXOPTEEYGII. 
  

  

  The 
  teeth 
  are 
  hollow 
  and 
  the 
  calcigerous 
  tubes 
  in 
  the 
  dentine 
  

   radiate 
  from 
  the 
  central 
  cavity. 
  The 
  cap 
  of 
  enamel 
  is 
  thick, 
  and 
  

   when 
  the 
  germ 
  of 
  the 
  successional 
  tooth 
  is 
  first 
  formed 
  it 
  lies 
  in 
  

   exactly 
  the 
  opposite 
  direction 
  to 
  that 
  of 
  the 
  functional 
  tooth, 
  thus 
  

   making 
  a 
  revolution 
  of 
  180° 
  while 
  the 
  root 
  of 
  the 
  old 
  tooth 
  is 
  

   absorbed 
  and 
  it 
  prepares 
  to 
  appear. 
  The 
  marginal 
  teeth, 
  forming 
  

   a 
  single 
  series 
  on 
  the 
  maxilla, 
  premaxilla, 
  and 
  dentary, 
  are 
  com- 
  

   paratively 
  small 
  and 
  prehensile 
  ; 
  those 
  within 
  are 
  more 
  robust 
  and 
  

   form 
  a 
  tritoral 
  pavement. 
  The 
  latter 
  are 
  arranged 
  with 
  more 
  or 
  

   less 
  regularity 
  on 
  the 
  coalesced 
  vomers, 
  the 
  pterygo-palatine 
  arcade, 
  

   and 
  the 
  splenial. 
  In 
  the 
  earlier 
  species 
  all 
  the 
  teeth 
  exhibit 
  

   comparatively 
  elongated 
  pedicles, 
  while 
  in 
  the 
  later 
  and 
  more 
  

   specialized 
  forms 
  the 
  inner 
  teeth 
  are 
  fixed 
  on 
  very 
  short 
  bases. 
  

  

  The 
  opercular 
  apparatus 
  is 
  complete 
  and 
  there 
  is 
  a 
  good 
  series 
  of 
  

   branchiostegal 
  rays, 
  the 
  uppermost 
  very 
  broad 
  ; 
  but 
  no 
  indications 
  

   of 
  a 
  gular 
  plate 
  have 
  yet 
  been 
  discovered. 
  The 
  last-named 
  fact 
  is 
  

   all 
  the 
  more 
  remarkable, 
  since 
  in 
  some 
  of 
  the 
  other 
  genera 
  of 
  

   Semionotidse 
  the 
  gular 
  plate 
  is 
  relatively 
  large 
  and 
  robust. 
  The 
  

   preoperculum 
  is 
  a 
  long 
  narrow 
  bone, 
  exposed 
  throughout 
  its 
  

   length 
  and 
  much 
  bent 
  forwards 
  below. 
  The 
  operculum 
  exhibits 
  

   a 
  sharp 
  elevation 
  on 
  its 
  inner 
  face 
  for 
  articulation 
  with 
  the 
  

   hyomandibular 
  process; 
  its 
  upper 
  border 
  is 
  overlapped 
  by 
  the 
  

   supratemporal 
  plates, 
  and 
  its 
  lower 
  border 
  deeply 
  overlaps 
  the 
  

   suboperculum. 
  The 
  latter 
  element 
  is 
  almost 
  sickle-shaped, 
  with 
  a 
  

   very 
  large 
  upwardly-directed 
  process 
  at 
  its 
  antero-superior 
  angle, 
  

   and 
  abruptly 
  truncated 
  in 
  front 
  for 
  union 
  with 
  the 
  large 
  elongate- 
  

   triangular 
  interoperculum. 
  

  

  The 
  branchial 
  arches 
  are 
  delicate 
  and 
  have 
  only 
  been 
  observed 
  in 
  

   a 
  fragmentary 
  state. 
  The 
  gill-filaments 
  are 
  shown 
  in 
  the 
  Oxfordian 
  

   species 
  to 
  have 
  been 
  supported 
  by 
  a 
  series 
  of 
  long 
  slender 
  rods, 
  

   apparently 
  quadrate 
  in 
  section, 
  and 
  each 
  serrated 
  on 
  one 
  margin; 
  

   the 
  gill-rakers 
  are 
  small, 
  stout, 
  and 
  pointed, 
  tipped 
  with 
  enamel, 
  

   more 
  or 
  less 
  falciform, 
  and 
  arranged 
  in 
  well-spaced 
  series. 
  

  

  The 
  endoskeleton 
  of 
  the 
  trunk 
  is 
  well 
  ossified, 
  but 
  owing 
  to 
  the 
  

   thickness 
  of 
  the 
  squamation 
  it 
  is 
  rarely 
  seen 
  and 
  is 
  as 
  yet 
  only 
  

   imperfectly 
  known. 
  The 
  notochord 
  is 
  persistent, 
  and 
  in 
  the 
  earlier 
  

   and 
  smaller 
  species 
  no 
  ossifications 
  have 
  been 
  observed 
  in 
  its 
  sheath 
  ; 
  

   but 
  in 
  the 
  large 
  Wealden, 
  and 
  perhaps 
  also 
  in 
  the 
  large 
  Oxfordian, 
  

   species 
  there 
  are 
  distinct 
  indications 
  of 
  ring-vertebrae, 
  at 
  least 
  in 
  

   the 
  abdominal 
  region. 
  The 
  rings 
  are 
  much 
  deeper 
  than 
  broad, 
  but 
  in 
  

   the 
  few 
  known 
  specimens 
  (e. 
  g. 
  nos. 
  2401, 
  P. 
  1124, 
  P. 
  6348 
  c) 
  their 
  

   precise 
  characters 
  are 
  obscure, 
  and 
  it 
  can 
  only 
  be 
  stated 
  that 
  each 
  

   ring 
  appears 
  to 
  consist 
  of 
  four 
  sectors, 
  the 
  lower 
  pair 
  bearing 
  short 
  

   processes 
  for 
  the 
  support 
  of 
  the 
  ribs. 
  The 
  right 
  and 
  left 
  halves 
  of 
  

  

  