﻿INTRODUCTION. 
  

  

  During 
  the 
  Mesozoic 
  Period 
  the 
  Actinopterygian 
  fishes 
  of 
  the 
  

   Chondrostean 
  type 
  were 
  gradually 
  replaced 
  by 
  others 
  which 
  advanced 
  

   towards 
  the 
  modern 
  Teleostean 
  type. 
  The 
  latter 
  finally 
  became 
  

   dominant 
  in 
  the 
  seas 
  of 
  the 
  epoch 
  of 
  the 
  Chalk. 
  In 
  the 
  course 
  of 
  

   this 
  change 
  in 
  the 
  character 
  of 
  the 
  fish-fauna, 
  the 
  successive 
  stages 
  

   were 
  represented 
  by 
  numerous 
  minor 
  modifications, 
  which 
  have 
  

   little 
  relation 
  to 
  the 
  direct 
  line 
  of 
  progress 
  but 
  present 
  many 
  

   features 
  of 
  great 
  morphological 
  interest. 
  It 
  is 
  the 
  object 
  of 
  the 
  

   present 
  volume 
  to 
  trace 
  these 
  phases 
  of 
  development 
  in 
  detail, 
  and 
  

   to 
  record 
  in 
  as 
  nearly 
  a 
  natural 
  order 
  as 
  possible 
  the 
  variations 
  of 
  

   each 
  at 
  the 
  time 
  of 
  its 
  dominance. 
  

  

  Chondrostei. 
  

  

  Firstly, 
  however, 
  there 
  is 
  the 
  question 
  of 
  the 
  fate 
  of 
  the 
  Chondrostei 
  

   themselves, 
  which 
  are 
  represented 
  even 
  at 
  the 
  present 
  day 
  by 
  a 
  

   few 
  survivors. 
  

  

  The 
  origin 
  of 
  these 
  fishes, 
  as 
  defined 
  in 
  the 
  present 
  Catalogue, 
  is 
  

   still 
  entirely 
  obscure. 
  Among 
  known 
  fossils 
  they 
  range 
  downwards 
  

   as 
  far 
  as 
  the 
  Crossopterygians, 
  while 
  there 
  is 
  as 
  yet 
  no 
  evidence 
  of 
  

   a 
  link 
  between 
  these 
  two 
  groups. 
  On 
  the 
  other 
  hand, 
  it 
  is 
  clear 
  

   that 
  the 
  Chondrostean 
  is 
  later 
  than 
  the 
  Crossopterygian 
  type 
  ; 
  for 
  

   the 
  former 
  is 
  represented 
  in 
  the 
  Devonian 
  solely 
  by 
  the 
  rare 
  genus 
  

   Cheirolepis, 
  while 
  the 
  latter 
  is 
  dominant 
  throughout, 
  and 
  the 
  

   members 
  of 
  the 
  Chondrostei 
  do 
  not 
  flourish 
  vigorously 
  until 
  those 
  

   of 
  the 
  Crossopterygii 
  begin 
  to 
  decline 
  in 
  the 
  Carboniferous 
  and 
  

   Permian. 
  The 
  modifications 
  by 
  which 
  a 
  Crossopterygian 
  could 
  be 
  

   changed 
  into 
  a 
  Chondrostean 
  are 
  also 
  readily 
  comprehensible. 
  In 
  

   the 
  latter 
  the 
  paired 
  fins 
  are 
  always 
  polybasal, 
  with 
  excessively 
  

   shortened 
  lobe 
  ; 
  and 
  among 
  Crossopterygii 
  the 
  genera 
  with 
  most 
  

  

  