﻿xvm 
  Introduction. 
  

  

  AspidorJiynchus, 
  with 
  prominent 
  rostrum, 
  exhibits 
  only 
  one 
  series 
  

   of 
  small 
  teeth 
  on 
  the 
  splenial 
  bone, 
  and 
  has 
  ring-vertebrae. 
  It 
  

   seems 
  to 
  be 
  confined 
  to 
  the 
  Jurassic 
  ; 
  but 
  a 
  second 
  genus, 
  Belono- 
  

   stomus, 
  ranges 
  from 
  the 
  Upper 
  Jurassic 
  to 
  the 
  Upper 
  Cretaceous 
  

   inclusive, 
  and 
  in 
  the 
  course 
  of 
  this 
  range 
  the 
  delicate 
  ring-vertebrae 
  

   gradually 
  pass 
  into 
  centra 
  which 
  are 
  pierced 
  only 
  by 
  a 
  very 
  delicate 
  

   thread 
  of 
  notochord. 
  It 
  is 
  also 
  noteworthy 
  that 
  the 
  enlarged 
  

   splenial, 
  covered 
  with 
  tubercular 
  teeth, 
  excludes 
  the 
  dentary 
  from 
  

   the 
  margin 
  of 
  the 
  mandible 
  in 
  the 
  English 
  Chalk 
  species, 
  Belono- 
  

   stomus 
  cinctus. 
  Otherwise 
  there 
  is 
  no 
  special 
  evidence 
  of 
  evolution 
  

   in 
  the 
  family. 
  

  

  It 
  is 
  just 
  possible 
  that 
  the 
  small 
  Triassic 
  fish 
  Pholidopleurus, 
  

   doubtfully 
  assigned 
  to 
  the 
  Pholidophoridae 
  in 
  the 
  Catalogue 
  (p. 
  479), 
  

   may 
  be 
  a 
  forerunner 
  of 
  the 
  Aspidorhynchidae 
  ; 
  but 
  its 
  cranial 
  

   osteology 
  is 
  as 
  yet 
  too 
  imperfectly 
  known 
  for 
  comparison. 
  

  

  Nearly 
  all 
  the 
  known 
  fossil 
  remains 
  of 
  Lepidosteidae 
  are 
  mere 
  

   fragments, 
  none 
  dating 
  back 
  further 
  than 
  the 
  Lower 
  Eocene. 
  The 
  

   only 
  interesting 
  fact 
  to 
  be 
  deduced 
  from 
  these 
  remains 
  is, 
  that 
  fishes 
  

   with 
  scales 
  and 
  vertebrae 
  identical 
  with 
  those 
  of 
  the 
  existing 
  

   American 
  Lepidosteus 
  inhabited 
  Western 
  Europe 
  during 
  the 
  period 
  

   of 
  deposition 
  of 
  the 
  Eocene 
  and 
  Lower 
  Miocene 
  deposits. 
  Their 
  

   range 
  in 
  the 
  Old 
  World 
  thus 
  corresponds 
  with 
  that 
  of 
  the 
  fossil 
  

   Amia 
  already 
  described. 
  It 
  is, 
  of 
  course, 
  likely 
  that 
  their 
  Mesozoic 
  

   ancestors 
  would 
  not 
  be 
  recognized 
  at 
  first 
  sight 
  on 
  account 
  of 
  

   the 
  normal 
  character 
  of 
  their 
  vertebral 
  centra 
  ; 
  but 
  the 
  markedly 
  

   forward 
  inclination 
  of 
  the 
  mandibular 
  suspensorium 
  seems 
  to 
  

   exclude 
  comparison 
  with 
  all 
  large-mouthed, 
  conical-toothed 
  

   Mesozoic 
  ganoids 
  except 
  the 
  Aspidorhynchidae, 
  and 
  these 
  cannot 
  be 
  

   their 
  direct 
  progenitors 
  for 
  many 
  obvious 
  reasons. 
  The 
  irregular 
  

   subdivision 
  of 
  the 
  facial 
  bones 
  has 
  been 
  shown 
  to 
  occur 
  commonly 
  

   among 
  highly-specialized 
  ganoids, 
  though 
  Lepidosteus 
  still 
  remains 
  

   unique 
  in 
  having 
  this 
  segmentation 
  extended 
  to 
  the 
  maxilla. 
  As 
  to 
  

   the 
  well-known 
  remarkable 
  features 
  in 
  the 
  soft 
  parts 
  of 
  the 
  existing 
  

   fish, 
  it 
  is 
  unfortunate 
  that 
  Palaeontology 
  cannot 
  afford 
  any 
  definite 
  

   information. 
  Only 
  one 
  point 
  may 
  be 
  mentioned, 
  namely, 
  that 
  so 
  

   far 
  as 
  can 
  be 
  judged 
  from 
  the 
  cranium, 
  the 
  course 
  of 
  the 
  olfactory 
  

   nerves 
  in 
  Dapedius 
  differs 
  from 
  the 
  course 
  of 
  these 
  nerves 
  in 
  the 
  

   closely-allied 
  genus 
  Lepidotus, 
  exactly 
  in 
  the 
  same 
  way 
  that 
  

  

  