﻿VEETEBKATES. 
  235 
  

  

  they 
  have 
  a 
  pectinated 
  appearance 
  at 
  their 
  edges, 
  in 
  token 
  of 
  the 
  

   tubercles 
  originally 
  ornamenting 
  their 
  crests. 
  They 
  are 
  so 
  crowded 
  

   upon 
  the 
  costae 
  as 
  to 
  compel 
  an 
  alternate 
  disposition 
  ; 
  in 
  the 
  an- 
  

   terior 
  portion 
  they 
  are 
  crowded 
  vertically, 
  but 
  over 
  the 
  greater 
  por- 
  

   tion 
  of 
  the 
  surface 
  they 
  are 
  spaced 
  by 
  about 
  their 
  own 
  diameter. 
  

   The 
  pulp 
  cavity 
  is 
  relatively 
  large 
  at 
  the 
  inferior 
  angle 
  of 
  the 
  pos- 
  

   terior 
  face, 
  beneath 
  wbich 
  it 
  first 
  opens 
  out, 
  exposing 
  the 
  deep 
  

   trough-like 
  excavation 
  extending 
  thence 
  to 
  the 
  proximal 
  extremity, 
  

   but 
  rapidly 
  diminishing 
  above 
  where 
  it 
  is 
  filled 
  by 
  the 
  peculiar 
  dep- 
  

   osition 
  of 
  dense 
  matter 
  analogous 
  to 
  the 
  inferior 
  homogeneous 
  layer 
  

   met 
  with 
  in 
  the 
  associated 
  teeth 
  of 
  Cochliodonts, 
  the 
  50 
  or 
  60 
  mili- 
  

   metres 
  of 
  the 
  distal 
  extremity 
  being 
  solid. 
  

  

  Mr. 
  Butters 
  has 
  obtained 
  from 
  the 
  St. 
  Louis 
  formation 
  a 
  single 
  

   representative 
  of 
  the 
  above 
  described 
  species, 
  representing 
  a 
  spine 
  

   20 
  to 
  25 
  centimetres 
  in 
  length, 
  of 
  which 
  the 
  posterior 
  face 
  consti- 
  

   tuted 
  half, 
  little 
  more 
  or 
  less. 
  Unfortunately, 
  the 
  specimen 
  was 
  

   much 
  mutilated 
  in 
  freeing 
  it 
  from 
  the 
  limestone 
  matrix, 
  both 
  

   extremities 
  being 
  broken, 
  and 
  preserving 
  a 
  length 
  of 
  about 
  15 
  cen- 
  

   timetres 
  of 
  the 
  middle 
  portion 
  of 
  the 
  spine. 
  It 
  is 
  a 
  fine 
  representa- 
  

   tive 
  of 
  a 
  peculiar 
  form 
  of 
  ichthyodorulite, 
  of 
  which 
  several 
  examples 
  

   are 
  known, 
  and 
  which 
  is 
  distinguished 
  by 
  the 
  great 
  fore 
  and 
  aft 
  

   extent 
  of 
  the 
  dorsal 
  line 
  and 
  corresponding 
  breadth 
  of 
  the 
  inserted 
  

   basal 
  portion. 
  In 
  the 
  foregoing 
  description, 
  allusion 
  was 
  made, 
  in 
  

   connection 
  with 
  the 
  latter 
  character, 
  to 
  the 
  probable 
  recumbent 
  

   position 
  the 
  spine 
  occupied 
  in 
  the 
  creature's 
  back; 
  but 
  this, 
  after 
  

   all, 
  may 
  not 
  necessarily 
  indicate 
  unusual 
  recumbency 
  in 
  view 
  of 
  the 
  

   fact 
  that, 
  in 
  certain 
  living 
  sharks, 
  the 
  integuments 
  are 
  built 
  up 
  

   about 
  the 
  bases 
  of 
  the 
  spines, 
  forming 
  low, 
  laterally 
  compressed 
  

   prominences 
  from 
  which 
  their 
  tips 
  protrude, 
  as 
  is 
  the 
  case 
  in 
  Cestra- 
  

   cion. 
  We 
  hesitate, 
  however, 
  in 
  recognizing 
  this 
  feature 
  as 
  possessing 
  

   generic 
  importance 
  in 
  contradistinction 
  to 
  the 
  narrow, 
  more 
  regu- 
  

   larly 
  and 
  gradually 
  tapering 
  spines 
  with 
  which 
  the 
  latter 
  have 
  been 
  

   genericaUy 
  associated. 
  

  

  The 
  specific 
  relations 
  of 
  the 
  present 
  specimen 
  with 
  previously 
  

   described 
  spines 
  are 
  perhaps 
  most 
  intimate 
  with 
  the 
  Kinderhook 
  

   species, 
  Ctenacanthus 
  speciosus. 
  St. 
  J. 
  and 
  W., 
  and 
  C. 
  spectabilis, 
  

   St. 
  J. 
  and 
  W. 
  Its 
  distinguishing 
  features, 
  however, 
  are 
  so 
  pro- 
  

   nounced 
  as 
  not 
  to 
  require 
  detailed 
  comparison. 
  

  

  Geological 
  position 
  and 
  locality: 
  St. 
  Louis 
  limestone 
  ; 
  Alton, 
  111. 
  

  

  