﻿INTRODUCTORY. 
  

  

  The 
  present 
  work 
  is 
  largely 
  though 
  not 
  exclusively 
  concerned 
  with 
  

   the 
  consideration 
  of 
  those 
  distinctively 
  carboniferous 
  families 
  of 
  

   ancient 
  Selachians, 
  the 
  Cochliodontidse 
  and 
  the 
  Psammodontidae. 
  

   Although 
  in 
  the 
  majority 
  of 
  instances 
  the 
  classification 
  now 
  adopted 
  

   has 
  necessitated 
  the 
  amending, 
  amplifying, 
  and 
  curtailment 
  of 
  the 
  attri- 
  

   butes 
  ascribed 
  to 
  genera, 
  it 
  has 
  not 
  been 
  deemed 
  necessary 
  to 
  give 
  a 
  

   detailed 
  diagnosis 
  of 
  all 
  the 
  genera 
  thus 
  revised. 
  In 
  the 
  latter 
  in- 
  

   stances 
  the 
  descriptions 
  of 
  species 
  will 
  afford 
  the 
  necessary 
  informa- 
  

   tion 
  relating 
  to 
  the 
  peculiarities 
  of 
  the 
  genera 
  to 
  which 
  they 
  belong, 
  

   and 
  a 
  glance 
  at 
  the 
  illustrations 
  will 
  often 
  convey 
  a 
  vivid 
  impression 
  

   of 
  those 
  distinctions. 
  

  

  It 
  has 
  been 
  impossible 
  to 
  avoid 
  repetition 
  of 
  details 
  in 
  the 
  notices 
  

   of 
  the 
  species, 
  especially 
  in 
  the 
  desire 
  to 
  render 
  for 
  each 
  species 
  a 
  

   comprehensive 
  diagnosis 
  by 
  itself 
  ; 
  otherwise 
  the 
  descriptions 
  would 
  

   mainly 
  consist 
  of 
  comparisons 
  of 
  their 
  distinctive 
  features 
  as 
  con- 
  

   trasted 
  with 
  one 
  another, 
  and 
  we 
  have 
  deemed 
  it 
  the 
  safer 
  plan 
  to 
  

   err 
  on 
  the 
  score 
  of 
  details 
  rather 
  than 
  brevity, 
  stating 
  the 
  facts 
  

   concisely 
  as 
  possible, 
  but 
  omitting 
  none 
  of 
  evident 
  consequence 
  as 
  

   aids 
  to 
  a 
  fair 
  conprehension 
  of 
  their 
  special 
  characteristics 
  and 
  their 
  

   resemblances 
  and 
  distinctions 
  compared 
  with 
  allied 
  species. 
  In 
  no 
  

   similar 
  investigation 
  have 
  so 
  many 
  and 
  varied 
  materials 
  relating 
  to 
  

   these 
  interesting 
  groups 
  of 
  Selachians 
  been 
  brought 
  forward 
  for 
  

   critical 
  consideration 
  at 
  the 
  same 
  time. 
  That 
  the 
  subject 
  was 
  some- 
  

   what 
  involved 
  may 
  be 
  readily 
  understood 
  by 
  all 
  students 
  ; 
  but 
  to 
  

   none 
  so 
  much 
  as 
  to 
  the 
  specialist 
  can 
  the 
  actual 
  state 
  of 
  things 
  in 
  

   this 
  relation 
  be 
  at 
  all 
  adequately 
  appreciated. 
  This 
  has 
  largely 
  re- 
  

   sulted 
  from 
  incomplete 
  data, 
  and 
  to 
  some 
  extent 
  also 
  to 
  the 
  lack 
  of 
  

   familiarity 
  with 
  the 
  association 
  of 
  forms 
  in 
  the 
  same 
  deposits. 
  For 
  

   it 
  must 
  be 
  confessed 
  that 
  in 
  the 
  very 
  few 
  exceptions 
  these 
  remains 
  

  

  