﻿262 
  

  

  THE 
  EXTINCT 
  VERTEBRATE 
  ANIMALS 
  

  

  fig. 
  3. 
  Besides 
  this, 
  in 
  some 
  cases, 
  some 
  comparatively 
  large, 
  

   irregularly 
  branching 
  and 
  anastomosing 
  grooves 
  are 
  seen 
  running 
  

   over 
  the 
  surface, 
  which 
  would 
  be 
  converted 
  into 
  canals 
  by 
  the 
  

   outer 
  layer 
  when 
  present. 
  

  

  The 
  outer 
  layer 
  is 
  very 
  thin, 
  and 
  evidently 
  has 
  been 
  liable 
  to 
  be 
  

   easily 
  detached, 
  as 
  it 
  is 
  wanting 
  in 
  most 
  of 
  the 
  specimens 
  from 
  the 
  

   Elgin 
  district. 
  When 
  present 
  it 
  is 
  seen 
  to 
  display 
  an 
  ornament 
  

   of 
  small 
  closely 
  placed 
  stellate 
  tubercles 
  (PI. 
  vi. 
  fig. 
  2 
  magnified 
  

   diameters) 
  which 
  are 
  coarser 
  and 
  more 
  irregular 
  in 
  shape 
  than 
  

   those 
  of 
  the 
  allied 
  Russian 
  species 
  P. 
  paradoxus 
  and 
  P. 
  arenatus. 
  

   But 
  it 
  is 
  worthy 
  of 
  note 
  that 
  in 
  one 
  specimen 
  I 
  have 
  found 
  the 
  

   surface 
  of 
  the 
  outer 
  layer 
  marked 
  off 
  into 
  polygonal 
  slightly 
  con- 
  

   vex 
  areas, 
  as 
  is 
  often 
  seen 
  in 
  P. 
  paradoxus, 
  these 
  areas 
  corresponding 
  

   to 
  the 
  polygonal 
  depressions 
  exhibited 
  in 
  other 
  specimens 
  when 
  

   the 
  outer 
  layer 
  is 
  wanting. 
  Clearly, 
  therefore, 
  the 
  outer 
  layer 
  in 
  

   such 
  cases 
  tends 
  to 
  fall 
  into 
  small 
  scale-like 
  polygonal 
  pieces 
  thicker 
  

   in 
  the 
  centre 
  than 
  at 
  the 
  margins 
  ( 
  = 
  lenticular), 
  and 
  which 
  are 
  

   indeed 
  sometimes 
  found 
  detached 
  in 
  the 
  Russian 
  deposits 
  which 
  

   yield 
  P. 
  paradoxus. 
  

  

  Remarks. 
  — 
  As 
  the 
  plates 
  of 
  Psammosteus 
  exhibit 
  on 
  microscopic 
  

   examination 
  no 
  bone-lacunae, 
  but 
  only 
  dentine 
  tubules, 
  the 
  genus 
  

   is 
  now 
  generally 
  referred 
  to 
  the 
  Elasmobranchii, 
  and 
  considered 
  to 
  

   represent 
  a 
  group 
  of 
  armoured 
  sharks, 
  of 
  which 
  we 
  have 
  of 
  course 
  

   no 
  representatives 
  at 
  the 
  present 
  day. 
  The 
  minute 
  stellate 
  

   tubercles 
  which 
  form 
  the 
  ornament 
  of 
  the 
  outer 
  layer 
  are 
  clearly 
  

   shagreen 
  bodies 
  which 
  have 
  coalesced, 
  as 
  very 
  similarly-shaped 
  

   bodies, 
  still 
  disunited, 
  may 
  be 
  seen 
  to 
  form 
  the 
  dermal 
  covering 
  in 
  

   Powrie's 
  Cephalopterus 
  Paget 
  1 
  from 
  the 
  Lower 
  Old 
  Red 
  of 
  Forfar- 
  

   shire 
  ; 
  the 
  subjacent 
  thickness 
  of 
  the 
  plate 
  being 
  formed 
  in 
  a 
  deeper 
  

   layer 
  of 
  the 
  skin. 
  Only 
  one 
  form 
  of 
  the 
  shield 
  has 
  been 
  discovered, 
  

   so 
  far 
  as 
  P. 
  Taylori 
  is 
  concerned 
  at 
  least, 
  and 
  that 
  it 
  was 
  a 
  body 
  and 
  

   not 
  a 
  head 
  shield, 
  a 
  dorsal 
  and 
  not 
  a 
  ventral 
  one, 
  are 
  about 
  the 
  

   best 
  surmises 
  we 
  can 
  make 
  as 
  to 
  its 
  position 
  on 
  the 
  animal. 
  In 
  

   the 
  same 
  way 
  we 
  may 
  guess 
  that 
  the 
  pointed 
  extremity 
  was 
  pos- 
  

   terior, 
  though 
  we 
  have 
  no 
  real 
  information 
  on 
  the 
  subject. 
  

  

  But 
  associated 
  with 
  the 
  plates 
  of 
  Psammosteus 
  in 
  Russian 
  localities 
  

  

  1 
  The 
  name 
  ' 
  Cephalopterus, 
  having 
  been 
  preoccupied 
  as 
  early 
  as 
  1809, 
  is 
  not 
  

   admissible 
  in 
  this 
  case. 
  I 
  therefore 
  propose 
  to 
  substitute 
  Turinia, 
  from 
  Turin 
  Hill, 
  

   the 
  locality 
  whence 
  Powrie's 
  specimen 
  was 
  derived. 
  

  

  