﻿INTRODUCTION. 
  Vll 
  

  

  has 
  practically 
  cycloidal 
  scales 
  without 
  peg-and-socket 
  articulation. 
  

   Moreover, 
  while 
  the 
  endoskeletal 
  supports 
  of 
  the 
  dorsal 
  and 
  anal 
  

   fins 
  are 
  in 
  two 
  series 
  in 
  such 
  Palaeozoic 
  genera 
  as 
  Elonichthys 
  

   and 
  Pygopterus, 
  they 
  are 
  distinctly 
  shown 
  to 
  be 
  confined 
  to 
  one 
  

   series 
  in 
  the 
  Mesozoic 
  Coccolepis. 
  

  

  The 
  last 
  term 
  of 
  the 
  Palaeoniscid 
  series 
  (Coccolepis) 
  thus 
  

   approaches 
  a 
  higher 
  grade 
  than 
  that 
  of 
  the 
  Chondrostei 
  in 
  the 
  

   character 
  of 
  its 
  dorsal 
  and 
  anal 
  fins. 
  In 
  every 
  other 
  respect, 
  

   however, 
  it 
  is 
  typically 
  and 
  essentially 
  Chondrostean 
  ; 
  the 
  baseosts 
  

   of 
  its 
  pelvic 
  fins 
  are 
  relatively 
  as 
  large 
  and 
  numerous 
  as 
  those 
  of 
  a 
  

   modern 
  sturgeon. 
  It 
  is 
  noteworthy 
  that 
  the 
  only 
  known 
  apprcxi* 
  

   mation 
  to 
  a 
  higher 
  grade 
  occurs 
  among 
  much 
  more 
  generalized 
  

   fishes, 
  those 
  of 
  the 
  Triassic 
  family 
  of 
  Catopteridae. 
  

  

  The 
  little 
  that 
  is 
  known 
  of 
  Catopterus 
  and 
  Dictyojpyge, 
  the 
  two 
  

   genera 
  of 
  Catopteridae, 
  forms 
  the 
  subject 
  of 
  the 
  opening 
  pages 
  of 
  

   the 
  present 
  volume. 
  Much 
  of 
  this 
  information 
  is 
  unsatisfactory 
  

   and 
  needs 
  verification 
  ; 
  but 
  it 
  may 
  be 
  asserted, 
  with 
  considerable 
  

   probability 
  of 
  correctness, 
  that 
  these 
  fishes 
  possess 
  a 
  Palaeoniscid 
  

   head 
  and 
  shoulder-girdle, 
  while 
  the 
  tail 
  is 
  only 
  hemi-heterocercal, 
  

   and 
  the 
  single 
  series 
  of 
  supports 
  in 
  the 
  dorsal 
  and 
  anal 
  fins 
  almost 
  

   equals 
  in 
  number 
  the 
  apposed 
  dermal 
  rays. 
  Such 
  being 
  the 
  case, 
  

   here 
  is 
  an 
  interesting 
  illustration 
  of 
  the 
  common 
  law, 
  that 
  the 
  links 
  

   between 
  a 
  lower 
  and 
  a 
  higher 
  group 
  are 
  not 
  to 
  be 
  sought 
  among 
  

   the 
  specialized 
  types 
  of 
  the 
  former 
  but 
  among 
  those 
  with 
  the 
  most 
  

   generalized 
  secondary 
  characters. 
  

  

  Palaeontology 
  seems 
  to 
  demonstrate 
  that 
  the 
  various 
  modifications 
  

   in 
  the 
  minor 
  characters 
  of 
  the 
  Palaeoniscid 
  fishes 
  eventually 
  led 
  to 
  

   the 
  development 
  of 
  a 
  series 
  of 
  types 
  which 
  can 
  only 
  be 
  described 
  as 
  

   extremely 
  degenerate, 
  namely 
  the 
  modern 
  sturgeons 
  (Acipenseridae) 
  

   and 
  paddle-fishes 
  (Polyodontidae). 
  

  

  There 
  is 
  much 
  probability, 
  indeed, 
  that 
  other 
  degenerate 
  series 
  

   originated 
  from 
  Chondrosteans 
  related 
  to 
  them 
  ; 
  and 
  it 
  is 
  interesting 
  

   in 
  this 
  connection 
  to 
  notice 
  the 
  case 
  of 
  the 
  Belonorhynchidas 
  

   (catalogued 
  on 
  pp. 
  9-23). 
  These 
  are 
  eel-shaped 
  fishes 
  with 
  a 
  long 
  

   pointed 
  head, 
  an 
  abbreviate-diphycercal 
  tail, 
  short 
  dorsal 
  and 
  anal 
  

   fins, 
  and 
  small 
  paired 
  fins. 
  The 
  trunk 
  is 
  nearly 
  always, 
  if 
  not 
  

   always, 
  destitute 
  of 
  ordinary 
  scales 
  ; 
  but 
  it 
  is 
  armoured 
  with 
  four 
  

   distinct 
  longitudinal 
  series 
  of 
  imbricating 
  scutes 
  — 
  one 
  median 
  

   dorsal, 
  one 
  median 
  ventral, 
  and 
  a 
  pair 
  supporting 
  the 
  "lateral 
  

   line." 
  The 
  dorsal 
  and 
  anal 
  fin-supports 
  are 
  much 
  fewer 
  than 
  their 
  

   apposed 
  rays, 
  though 
  apparently 
  only 
  in 
  a 
  single 
  series 
  ; 
  and 
  the 
  

  

  