﻿XX 
  INTKODTJCTION. 
  

  

  hitherto 
  been 
  discovered 
  in 
  the 
  jaw. 
  It 
  is 
  thus 
  very 
  probable 
  that 
  

   the 
  Pholidophoridse 
  are 
  the 
  earliest 
  known 
  family 
  of 
  Isospondyli 
  ; 
  

   and 
  if 
  more 
  conclusive 
  specimens 
  eventually 
  confirm 
  this 
  inference, 
  

   the 
  suborder 
  is 
  proved 
  to 
  date 
  back 
  at 
  least 
  to 
  the 
  Upper 
  Trias. 
  

   In 
  any 
  case 
  it 
  is 
  remarkable 
  that 
  Leptolepis 
  and 
  its 
  immediate 
  

   allies 
  were 
  a 
  comparatively 
  insignificant 
  race 
  until 
  the 
  Cretaceous 
  

   period, 
  when 
  the 
  Isospondyli 
  and 
  higher 
  suborders 
  became 
  domi- 
  

   nant 
  ; 
  and 
  they 
  form 
  an 
  interesting 
  example 
  of 
  the 
  long 
  persistence 
  

   of 
  a 
  newly-acquired 
  character 
  before 
  its 
  possession 
  becomes 
  a 
  factor 
  

   of 
  real 
  importance 
  in 
  favouring 
  the 
  supremacy 
  of 
  the 
  type. 
  

  

  The 
  Pholidophoridae, 
  as 
  defined 
  in 
  the 
  Catalogue, 
  are 
  the 
  only 
  

   fishes 
  with 
  articulated 
  rhombic 
  scales 
  which 
  can 
  be 
  reasonably 
  

   claimed 
  as 
  Isospondyli 
  ; 
  and 
  even 
  the 
  peg-and-socket 
  articulation 
  

   is 
  inconstant 
  if 
  the 
  genus 
  Archceomcene 
  (p. 
  488) 
  is 
  correctly 
  associated 
  

   with 
  them. 
  The 
  most 
  generalized 
  member 
  of 
  the 
  family, 
  Pholido- 
  

   phorus 
  itself, 
  ranges 
  practically 
  unchanged 
  from 
  the 
  Upper 
  Trias, 
  

   throughout 
  the 
  Jurassic, 
  as 
  far 
  as 
  the 
  Purbeckian. 
  An 
  apparently 
  

   allied 
  genus 
  (Thoracojpterus), 
  with 
  excessively 
  enlarged 
  pectoral 
  fins, 
  

   is 
  restricted 
  to 
  the 
  Upper 
  Trias. 
  The 
  Triassic 
  Pholidopleurus, 
  as 
  

   already 
  mentioned, 
  is 
  very 
  doubtfully 
  placed 
  here 
  ; 
  but 
  two 
  other 
  

   genera 
  with 
  much-deepened 
  flank-scales, 
  namely, 
  Peltopleurus 
  of 
  the 
  

   Upper 
  Trias, 
  and 
  Pleuropholis 
  of 
  the 
  Upper 
  Jurassic 
  and 
  Pur- 
  

   beckian, 
  may 
  be 
  ascribed 
  to 
  the 
  Pholidophoridse 
  with 
  less 
  hesitation 
  ; 
  

   and 
  there 
  is 
  possibly 
  an 
  almost 
  scaleless 
  representative 
  of 
  the 
  family 
  

   (Ceramurus) 
  in 
  the 
  Purbeckian. 
  One 
  unique 
  feature 
  is 
  also 
  worthy 
  

   of 
  special 
  note 
  in 
  PleurojpJiolis, 
  namely, 
  that 
  the 
  canal 
  of 
  the 
  

   " 
  lateral 
  line 
  " 
  is 
  deflected 
  immediately 
  behind 
  the 
  pectoral 
  arch 
  

   and 
  extends 
  not 
  along 
  the 
  deepest 
  flank-scales 
  but 
  along 
  the 
  

   adjoining 
  series 
  below. 
  

  

  OUgojpleuridce. 
  

  

  The 
  few 
  Upper 
  Jurassic 
  and 
  Cretaceous 
  fishes 
  included 
  in 
  this 
  

   family 
  bear 
  much 
  superficial 
  resemblance 
  to 
  the 
  Arniidse. 
  All 
  of 
  

   them, 
  however, 
  exhibit 
  completely 
  ossified 
  vertebral 
  centra, 
  never 
  

   any 
  alternating 
  pleurocentral 
  and 
  hypocentral 
  discs 
  even 
  in 
  the 
  

   caudal 
  region 
  ; 
  while 
  the 
  mandible 
  appears 
  to 
  be 
  destitute 
  of 
  

   splenial 
  and 
  coronoid 
  elements, 
  and 
  thus 
  confirms 
  the 
  suspicion 
  

   suggested 
  by 
  the 
  first 
  character, 
  that 
  these 
  fishes 
  must 
  be 
  referred 
  

   to 
  a 
  higher 
  suborder 
  than 
  that 
  of 
  the 
  Protospondyli. 
  The 
  fins, 
  like 
  

   those 
  of 
  the 
  Pholidophoridse, 
  are 
  fringed 
  with 
  fulcra 
  ; 
  and 
  it 
  seems 
  

   most 
  reasonable 
  in 
  the 
  present 
  state 
  of 
  knowledge 
  to 
  place 
  the 
  

  

  