﻿Vlll 
  INTRODUCTION. 
  

  

  whole 
  of 
  the 
  branchiostegal 
  apparatus 
  is 
  wanting, 
  while 
  the 
  opercular 
  

   series 
  is 
  reduced 
  to 
  a 
  single 
  small 
  operculum. 
  If 
  these 
  are 
  not 
  

   degenerate 
  Chondrosteans 
  they 
  must 
  be 
  abnormally 
  modified 
  

   Crossopterygians, 
  as 
  suggested 
  by 
  0. 
  M. 
  Reis 
  \ 
  

  

  Degeneracy 
  of 
  the 
  Palaeoniscid 
  type 
  in 
  the 
  direction 
  of 
  the 
  

   modern 
  sturgeons 
  is 
  distinctly 
  observed 
  in 
  the 
  Chondrosteidse 
  of 
  

   the 
  English 
  Lower 
  Lias. 
  CJiondrosteus 
  resembles 
  the 
  Palseoniscidse 
  

   in 
  the 
  arrangement 
  of 
  its 
  cranial 
  roof 
  and 
  the 
  opercular 
  apparatus, 
  

   except 
  that 
  the 
  gular 
  plate 
  and 
  possibly 
  a 
  few 
  of 
  the 
  branchiostegal 
  

   rays 
  have 
  disappeared 
  ; 
  its 
  trunk 
  is 
  like 
  that 
  of 
  the 
  abnormal 
  

   Palaeoniscid 
  Phaner 
  'osteon, 
  and 
  equally 
  similar 
  to 
  that 
  of 
  the 
  modern 
  

   Polyodon 
  ; 
  its 
  reduced 
  toothless 
  jaws 
  are 
  much 
  simpler 
  than 
  those 
  

   of 
  the 
  Palaeoniscidse, 
  and 
  the 
  premaxillaa 
  have 
  disappeared 
  as 
  in 
  all 
  

   the 
  modern 
  sturgeons 
  and 
  paddle-fishes. 
  A 
  more 
  clearly 
  inter- 
  

   mediate 
  type 
  can 
  scarcely 
  be 
  conceived 
  ; 
  and 
  the 
  fact 
  that 
  the 
  

   Polyodontidse 
  at 
  least 
  once 
  possessed 
  a 
  regular 
  squamation 
  is 
  

   demonstrated 
  by 
  the 
  Eocene 
  genus 
  Crossopholis. 
  

  

  Peotospondtli. 
  

  

  It 
  has 
  already 
  been 
  remarked 
  that 
  the 
  Catopteridse 
  of 
  the 
  Trias 
  

   incline 
  towards 
  a 
  higher 
  type 
  of 
  fish 
  than 
  the 
  Chondrostei, 
  to 
  which 
  

   they 
  technically 
  belong 
  ; 
  but 
  the 
  two 
  known 
  genera 
  cannot 
  be 
  the 
  

   ancestors 
  of 
  this 
  more 
  advanced 
  race, 
  for 
  at 
  least 
  one 
  of 
  its 
  repre- 
  

   sentatives 
  (Acentrojphorus) 
  has 
  already 
  been 
  found 
  in 
  the 
  Upper 
  

   Permian, 
  while 
  numerous 
  and 
  varied 
  forms 
  are 
  commonly 
  met 
  

   with 
  in 
  the 
  Trias 
  and 
  Ehastic. 
  It 
  can 
  only 
  be 
  affirmed 
  that 
  as 
  

   soon 
  as 
  six 
  important 
  modifications 
  had 
  simultaneously 
  affected 
  

   the 
  Chondrostean 
  skeleton, 
  another 
  vigorous 
  race 
  arose 
  and 
  a 
  new 
  

   impetus 
  seems 
  to 
  have 
  been 
  given 
  to 
  variation. 
  These 
  changes 
  

   comprised 
  (i.) 
  the 
  almost 
  complete 
  atrophy 
  of 
  the 
  upper 
  caudal 
  lobe, 
  

   (ii.) 
  the 
  reduction 
  of 
  the 
  dorsal 
  and 
  anal 
  fin-rays 
  to 
  exactly 
  the 
  

   same 
  number 
  as 
  their 
  supports, 
  (iii.) 
  the 
  disappearance 
  of 
  the 
  

   infraclavicular 
  plates, 
  (iv.) 
  the 
  loss 
  of 
  the 
  pelvic 
  baseosts, 
  (v.) 
  the 
  

   subdivision 
  of 
  the 
  hinder 
  expansion 
  of 
  the 
  maxilla, 
  and 
  (vi.) 
  the 
  

   withdrawal 
  of 
  the 
  preoperculum 
  from 
  its 
  extension 
  over 
  the 
  cheek. 
  

   Numerous 
  types, 
  in 
  some 
  respects 
  parallel 
  to 
  those 
  already 
  noticed 
  

   among 
  the 
  Chondrostei, 
  are 
  to 
  be 
  recognized 
  in 
  this 
  later 
  race 
  ; 
  and 
  

   the 
  only 
  difficulty 
  is 
  that, 
  owing 
  to 
  the 
  imperfection 
  of 
  the 
  geolo- 
  

   gical 
  record, 
  very 
  few 
  of 
  these 
  types 
  are 
  revealed 
  until 
  approaching 
  

  

  1 
  O. 
  M. 
  Keis, 
  " 
  Zur 
  Osteologie 
  und 
  Systematik 
  der 
  Belonorhynchiden 
  und 
  

   Tetragonolepiden," 
  Geogn. 
  Jahresh. 
  1891 
  (1892), 
  p. 
  157, 
  

  

  