﻿196 
  ACTINOPTERYGII. 
  

  

  is 
  then 
  a 
  false 
  appearance 
  of 
  multiplicity 
  of 
  rays. 
  All 
  the 
  rays 
  are 
  

   more 
  or 
  less 
  articulated, 
  and 
  they 
  also 
  usually 
  bifurcate 
  ; 
  but 
  

   there 
  are 
  no 
  traces 
  of 
  fulcra, 
  unless 
  a 
  few 
  short, 
  undivided 
  basal 
  

   rays 
  at 
  the 
  origin 
  of 
  the 
  caudal 
  fin 
  are 
  placed 
  in 
  this 
  category. 
  

  

  The 
  scales 
  are 
  all 
  quadrangular 
  and 
  somewhat 
  deepened 
  on 
  the 
  

   flank. 
  Their 
  anterior 
  margin 
  is 
  strengthened 
  by 
  a 
  vertical 
  inner 
  

   rib, 
  which 
  forms 
  a 
  peg-and-socket 
  articulation 
  to 
  unite 
  the 
  scales 
  

   of 
  each 
  transverse 
  (vertical) 
  series 
  ; 
  and 
  there 
  is 
  one 
  exceptional 
  

   instance 
  (Mesturus) 
  in 
  which 
  this 
  union 
  is 
  still 
  further 
  strengthened 
  

   by 
  an 
  interlocking 
  jagged 
  suture 
  of 
  the 
  superposed 
  upper 
  and 
  lower 
  

   margins. 
  The 
  series 
  are 
  for 
  the 
  most 
  part 
  regular 
  ; 
  but 
  in 
  Mesturus 
  

   there 
  are 
  a 
  few, 
  short, 
  irregular 
  intercalary 
  series 
  both 
  dorsally 
  and 
  

   ventrally, 
  while 
  in 
  this 
  genus, 
  Gyrodus, 
  and 
  Palmohalistum 
  there 
  is 
  

   a 
  fine 
  subdivision 
  of 
  the 
  scales 
  at 
  the 
  base 
  of 
  the 
  dorsal 
  and 
  anal 
  

   fins. 
  "When 
  the 
  squamation 
  degenerates 
  on 
  the 
  caudal 
  region, 
  it 
  

   does 
  not 
  appear 
  to 
  pass 
  through 
  the 
  stage 
  represented 
  in 
  the 
  

   Dapedioid 
  genus 
  Aetheolepis 
  (see 
  p. 
  157) 
  ; 
  the 
  rhombic 
  scales 
  seem 
  

   to 
  disappear 
  directly 
  without 
  first 
  becoming 
  cycloidal, 
  for 
  remnants 
  

   of 
  their 
  anterior 
  thickened 
  rib 
  are 
  often 
  observed 
  on 
  the 
  otherwise 
  

   naked 
  tail. 
  The 
  " 
  lateral 
  line 
  " 
  extends 
  along 
  the 
  flank 
  from 
  the 
  outer 
  

   supratemporal 
  to 
  the 
  middle 
  of 
  the 
  caudal 
  pedicle 
  ; 
  and 
  there 
  is 
  also 
  

   always 
  an 
  upper 
  canal 
  passing 
  along 
  the 
  top 
  of 
  the 
  flank 
  to 
  the 
  

   origin 
  of 
  the 
  dorsal 
  fin. 
  The 
  ridge-scales 
  are 
  usually 
  small 
  and 
  

   inconspicuous 
  ; 
  but 
  in 
  Palceohalistum 
  and 
  Pycnodus 
  those 
  of 
  the 
  

   ventral 
  ridge 
  appear 
  to 
  have 
  considerably 
  extended 
  limbs. 
  

  

  The 
  hinder 
  end 
  of 
  the 
  abdominal 
  cavity 
  is 
  nearly 
  always 
  marked 
  

   by 
  a 
  long, 
  curved 
  bone, 
  extending 
  from 
  a 
  slight 
  expansion 
  at 
  the 
  

   ventral 
  border 
  just 
  in 
  advance 
  of 
  the 
  anal 
  fin, 
  and 
  tapering 
  to 
  a 
  

   point 
  immediately 
  below 
  the 
  notochordal 
  axis. 
  In 
  Gyrodus 
  (PI. 
  XYI. 
  

   fig. 
  2, 
  x) 
  this 
  has 
  much 
  the 
  aspect 
  of 
  an 
  azygous 
  internal 
  bone 
  (as 
  

   described 
  by 
  B. 
  Tetter, 
  Mittheil. 
  k. 
  mineral.-geol. 
  Mus. 
  Dresden, 
  

   pt. 
  iv. 
  1881, 
  p. 
  36) 
  ; 
  while 
  in 
  Coelodus, 
  Pcdceohalistum 
  and 
  Pycnodus 
  

   it 
  appears 
  rather 
  as 
  a 
  great 
  paired 
  dermal 
  scute 
  (as 
  described 
  by 
  J. 
  J. 
  

   Heckel, 
  Denkschr. 
  k. 
  Akad. 
  Wiss., 
  math.-naturw. 
  CI. 
  vol. 
  xi. 
  1856, 
  

   pp. 
  216, 
  221, 
  241). 
  The 
  element 
  is 
  certainly 
  not 
  an 
  enlarged 
  support 
  

   of 
  the 
  anal 
  fin, 
  and 
  has 
  no 
  connection 
  with 
  the 
  pelvic 
  fins. 
  It 
  must 
  

   still 
  remain 
  problematical. 
  

  

  In 
  arranging 
  the 
  genera 
  of 
  Pycnodonts, 
  the 
  presence 
  or 
  absence 
  

   of 
  scales 
  on 
  the 
  caudal 
  region 
  cannot 
  be 
  regarded 
  as 
  of 
  prime 
  

   importance, 
  owing 
  to 
  the 
  uncertainty 
  of 
  the 
  character 
  in 
  some 
  

   degree 
  in 
  Microdon 
  and 
  in 
  a 
  greater 
  degree 
  in 
  Pcdceohalistum. 
  

   Specialization 
  seems 
  to 
  result 
  chiefly 
  (i.) 
  in 
  the 
  strengthening 
  of 
  

   the 
  backbone 
  by 
  the 
  interlocking 
  of 
  its 
  arches 
  ; 
  (ii.) 
  in 
  the 
  reduc- 
  

  

  