﻿XVI 
  INTRODUCTION. 
  

  

  Amiidce. 
  

  

  The 
  separation 
  of 
  the 
  Amiidae 
  from 
  the 
  Eugnathidae 
  is 
  perhaps 
  

   arbitrary, 
  but 
  it 
  is 
  at 
  least 
  convenient. 
  The 
  genus 
  Eurycormus, 
  

   with 
  cycloidal 
  scales 
  and 
  ring-vertebrae, 
  may 
  be 
  as 
  appropriately 
  

   placed 
  in 
  one 
  family 
  as 
  in 
  the 
  other 
  ; 
  so 
  also 
  may 
  the 
  small 
  noto- 
  

   chordal 
  fishes 
  named 
  Liodesmus. 
  The 
  former, 
  however, 
  is 
  assigned 
  

   to 
  the 
  Eugnathidae 
  because 
  it 
  exhibits 
  fulcra 
  od 
  the 
  fins 
  ; 
  the 
  latter 
  

   to 
  the 
  Amiidae, 
  because 
  all 
  its 
  fins 
  seem 
  to 
  be 
  destitute 
  of 
  fulcra. 
  

  

  Amia 
  itself 
  does 
  not 
  date 
  back 
  beyond 
  the 
  Upper 
  Eocene, 
  but 
  the 
  

   extinct 
  species 
  and 
  possibly 
  allied 
  genera 
  are 
  very 
  unsatisfactorily 
  

   known, 
  owing 
  to 
  the 
  fragmentary 
  nature 
  of 
  the 
  fossils. 
  Never- 
  

   theless, 
  the 
  vertebral 
  centra 
  are 
  so 
  characteristic 
  that 
  it 
  is 
  possible 
  

   to 
  obtain 
  considerable 
  information 
  as 
  to 
  its 
  former 
  range 
  ; 
  and 
  the 
  

   interesting 
  result 
  is 
  arrived 
  at, 
  that 
  Amia 
  (or 
  fishes 
  represented 
  by 
  

   fragments 
  indistinguishable 
  from 
  it) 
  was 
  distributed 
  throughout 
  

   Western 
  Europe 
  in 
  the 
  Eocene 
  period, 
  and 
  survived 
  in 
  some 
  areas 
  

   so 
  late 
  as 
  the 
  Lower 
  Miocene. 
  The 
  supposed 
  Amioid 
  from 
  the 
  

   Miocene 
  of 
  Oeningen, 
  Switzerland, 
  proves 
  to 
  have 
  been 
  wrongly 
  

   determined, 
  and 
  is 
  a 
  truly 
  "Teleostean" 
  fish. 
  

  

  Pachycormidce. 
  

  

  A 
  remarkable 
  modification 
  of 
  the 
  thin-scaled 
  type 
  of 
  Eugnathidae 
  

   suddenly 
  appears 
  in 
  the 
  Upper 
  Lias. 
  The 
  segments 
  of 
  the 
  axial 
  

   skeleton 
  of 
  the 
  trunk 
  multiply 
  considerably 
  ; 
  the 
  rostrum 
  begins 
  to 
  

   project 
  and 
  prevents 
  the 
  premaxillae 
  from 
  meeting 
  in 
  the 
  middle 
  

   line 
  ; 
  while 
  the 
  fins, 
  especially 
  the 
  pectorals 
  and 
  caudal, 
  are 
  adapted 
  

   for 
  rapid 
  progression. 
  This 
  modification, 
  in 
  fact, 
  eventually 
  results 
  

   in 
  genera 
  having 
  the 
  outward 
  form 
  of 
  the 
  modern 
  " 
  sword-fishes 
  " 
  ; 
  

   and 
  they 
  are 
  particularly 
  interesting 
  as 
  showing 
  how 
  the 
  Proto- 
  

   spondylic 
  type 
  of 
  axial 
  skeleton 
  may 
  be 
  modified 
  to 
  produce 
  the 
  

   same 
  mechanical 
  effect 
  as 
  a 
  robust 
  chain 
  of 
  vertebrae 
  terminated 
  

   behind 
  by 
  its 
  complex 
  hypural 
  bone. 
  All 
  these 
  fishes 
  are 
  proved 
  

   to 
  have 
  had 
  a 
  persistent 
  notochord, 
  and 
  it 
  is 
  not 
  improbably 
  to 
  com- 
  

   pensate 
  for 
  this 
  weakness 
  that 
  the 
  segments 
  of 
  the 
  axial 
  skeleton 
  

   are 
  so 
  remakably 
  shortened 
  and 
  multiplied. 
  In 
  most 
  genera 
  also 
  

   a 
  single, 
  much-expanded 
  haemal 
  arch 
  has 
  been 
  observed 
  at 
  the 
  base 
  

   of 
  the 
  lower 
  lobe 
  of 
  the 
  powerful, 
  deeply-forked 
  tail 
  ; 
  this 
  admir- 
  

   ably 
  serving 
  the 
  purpose 
  of 
  a 
  hypural, 
  and 
  yet 
  not 
  fundamentally 
  

   altering 
  the 
  normal 
  " 
  stegurous 
  " 
  tail 
  such 
  as 
  characterizes 
  all 
  

   Protospondyli. 
  

  

  