﻿INTRODUCTION 
  - 
  . 
  XXI 
  

  

  Oligopleuridae 
  with 
  the 
  latter 
  family 
  at 
  the 
  base 
  of 
  the 
  Isospondylic 
  

   series. 
  

  

  Leptolepidce. 
  

  

  The 
  Leptolepidaa 
  differ 
  from 
  the 
  two 
  preceding 
  families 
  in 
  the 
  

   absence 
  of 
  fulcra 
  on 
  the 
  fins, 
  and 
  are 
  remarkable 
  as 
  being 
  the 
  

   earliest 
  family 
  in 
  which 
  intermuscular 
  bones 
  occur. 
  These 
  

   elements, 
  forming 
  so 
  conspicuous 
  a 
  feature 
  among 
  modern 
  fishes, 
  

   appear 
  to 
  be 
  arranged 
  here 
  only 
  in 
  a 
  single 
  series 
  above 
  the 
  vertebral 
  

   column 
  in 
  the 
  abdominal 
  region 
  ; 
  though 
  there 
  may 
  perhaps 
  be 
  

   traces 
  of 
  them 
  sometimes 
  also 
  in 
  the 
  lower 
  half 
  of 
  the 
  caudal 
  

   region. 
  The 
  vertebral 
  centra 
  in 
  Leptolevis 
  itself 
  exhibit 
  interesting 
  

   gradations 
  in 
  the 
  degree 
  of 
  development 
  according 
  to 
  the 
  geological 
  

   age 
  of 
  the 
  species 
  ; 
  these 
  centra 
  never 
  being 
  more 
  than 
  delicate 
  

   constricted 
  rings 
  or 
  cylinders 
  in 
  the 
  Upper 
  Lias, 
  and 
  always 
  

   strengthened 
  by 
  secondary 
  peripheral 
  calcifications 
  in 
  the 
  Oxfordian 
  

   and 
  upwards. 
  In 
  TTirissops, 
  which 
  ranges 
  as 
  far 
  at 
  least 
  as 
  the 
  

   Lower 
  Cretaceous, 
  the 
  vertebral 
  centra 
  are 
  still 
  more 
  robust. 
  

  

  These 
  fishes, 
  it 
  will 
  be 
  noticed, 
  approach 
  very 
  closely 
  the 
  

   Clupeidae, 
  among 
  which 
  they 
  are 
  sometimes 
  included 
  ; 
  but 
  they 
  

   are 
  distinguished 
  by 
  the 
  meeting 
  of 
  the 
  parietal 
  bones 
  in 
  the 
  

   median 
  line, 
  by 
  the 
  non-fusion 
  of 
  the 
  haemal 
  spines 
  at 
  the 
  base 
  of 
  

   the 
  tail, 
  and 
  by 
  the 
  presence 
  of 
  a 
  thin 
  film 
  of 
  ganoine 
  on 
  the 
  

   scales. 
  

  

  The 
  interpretation 
  of 
  the 
  relationships 
  of 
  the 
  Mesozoic 
  Actino- 
  

   pterygian 
  fishes 
  thus 
  briefly 
  summarized, 
  differs 
  so 
  much 
  from 
  any 
  

   hitherto 
  proposed 
  that 
  it 
  has 
  not 
  been 
  found 
  possible 
  to 
  arrange 
  a 
  

   synonymy 
  under 
  the 
  subordinal 
  and 
  family-headings 
  in 
  the 
  

   Catalogue. 
  Family-names 
  not 
  derived 
  from 
  generic 
  names 
  have 
  

   also 
  been 
  ignored. 
  The 
  existing 
  Acijpenser, 
  Polyodon, 
  Sjoatularia, 
  

   Lepidosteus, 
  and 
  Amia 
  are 
  now 
  shown 
  to 
  afford 
  a 
  very 
  inadequate 
  

   and 
  misleading 
  idea 
  of 
  the 
  actinopterygian 
  ganoids, 
  on 
  account 
  of 
  

   their 
  remarkably 
  specialized 
  nature. 
  The 
  characters 
  of 
  the 
  appen- 
  

   dicular 
  skeleton 
  are 
  observed 
  to 
  be 
  much 
  more 
  important 
  than 
  those 
  

   even 
  of 
  the 
  axial 
  skeleton 
  of 
  the 
  trunk, 
  while 
  the 
  form 
  and 
  absence 
  

   or 
  presence 
  of 
  the 
  scales, 
  as 
  also 
  the 
  degree 
  of 
  development 
  of 
  the 
  

   branchiostegal 
  apparatus, 
  are 
  features 
  comparatively 
  insignificant. 
  

   It 
  is 
  thus 
  no 
  longer 
  scientific 
  to 
  regard 
  the 
  " 
  Acipenseroidei 
  " 
  as 
  

   typical 
  members 
  of 
  the 
  group 
  to 
  which 
  they 
  belong 
  ; 
  they 
  are 
  mere 
  

  

  