﻿INTRODUCTION. 
  IX 
  

  

  full 
  development. 
  It 
  is 
  true 
  that 
  there 
  are 
  links 
  between 
  most 
  of 
  

   the 
  families, 
  rendering 
  precise 
  definitions 
  almost 
  impossible 
  ; 
  and 
  

   evidences 
  of 
  evolution 
  can 
  be 
  detected 
  in 
  a 
  slight 
  degree 
  as 
  the 
  

   different 
  groups 
  are 
  traced 
  upwards 
  in 
  their 
  range. 
  All 
  the 
  

   families, 
  however, 
  except 
  the 
  modern 
  Lepidosteidae 
  and 
  Amiidse, 
  

   had 
  already 
  become 
  differentiated 
  before 
  the 
  period 
  of 
  the 
  Lower 
  

   Oolites. 
  

  

  Semionotidce. 
  

  

  Eobust 
  Protospondyli 
  with 
  a 
  small 
  mouth 
  and 
  grinding 
  teeth, 
  

   and 
  predaceous 
  forms 
  with 
  a 
  large 
  mouth 
  and 
  conical 
  teeth, 
  appear 
  

   abundantly 
  in 
  the 
  Trias; 
  but 
  as 
  the 
  former 
  do 
  not 
  seem 
  to 
  pass 
  

   into 
  modern 
  bony 
  fishes, 
  while 
  the 
  latter 
  can 
  be 
  distinctly 
  traced 
  

   towards 
  the 
  characteristic 
  types 
  of 
  Upper 
  Cretaceous 
  and 
  Tertiary 
  

   times, 
  those 
  with 
  a 
  small 
  mouth 
  and 
  more 
  or 
  less 
  tritoral 
  dentition 
  

   are 
  treated 
  first. 
  

  

  This 
  arrangement 
  seems 
  all 
  the 
  more 
  natural 
  since 
  the 
  family 
  

   of 
  SemionotidaB 
  — 
  the 
  most 
  generalized 
  of 
  the 
  series 
  — 
  is 
  first 
  re- 
  

   presented 
  by 
  Acentropliorus 
  in 
  the 
  Upper 
  Permian. 
  No 
  other 
  

   representative 
  of 
  the 
  Protospondyli 
  has 
  hitherto 
  been 
  discovered 
  

   in 
  Palaeozoic 
  formations, 
  and 
  it 
  is 
  interesting 
  to 
  notice 
  that 
  this 
  

   unique 
  fish 
  is 
  the 
  most 
  generalized 
  genus 
  of 
  the 
  family 
  to 
  which 
  it 
  

   belongs. 
  Before 
  the 
  end 
  of 
  the 
  Triassic 
  period, 
  however, 
  all 
  the 
  

   principal 
  types 
  of 
  Semionotidse 
  had 
  appeared 
  ; 
  and 
  the 
  most 
  specialized 
  

   genus 
  Tetragonolepis 
  occurs 
  in 
  the 
  Upper 
  Lias. 
  

  

  The 
  Triassic 
  genera 
  most 
  closely 
  related 
  to 
  Acentrophorus, 
  

   namely, 
  Semionotus 
  and 
  Pristisomus, 
  scarcely 
  differ 
  from 
  it 
  except 
  

   in 
  possessing 
  well-developed 
  ridge-scales. 
  The 
  former 
  is 
  confined 
  to 
  

   the 
  Trias 
  in 
  North 
  America, 
  and 
  is 
  not 
  known 
  to 
  range 
  above 
  the 
  

   Ehaetic 
  in 
  Europe. 
  There 
  is, 
  however, 
  an 
  interesting 
  allied 
  genus, 
  

   Aphnelejais, 
  in 
  the 
  supposed 
  Jurassic 
  of 
  New 
  South 
  Wales 
  ; 
  and 
  

   this 
  exhibits 
  comparatively 
  thin 
  scales, 
  which 
  suddenly 
  become 
  

   extremely 
  delicate 
  on 
  the 
  caudal 
  region 
  behind 
  a 
  line 
  connecting 
  

   the 
  points 
  of 
  origin 
  of 
  the 
  dorsal 
  and 
  anal 
  fins. 
  

  

  The 
  Triassic 
  genus 
  Colobodus 
  scarcely 
  differs 
  from 
  Semionotus 
  

   except 
  in 
  its 
  powerful 
  grinding 
  dentition 
  and 
  the 
  reduction 
  of 
  its 
  

   fin-fulcra 
  to 
  a 
  fine 
  and 
  close 
  series. 
  In 
  many 
  respects 
  it 
  resembles 
  

   Lepidotus 
  : 
  but 
  the 
  tooth-pavement 
  does 
  not 
  appear 
  to 
  exhibit 
  any 
  

   regular 
  arrangement, 
  and 
  the 
  scale-articulation 
  is 
  not 
  strengthened 
  

   by 
  the 
  development 
  of 
  spurs 
  from 
  the 
  overlapped 
  margin 
  such 
  as 
  

   characterize 
  even 
  the 
  earliest 
  species 
  of 
  Lepidotus, 
  

  

  