﻿INTRODUCTION 
  XIX 
  

  

  the 
  corresponding 
  feature 
  in 
  Lepidosteus 
  differs 
  from 
  that 
  in 
  all 
  

   other 
  surviving 
  ganoids 
  (cf. 
  Proc. 
  Zool. 
  Soc. 
  1893, 
  p. 
  565). 
  

  

  ISOSPONDYLI. 
  

  

  It 
  is 
  interesting 
  to 
  note 
  that 
  all 
  the 
  higher 
  fishes, 
  like 
  the 
  highest 
  

   of 
  the 
  terrestrial 
  Yertebrata, 
  are 
  characterized 
  by 
  a 
  comparatively 
  

   simple 
  mandible. 
  Only 
  doubtful 
  or 
  extremely 
  rare 
  exceptions 
  are 
  

   recorded 
  x 
  to 
  the 
  rulej 
  that 
  each 
  mandibular 
  ramus 
  in 
  these 
  types 
  

   consists 
  of 
  two 
  or 
  three 
  elements, 
  an 
  articulo-angular 
  (or 
  articular 
  

   and 
  angular) 
  behind, 
  a 
  dentary 
  in 
  front. 
  The 
  circumstance 
  is 
  all 
  

   the 
  more 
  remarkable 
  because, 
  on 
  acquiring 
  this 
  simplification 
  of 
  the 
  

   jaw, 
  the 
  Teleostomes 
  seem 
  to 
  be 
  infused 
  with 
  new 
  vigour 
  : 
  vertebral 
  

   centra 
  invariably 
  occur, 
  at 
  first 
  as 
  simple 
  rings, 
  then 
  as 
  robust 
  

   amphiccslous 
  bodies 
  ; 
  and 
  a 
  still 
  more 
  varied 
  series 
  of 
  families 
  arises, 
  

   including 
  analogues 
  of 
  all 
  the 
  principal 
  modifications 
  observed 
  among 
  

   the 
  lower 
  races, 
  these 
  being 
  superinduced 
  upon 
  the 
  new 
  and 
  

   advanced 
  type 
  of 
  skeletal 
  frame. 
  

  

  The 
  first 
  and 
  least 
  specialized 
  suborder 
  of 
  these 
  higher 
  fishes 
  is 
  

   that 
  of 
  the 
  Isospondyli, 
  thus 
  named 
  by 
  Prof. 
  E. 
  D. 
  Cope 
  in 
  allusion 
  

   to 
  the 
  fact 
  that 
  the 
  vertebrae 
  are 
  simple, 
  not 
  fused 
  into 
  a 
  group 
  

   behind 
  the 
  head 
  or 
  related 
  in 
  any 
  way 
  to 
  arrangements 
  for 
  an 
  

   osseous 
  connection 
  between 
  the 
  air-bladder 
  and 
  the 
  organs 
  of 
  

   hearing. 
  It 
  comprises 
  a 
  large 
  number 
  of 
  families, 
  mostly 
  Tertiary 
  

   and 
  Recent, 
  and 
  only 
  the 
  few 
  Jurassic 
  representatives 
  of 
  the 
  sub- 
  

   order 
  with 
  some 
  of 
  those 
  of 
  Cretaceous 
  age 
  are 
  treated 
  in 
  the 
  

   present 
  Part 
  of 
  the 
  Catalogue. 
  

  

  Pholidoplioridce. 
  

  

  The 
  earliest 
  clearly-proved 
  instance 
  of 
  the 
  simple 
  mandible 
  just 
  

   referred 
  to, 
  is 
  that 
  afforded 
  by 
  the 
  small 
  fishes 
  of 
  the 
  genus 
  Lepto- 
  

   lepis 
  occurring 
  in 
  the 
  European 
  Upper 
  Lias. 
  As 
  already 
  observed 
  

   by 
  Agassiz, 
  however, 
  the 
  genus 
  Pliolidopliorus 
  exhibits 
  a 
  very 
  close 
  

   resemblance 
  to 
  Leptolepis 
  in 
  general 
  aspect, 
  the 
  osteology 
  of 
  the 
  

   head 
  being 
  remarkably 
  similar, 
  vertebral 
  rings 
  being 
  tolerably 
  well 
  

   ossified, 
  the 
  fin-fulcra 
  very 
  small 
  and 
  usually 
  lost, 
  while 
  the 
  scales 
  

   are 
  often 
  extremely 
  thin 
  and 
  deeply 
  overlapping 
  though 
  for 
  the 
  

   most 
  part 
  united 
  by 
  a 
  peg-and-socket 
  articulation 
  ; 
  and 
  it 
  is 
  note- 
  

   worthy 
  that 
  no 
  indications 
  of 
  splenial 
  and 
  coronoid 
  elements 
  have 
  

  

  1 
  E. 
  g., 
  Arapaima, 
  according 
  to 
  Owen, 
  ' 
  Anatomy 
  of 
  Vertebrates,' 
  vol. 
  i. 
  

   p. 
  123, 
  fig. 
  88 
  (1866). 
  

  

  62 
  

  

  