﻿196 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [Mar. 
  9, 
  

  

  The 
  scales 
  which 
  are 
  thus 
  preserved 
  agree 
  in 
  general 
  form 
  and 
  

   arrangement 
  with 
  the 
  uppermost, 
  or 
  dorsal, 
  series 
  of 
  Cephalaspis. 
  

   The 
  scales 
  of 
  this 
  latter 
  genus 
  consist 
  of 
  three 
  vertical 
  series 
  on 
  either 
  

   side 
  of 
  the 
  median 
  line 
  * 
  ; 
  namely, 
  a 
  dorsal 
  range 
  of 
  four 
  rows 
  of 
  

   rhomboidal 
  scales 
  (fig. 
  8), 
  a 
  second 
  series 
  of 
  elongated 
  plates 
  (fig. 
  9) 
  

   similar 
  to 
  those 
  of 
  the 
  recent 
  genus 
  Callichthys, 
  and 
  a 
  third 
  series 
  

   formed 
  by 
  two 
  rows 
  of 
  smaller 
  plates 
  placed 
  in 
  an 
  obliquely 
  pos- 
  

   terior 
  direction. 
  In 
  Pteraspis 
  (figs. 
  6 
  & 
  7), 
  a 
  single 
  row 
  of 
  large 
  

   square 
  scales 
  runs 
  along 
  the 
  median 
  line, 
  from 
  the 
  sides 
  of 
  each 
  of 
  

   which 
  arise 
  a 
  row 
  of 
  rhomboidal 
  scales. 
  The 
  successive 
  rows 
  are 
  so 
  

   disposed 
  that 
  the 
  anterior 
  margin 
  of 
  each 
  scale 
  is 
  overlapped 
  by 
  the 
  

   posterior 
  margin 
  of 
  the 
  one 
  in 
  the 
  preceding 
  row, 
  directly 
  in 
  front 
  

   of 
  it; 
  at 
  the 
  same 
  time 
  the 
  superior 
  margin 
  is 
  concealed 
  by 
  the 
  

   overlapping 
  of 
  the 
  inferior 
  margin 
  of 
  the 
  preceding 
  scale 
  in 
  the 
  same 
  

   row. 
  This 
  constitutes 
  a 
  general 
  imbricated 
  arrangement, 
  whereby 
  

   the 
  inferior 
  and 
  posterior 
  margins 
  only 
  are 
  exposed. 
  There 
  are 
  

   no 
  traces 
  of 
  large 
  lateral 
  plates 
  in 
  my 
  specimen, 
  like 
  those 
  of 
  Ce- 
  

   phalaspis, 
  although 
  it 
  is 
  quite 
  possible 
  that 
  these 
  may 
  have 
  existed. 
  

   It 
  would, 
  however, 
  be 
  unsafe 
  to 
  predicate 
  anything 
  concerning 
  the 
  

   scales 
  of 
  Pteraspis 
  from 
  what 
  we 
  know 
  of 
  Cephalaspis, 
  since 
  the 
  

   analogies 
  between 
  them 
  are 
  merely 
  general, 
  and 
  not 
  of 
  a 
  generic 
  

   nature. 
  

  

  The 
  ornamentation 
  of 
  the 
  scales 
  (fig. 
  1) 
  of 
  Pteraspis 
  is 
  shown 
  

   but 
  imperfectly 
  in 
  the 
  specimen 
  under 
  description. 
  Two, 
  however, 
  

   of 
  the 
  scales 
  preserved 
  show 
  delicate 
  striations, 
  which 
  appear 
  to 
  

   exist 
  on 
  a 
  superficial 
  layer 
  of 
  bony 
  matter, 
  which 
  easily 
  separates 
  

   from 
  the 
  rest 
  of 
  the 
  scale, 
  and 
  is 
  destroyed. 
  The 
  arrangement 
  of 
  

   the 
  markings 
  is 
  drawn 
  in 
  fig. 
  1. 
  No 
  trace 
  of 
  a 
  fin 
  is 
  observable, 
  

   although 
  it 
  seemed 
  probable 
  that 
  the 
  posterior 
  spine 
  was 
  intended 
  

   for 
  the 
  support 
  of 
  some 
  such 
  appendage. 
  The 
  scar 
  only 
  remains 
  

   where 
  the 
  spine 
  was 
  inserted, 
  both 
  it 
  and 
  the 
  fin, 
  if 
  it 
  ever 
  existed, 
  

   having 
  been 
  destroyed. 
  

  

  The 
  restoration 
  of 
  the 
  cephalic 
  shield 
  of 
  Pteraspis 
  has 
  already 
  

   been 
  given 
  by 
  Prof. 
  Huxley 
  in 
  this 
  Journal 
  f 
  in 
  outline. 
  I 
  have 
  no 
  

   additions 
  to 
  make 
  to 
  this 
  ; 
  but 
  since 
  the 
  markings 
  on 
  the 
  surface 
  of 
  

   the 
  test 
  are 
  extremely 
  curious, 
  and 
  have 
  in 
  certain 
  species 
  % 
  from 
  

   older 
  rocks 
  been 
  made 
  the 
  grounds 
  for 
  specific 
  separation, 
  a 
  descrip- 
  

   tion 
  of 
  them 
  in 
  the 
  Pteraspis 
  rostratus 
  will 
  no 
  doubt 
  prove 
  interesting. 
  

   Fig. 
  10 
  represents 
  the 
  cephalic 
  shield 
  restored. 
  The 
  striations 
  

   which 
  mark 
  the 
  whole 
  surface 
  are 
  very 
  minute, 
  and 
  about 
  the 
  jpth 
  

   of 
  an 
  inch 
  apart. 
  In 
  the 
  median 
  line, 
  above 
  the 
  insertion 
  of 
  the 
  

   posterior 
  spine, 
  a 
  slight 
  elevation 
  exists. 
  This 
  is 
  the 
  centre 
  whence 
  

   a 
  series 
  of 
  concentric 
  elliptical 
  striations 
  proceed, 
  gradually 
  as- 
  

   suming 
  the 
  form 
  of 
  the 
  outline 
  of 
  the 
  scute 
  as 
  they 
  approach 
  its 
  

   margin 
  (a). 
  A 
  slight 
  groove 
  or 
  depression 
  marks 
  the 
  anterior 
  por- 
  

   tion 
  along 
  the 
  median 
  line, 
  disappearing 
  towards 
  the 
  elevation 
  of 
  the 
  

   central 
  boss, 
  whence 
  the 
  lines 
  of 
  striation 
  arise. 
  The 
  lateral 
  cornua 
  (b) 
  

  

  * 
  ' 
  Poissons 
  Fossiles,' 
  vol. 
  i. 
  p. 
  137. 
  

  

  t 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xvii. 
  p. 
  163. 
  

  

  t 
  Ibid. 
  vol. 
  xii. 
  p. 
  100. 
  

  

  