﻿XIV 
  INTRODUCTION. 
  

  

  dontidae 
  bear 
  much 
  the 
  same 
  relation 
  to 
  the 
  Permian 
  AcentropJwrus 
  

   as 
  the 
  modern 
  sturgeons 
  to 
  the 
  Palaeozoic 
  Palaeoniscidae 
  ; 
  the 
  latter 
  

   having 
  only 
  advanced 
  a 
  stage 
  further 
  in 
  the 
  matter 
  of 
  teeth, 
  which 
  

   are 
  in 
  them 
  not 
  only 
  destitute 
  of 
  successors, 
  but 
  are 
  also 
  lost 
  before 
  

   the 
  fish 
  becomes 
  adult. 
  Among 
  Pycnodontidae 
  as 
  a 
  race, 
  tooth- 
  

   specialization 
  consists 
  in 
  the 
  reduction 
  of 
  the 
  tritors 
  into 
  a 
  few- 
  

   regular 
  longitudinal 
  series, 
  and 
  there 
  is 
  only 
  one 
  known 
  genus 
  (the 
  

   Cretaceous 
  Anomoeodus) 
  in 
  which 
  there 
  is 
  much 
  tendency 
  towards 
  

   the 
  loss 
  of 
  this 
  armature. 
  

  

  The 
  facts 
  detailed 
  in 
  the 
  Catalogue, 
  as 
  just 
  briefly 
  analysed, 
  seem 
  

   to 
  render 
  it 
  unnecessary 
  to 
  discuss 
  the 
  still 
  widely 
  prevalent 
  belief 
  

   that 
  the 
  Pycnodonts 
  are 
  the 
  direct 
  successors 
  of 
  the 
  Platysomidae. 
  

   The 
  elaborate 
  researches 
  and 
  careful 
  reasoning 
  of 
  Traquair 
  sixteen 
  

   years 
  ago, 
  1 
  ought 
  to 
  have 
  sufficed 
  ere 
  this 
  to 
  banish 
  the 
  theory 
  even 
  

   from 
  popular 
  handbooks 
  ; 
  nevertheless, 
  it 
  survives 
  in 
  a 
  shape 
  

   scarcely 
  different 
  from 
  that 
  current 
  at 
  the 
  time 
  when 
  comparatively 
  

   nothing 
  was 
  known 
  of 
  the 
  Platysomid 
  skeleton. 
  It 
  must 
  suffice 
  to 
  

   repeat, 
  that 
  the 
  Platysomidae 
  never 
  make 
  the 
  faintest 
  approach 
  to 
  

   the 
  Pycnodontidae 
  in 
  a 
  single 
  essential 
  character. 
  The 
  trunk 
  and 
  

   fins 
  are 
  as 
  thoroughly 
  Chondrostean 
  in 
  the 
  most 
  specialized 
  as 
  in 
  

   the 
  most 
  generalized 
  type 
  ; 
  the 
  cranial 
  osteology 
  and 
  arrangement 
  

   of 
  the 
  jaws 
  are 
  fundamentally 
  different 
  even 
  when 
  the 
  dentition 
  is 
  

   most 
  powerful. 
  

  

  If 
  speculation 
  were 
  permitted 
  in 
  seeking 
  for 
  the 
  direct 
  ancestors 
  

   of 
  the 
  Pycnodonts, 
  it 
  might 
  be 
  most 
  profitable 
  to 
  turn 
  towards 
  the 
  

   earliest 
  Mesozoic 
  fishes 
  of 
  the 
  Colobodus-ty^e. 
  The 
  appearance 
  of 
  

   the 
  family, 
  however, 
  still 
  remains 
  as 
  mysterious 
  as 
  that 
  of 
  the 
  

   Ccelacanthidae 
  among 
  the 
  Crossopterygians 
  ; 
  and 
  the 
  long-continued 
  

   permanence 
  of 
  so 
  remarkably 
  specialized 
  a 
  type 
  (Lower 
  Lias 
  — 
  Eocene) 
  

   is 
  curiously 
  paralleled 
  in 
  the 
  range 
  of 
  the 
  more 
  primitive 
  family 
  

   just 
  mentioned 
  (Lower 
  Carboniferous 
  — 
  Chalk). 
  Persistent 
  types 
  of 
  

   this 
  character 
  are 
  rarely 
  met 
  with. 
  

  

  Eugnathidce. 
  

  

  It 
  has 
  already 
  been 
  mentioned 
  that 
  the 
  small-mouthed 
  Proto- 
  

   spondyli 
  considered 
  above 
  appear 
  to 
  have 
  left 
  no 
  descendants. 
  

   At 
  least, 
  no 
  definite 
  links 
  can 
  be 
  recognized 
  between 
  these 
  and 
  

   modern 
  fishes. 
  The 
  large-mouthed 
  Protospondyli 
  with 
  conical 
  

   teeth, 
  on 
  the 
  other 
  hand, 
  can 
  be 
  traced 
  from 
  the 
  Trias 
  upwards 
  

  

  1 
  B.. 
  H. 
  Traquair, 
  " 
  On 
  the 
  Structure 
  and 
  Affinities 
  of 
  the 
  Platysomidse," 
  

   Trans. 
  Eoy. 
  Soc. 
  Edinb. 
  vol. 
  xxix. 
  pp. 
  343-391, 
  pis. 
  iii.-vi. 
  (1879). 
  

  

  