﻿INTHODTJCTION. 
  X1U 
  

  

  necessity, 
  and 
  not 
  a 
  character 
  of 
  taxonomic 
  importance. 
  The 
  

   absence 
  of 
  a 
  maxillary 
  dentition 
  is 
  not 
  remarkable, 
  considering 
  its 
  

   feebleness 
  in 
  the 
  species 
  of 
  Lepidotus 
  with 
  most 
  specialized 
  grinding- 
  

   teeth. 
  

  

  Secondly, 
  there 
  is 
  the 
  question 
  of 
  the 
  mode 
  of 
  growth 
  of 
  the 
  

   dentition. 
  There 
  may 
  sometimes 
  be 
  vertical 
  successional 
  teeth, 
  but 
  

   the 
  present 
  writer 
  has 
  never 
  observed 
  satisfactory 
  evidence 
  of 
  them 
  ; 
  

   even 
  if 
  occurring, 
  they 
  must 
  thus 
  be 
  extremely 
  unusual. 
  The 
  contrast 
  

   with 
  Lepidotus 
  is 
  indeed 
  very 
  marked 
  ; 
  but 
  the 
  loss 
  of 
  successional 
  

   teeth 
  in 
  the 
  higher 
  Yertebrata 
  (e. 
  g. 
  in 
  elephants 
  and 
  kangaroos) 
  is 
  

   a 
  sign 
  of 
  great 
  specialization, 
  and 
  it 
  is 
  not 
  improbably 
  the 
  same 
  

   among 
  these 
  fishes. 
  So 
  far 
  as 
  the 
  dentition 
  is 
  concerned, 
  Lepidotus 
  

   need 
  not 
  be 
  more 
  widely 
  separated 
  from 
  Mesodon 
  than 
  is 
  the 
  extinct 
  

   Dinotlierium 
  from 
  the 
  modern 
  elephant. 
  

  

  Thirdly, 
  the 
  enormous 
  development 
  of 
  the 
  preoperculum 
  and 
  the 
  

   concomitant 
  reduction 
  of 
  the 
  operculum 
  are 
  again 
  paralleled 
  in 
  

   distantly-related 
  fishes, 
  which 
  possess 
  similarly 
  powerful, 
  short 
  jaws 
  

   in 
  a 
  very 
  forward 
  position. 
  The 
  existing 
  genera 
  Tetrodon 
  and 
  

   Lampris 
  may 
  be 
  cited 
  as 
  illustrations. 
  Once 
  more 
  it 
  is 
  thus 
  evi- 
  

   dent 
  that 
  we 
  are 
  concerned 
  with 
  a 
  physiological 
  correlation 
  of 
  no 
  

   fundamental 
  significance, 
  and 
  one 
  which 
  is 
  the 
  mere 
  outcome 
  of 
  

   specializations 
  towards 
  which 
  there 
  is 
  already 
  a 
  tendency 
  in 
  the 
  

   Semionotidae. 
  

  

  Pin 
  ally, 
  the 
  anomalous 
  character 
  of 
  the 
  cranial 
  roof 
  is 
  at 
  first 
  

   more 
  difficult 
  to 
  understand. 
  It 
  will, 
  however, 
  be 
  observed 
  that 
  

   there 
  is 
  a 
  most 
  striking 
  resemblance 
  between 
  this 
  roof 
  as 
  known 
  

   in 
  Mesturas 
  (p. 
  192, 
  fig. 
  30) 
  and 
  that 
  of 
  the 
  modern 
  sturgeon 
  

   (Acipenser). 
  Moreover, 
  the 
  disposition 
  of 
  the 
  sutures 
  is 
  evidently 
  

   as 
  capricious 
  as 
  that 
  in 
  Acipenser 
  ; 
  for 
  not 
  only 
  do 
  the 
  parietal 
  

   plates 
  lack 
  bilateral 
  symmetry 
  in 
  the 
  specimen 
  figured, 
  but 
  there 
  

   is 
  also 
  considerable 
  difference 
  in 
  the 
  arrangement 
  of 
  the 
  median 
  

   plates 
  in 
  a 
  second 
  specimen 
  in 
  the 
  Leeds 
  Collection, 
  which 
  is 
  

   otherwise 
  very 
  similar. 
  Now, 
  it 
  has 
  already 
  been 
  shown 
  (p. 
  viii) 
  

   that 
  Acipenser 
  is 
  merely 
  the 
  surviving 
  terminal 
  form 
  of 
  a 
  long 
  

   series 
  of 
  Chondrostean 
  fishes, 
  which 
  in 
  the 
  Liassic 
  period 
  still 
  re- 
  

   tained 
  a 
  normal 
  cranial 
  roof. 
  Such 
  having 
  been 
  demonstrated 
  

   among 
  Chondrostei, 
  it 
  would 
  not 
  be 
  surprising 
  to 
  find 
  a 
  similar 
  case 
  

   of 
  development 
  or 
  degeneracy 
  among 
  Protospondyli. 
  In 
  the 
  present 
  

   state 
  of 
  knowledge 
  it 
  appears 
  likely 
  that 
  this 
  parallel 
  case 
  is 
  to 
  be 
  

   recognized 
  among 
  the 
  Pycnodonts. 
  

  

  The 
  result 
  of 
  these 
  considerations 
  is, 
  therefore, 
  that 
  the 
  Pycno- 
  

  

  