﻿VI 
  INTKODUCTTON. 
  

  

  elongated 
  lobate 
  fins 
  flourish 
  the 
  earliest, 
  all 
  survivors 
  above 
  the 
  

   Devonian 
  having 
  the 
  lobe 
  comparatively 
  abbreviate. 
  The 
  large 
  

   pair 
  of 
  gular 
  plates 
  so 
  characteristic 
  of 
  the 
  Crossopterygii 
  is 
  always 
  

   wanting 
  in 
  the 
  Chondrostei; 
  but 
  the 
  lateral 
  gulars 
  are 
  merely 
  

   further 
  subdivided 
  to 
  become 
  the 
  ordinary 
  paired 
  series 
  of 
  branchio- 
  

   stegal 
  rays, 
  and 
  in 
  this 
  connection 
  it 
  is 
  noteworthy 
  that 
  both 
  in 
  

   Clieirolepis 
  and 
  many 
  later 
  Palseoniscidse 
  the 
  subdivision 
  is 
  not 
  

   complete 
  anteriorly 
  where 
  a 
  pair 
  of 
  very 
  large 
  plates 
  persists. 
  The 
  

   ridge-scales 
  on 
  the 
  upper 
  caudal 
  lobe 
  of 
  Chondrostei 
  represent 
  the 
  

   superior 
  rays 
  of 
  the 
  caudal 
  fin 
  in 
  Crossopterygii, 
  as 
  is 
  indicated 
  by 
  

   the 
  presence 
  of 
  the 
  endoskeletal 
  supports 
  in 
  Chondrosteus 
  and 
  the 
  

   existing 
  genera 
  ; 
  while 
  it 
  is 
  not 
  unlikely 
  that 
  the 
  fulcra 
  are 
  modified 
  

   enamelled 
  anterior 
  fin-rays 
  such 
  as 
  occur, 
  for 
  example, 
  in 
  Osteo- 
  

  

  When 
  the 
  Chondrostei 
  suddenly 
  become 
  dominant, 
  as 
  they 
  do 
  in 
  

   the 
  Lower 
  Carboniferous, 
  they 
  already 
  exhibit 
  a 
  remarkable 
  series 
  

   of 
  modifications, 
  which 
  are 
  enumerated 
  in 
  Part 
  II. 
  of 
  this 
  Catalogue. 
  

   The 
  family 
  of 
  Palseoniscidse 
  is 
  represented 
  chiefly 
  by 
  fishes 
  with 
  

   regular 
  rhombic 
  scales 
  and 
  distally-bifur 
  eating 
  fin-rays 
  ; 
  but 
  there 
  

   is 
  one 
  case 
  in 
  which 
  the 
  scales 
  are 
  cycloidal 
  and 
  deeply 
  imbricating 
  

   (CryjiJiiolepis), 
  another 
  in 
  which 
  the 
  scales 
  are 
  absent 
  except 
  upon 
  

   the 
  caudal 
  lobe 
  (Phaner 
  osteon), 
  and 
  a 
  third 
  in 
  which 
  the 
  fin-rays 
  

   never 
  branch 
  (Holurus). 
  Some, 
  moreover, 
  have 
  a 
  large 
  mouth 
  with 
  

   powerful 
  conical 
  teeth 
  ; 
  others, 
  a 
  small 
  mouth 
  with 
  comparatively 
  

   insignificant 
  teeth. 
  There 
  is 
  also 
  a 
  good 
  deal 
  of 
  variety 
  among 
  the 
  

   Platysomidse, 
  which 
  are 
  deep-bodied 
  fishes 
  closely 
  related 
  to 
  the 
  

   Palseoniscidse. 
  

  

  As 
  these 
  fishes 
  are 
  traced 
  upwards, 
  they 
  exhibit 
  very 
  little 
  

   essential 
  change. 
  The 
  upper 
  caudal 
  lobe 
  never 
  appears 
  to 
  atrophy 
  

   in 
  the 
  least 
  ; 
  the 
  supports 
  of 
  the 
  dorsal 
  and 
  anal 
  fins 
  never 
  equal 
  

   in 
  number 
  the 
  appended 
  dermal 
  rays 
  ; 
  the 
  infraclavicular 
  plates 
  

   always 
  remain 
  ; 
  the 
  ribs, 
  if 
  present, 
  never 
  ossify. 
  The 
  Platysomidae, 
  

   indeed, 
  become 
  extinct 
  in 
  the 
  Upper 
  Permian, 
  where, 
  in 
  addition 
  

   to 
  the 
  normal 
  genus 
  Platysomus, 
  there 
  is 
  a 
  scaleless 
  fish 
  (Dorypterus) 
  

   which 
  seems 
  to 
  represent 
  them. 
  The 
  large-mouthed 
  and 
  small- 
  

   mouthed, 
  rhombic-scaled 
  and 
  round-scaled, 
  perhaps 
  also 
  the 
  scaleless, 
  

   Palaeoniscidse 
  range 
  as 
  far 
  upwards 
  as 
  the 
  Permian 
  ; 
  and 
  then 
  the 
  

   family 
  presents 
  less 
  variety 
  in 
  its 
  representatives. 
  All 
  the 
  Mesozoic 
  

   genera 
  are 
  predaceous 
  fishes 
  with 
  a 
  large 
  mouth 
  and 
  deeply 
  over- 
  

   lapping 
  scales 
  which 
  are 
  usually 
  very 
  thin 
  ; 
  and 
  the 
  genus 
  Cocco- 
  

   lepis, 
  which 
  is 
  the 
  only 
  known 
  Palaeoniscid 
  ranging 
  above 
  the 
  Lias, 
  

  

  