﻿X 
  INTRODUCTION 
  

  

  Lepidotus 
  itself, 
  exhibiting 
  all 
  its 
  typical 
  characters, 
  has 
  also 
  

   been 
  lately 
  discovered 
  in 
  the 
  Upper 
  Trias 
  or 
  Rheetic 
  (Prolepidotus 
  

   gallineki 
  of 
  Michael). 
  Moreover, 
  it 
  is 
  clear 
  that 
  this 
  genus 
  cannot 
  

   be 
  a 
  specialized 
  form 
  of 
  Colobodus 
  ; 
  for 
  all 
  the 
  early 
  species 
  to 
  the 
  

   summit 
  of 
  the 
  Lias 
  exhibit 
  the 
  inner 
  teeth 
  of 
  moderate 
  size 
  on 
  

   comparatively 
  long 
  pedicles, 
  and 
  the 
  powerful 
  tritoral 
  arrangement 
  

   is 
  not 
  observed 
  before 
  the 
  Upper 
  Jurassic. 
  Some 
  of 
  the 
  Wealden 
  

   species 
  are 
  distinguished 
  by 
  the 
  possession 
  of 
  ring-vertebrse 
  ; 
  but 
  

   all 
  still 
  retain 
  the 
  thick 
  scales, 
  large 
  biserial 
  fin-fulcra, 
  and 
  com- 
  

   plete 
  opercular 
  apparatus. 
  The 
  comparatively 
  regular 
  arrangement 
  

   of 
  this 
  slowly-acquired 
  tritoral 
  dentition 
  is 
  striking, 
  when 
  compared 
  

   with 
  the 
  early 
  approach 
  to 
  a 
  similar 
  type 
  in 
  the 
  Triassic 
  Colobodus. 
  

  

  The 
  deep-bodied 
  fishes 
  which 
  begin 
  with 
  Dapedius 
  in 
  the 
  Upper 
  

   Trias 
  are 
  noteworthy 
  for 
  the 
  rapid 
  changes 
  exhibited 
  in 
  their 
  form 
  

   and 
  squamation. 
  Cleithrolepis 
  from 
  South 
  Africa 
  and 
  Australia, 
  

   and 
  Tetragonolepis 
  from 
  the 
  European 
  Upper 
  Lias 
  and 
  the 
  Indian 
  

   Kota 
  Formation, 
  have 
  an 
  excessively 
  deepened 
  trunk, 
  with 
  scales 
  

   strengthened 
  by 
  an 
  unusually 
  thickened 
  anterior 
  margin 
  ; 
  and 
  the 
  

   latter 
  genus 
  is 
  characterized 
  by 
  very 
  thin 
  scales 
  en 
  the 
  hinder 
  part 
  

   of 
  the 
  caudal 
  region. 
  Still 
  more 
  interesting 
  is 
  the 
  newly-established 
  

   genus 
  AetJieolepis, 
  from 
  the 
  Upper 
  Hawkesbury-Wianamatta 
  Series 
  of 
  

   New 
  South 
  Wales, 
  which 
  has 
  the 
  ordinary 
  deepened 
  rhombic 
  scales, 
  

   united 
  by 
  peg-and-socket, 
  on 
  the 
  abdominal 
  region, 
  while 
  these 
  

   gradually 
  lose 
  their 
  articulation, 
  become 
  cycloidal 
  and 
  deeply 
  over- 
  

   lapping 
  as 
  they 
  pass 
  backwards 
  towards 
  the 
  pedicle 
  of 
  the 
  tail. 
  

  

  Among 
  Semionotidse, 
  therefore, 
  a 
  precocious 
  though 
  irregular 
  

   development 
  of 
  grinding-teeth 
  is 
  observed 
  in 
  a 
  genus 
  of 
  the 
  Trias 
  

   (Colobodus); 
  deep-bodied 
  fishes 
  with 
  styliform 
  rather 
  than 
  tritoral 
  

   teeth, 
  lose 
  their 
  caudal 
  scales 
  in 
  the 
  Upper 
  Lias 
  (Tetragonolepis) 
  ; 
  

   and 
  a 
  more 
  normal 
  series 
  (Lepidotus), 
  abundantly 
  represented 
  

   throughout 
  the 
  Jurassic 
  period, 
  retains 
  most 
  of 
  its 
  primitive 
  out- 
  

   ward 
  features, 
  while 
  the 
  dentition 
  forms 
  a 
  nearly 
  regular 
  tritoral 
  

   pavement, 
  ring-vertebrae 
  are 
  acquired, 
  and 
  the 
  cheek-plates 
  and 
  

   supratemp 
  orals 
  are 
  irregularly 
  subdivided 
  in 
  the 
  latest 
  Wealden 
  

   forms. 
  

  

  Meter 
  osemiidce. 
  

  

  Another 
  series 
  of 
  Protospondyli, 
  with 
  small 
  mouth 
  and 
  styliform 
  

   rather 
  than 
  conical 
  teeth, 
  also 
  appears 
  first 
  in 
  the 
  Trias, 
  and 
  is 
  

   represented 
  throughout 
  the 
  Jurassic 
  period 
  by 
  numerous 
  long- 
  

   bodied 
  fishes 
  with 
  extended 
  dorsal 
  fin, 
  while 
  at 
  least 
  one 
  genus 
  

   survives 
  even 
  in 
  the 
  Upper 
  Cretaceous. 
  

  

  