﻿420 
  PROCEEDINGS 
  OP 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [JuilC 
  22 
  

  

  value. 
  " 
  More 
  perfect 
  specimens, 
  since 
  procured, 
  not 
  only 
  fully 
  bear 
  

   this 
  out, 
  but 
  also 
  show 
  so 
  many 
  other 
  points 
  widely 
  diverging 
  from 
  

   those 
  of 
  Diplacanthus, 
  that 
  I 
  here 
  venture 
  to 
  remove 
  it 
  altogether 
  

   from 
  that 
  genus, 
  and 
  to 
  suggest 
  the 
  new 
  generic 
  name 
  Ischnacan- 
  

   thus 
  (slender-spine). 
  The 
  above 
  generic 
  description 
  will 
  show 
  that 
  

   neither 
  the 
  position 
  nor 
  comparative 
  size 
  of 
  the 
  spines 
  in 
  any 
  respect 
  

   corresponds 
  with 
  those 
  of 
  Dip] 
  'acanthus. 
  The 
  form 
  of 
  the 
  head 
  and 
  

   that 
  of 
  the 
  body 
  are 
  equally 
  different 
  : 
  in 
  Diplacanthus 
  these 
  are 
  

   compressed 
  laterally, 
  the 
  head 
  short, 
  the 
  body 
  short, 
  stout, 
  and 
  ellipti- 
  

   cal 
  in 
  shape 
  ; 
  in 
  Ischnacanthus 
  the 
  body 
  is 
  long, 
  slender, 
  and 
  fusiform 
  ; 
  

   the 
  head 
  long, 
  rather 
  rhomboidal 
  in 
  shape, 
  and 
  somewhat 
  depressed. 
  

   The 
  scales 
  in 
  all 
  species 
  of 
  Diplacanthus 
  appear 
  to 
  have 
  been 
  sculp- 
  

   tured 
  on 
  the 
  under 
  surface 
  ; 
  in 
  Ischnacanthus 
  they 
  are 
  quite 
  smooth 
  

   on 
  both 
  outer 
  and 
  inner 
  surfaces. 
  The 
  head 
  in 
  Diplacanthus 
  is 
  

   covered 
  by 
  scales 
  similar 
  to, 
  but 
  smaller 
  than, 
  those 
  of 
  the 
  body 
  ; 
  in 
  

   Ischnacanthus 
  the 
  indistinctly 
  ossified 
  cranial 
  bones 
  have 
  a 
  coarsely 
  

   granular 
  covering. 
  In 
  Diplacanthus 
  the 
  teeth 
  are 
  described 
  as 
  very 
  

   small 
  ; 
  in 
  Ischnacanthus 
  they 
  are 
  very 
  large. 
  

  

  In 
  Ischnacanthus 
  gracilis 
  the 
  eye-orbits 
  are 
  large, 
  placed 
  widely 
  

   apart 
  and 
  nearly 
  centrical 
  in 
  the 
  head. 
  The 
  nasal 
  orifice 
  is 
  large 
  

   and 
  far 
  forward. 
  The 
  branchial 
  apparatus 
  is 
  obscure, 
  but 
  seems 
  to 
  

   have 
  consisted 
  of 
  a 
  few 
  exposed 
  arches. 
  The 
  fins 
  appear 
  to 
  have 
  

   been 
  membranous 
  and 
  covered 
  with 
  very 
  minute 
  scales. 
  One 
  soli- 
  

   tary 
  specimen 
  shows 
  a 
  pectoral 
  fin 
  apparently 
  having 
  true 
  rays. 
  

  

  Fragments 
  in 
  my 
  possession 
  indicate 
  another 
  species, 
  shorter 
  and 
  

   stouter 
  in 
  form, 
  and 
  having 
  larger 
  and 
  very 
  strong 
  teeth, 
  longer 
  and 
  

   more 
  slender 
  spines, 
  and 
  rather 
  larger 
  scales. 
  This, 
  as 
  yet, 
  wants 
  

   confirmation. 
  

  

  Next 
  to 
  Acanthodes 
  Mitclielli, 
  Ischnacanthus 
  gracilis 
  is 
  the 
  most 
  

   abundant 
  of 
  our 
  Pishes. 
  It 
  is 
  found 
  of 
  all 
  sizes, 
  from 
  little 
  more 
  

   than 
  1 
  inch 
  to 
  over 
  9 
  inches 
  in 
  length. 
  It 
  seems 
  to 
  have 
  been 
  the 
  

   Shark 
  of 
  these 
  olden 
  waters; 
  for 
  while 
  much 
  less 
  strongly 
  protected 
  

   by 
  defensive 
  armour 
  than 
  most 
  of 
  its 
  congeners, 
  its 
  powerful 
  jaws 
  

   and 
  long 
  sharp 
  curved 
  teeth 
  eminently 
  fitted 
  it 
  for 
  preying 
  on 
  the 
  

   other 
  Fishes, 
  while 
  its 
  slender 
  form 
  and 
  large 
  tail 
  and 
  fins 
  must 
  have 
  

   given 
  it 
  amazing 
  swiftness 
  in 
  pursuit. 
  

  

  The 
  name 
  Ictinocephalus 
  granulatus 
  was 
  given 
  by 
  Mr. 
  Page 
  to 
  

   some 
  jaws, 
  scales, 
  and 
  other 
  fragments 
  from 
  Balruddery, 
  evidently 
  

   belonging 
  to 
  this 
  species 
  ; 
  it 
  is 
  by 
  mistake 
  in 
  the 
  Decade 
  assigned 
  

   as 
  a 
  synonym 
  to 
  Olimatius 
  scutiger. 
  It 
  has 
  thus 
  every 
  claim 
  to 
  

   priority. 
  I 
  have, 
  however, 
  Mr. 
  Page's 
  authority 
  for 
  substituting 
  

   Ischnacanthus 
  as 
  more 
  appropriate 
  and 
  more 
  in 
  keeping 
  with 
  the 
  

   nomenclature 
  of 
  the 
  Acanthodian 
  Fishes. 
  

  

  Climatius, 
  Agassiz. 
  

  

  Body 
  more 
  or 
  less 
  fusiform 
  ; 
  head 
  large, 
  rounded 
  anteriorly, 
  some- 
  

   what 
  depressed 
  ; 
  tail 
  elongated, 
  heterocercal 
  ; 
  fins 
  membranous, 
  

   covered 
  with 
  very 
  minute 
  scales 
  in 
  parallel 
  rows, 
  each 
  preceded 
  by 
  a 
  

   spine 
  ; 
  spines 
  stout, 
  rather 
  short, 
  conical, 
  curved 
  or 
  straight, 
  serrated 
  

   longitudinally 
  ; 
  intermediate 
  ridges 
  crenulated 
  or 
  smooth 
  ; 
  two 
  dor- 
  

  

  