﻿VEKTEBEATES. 
  

  

  237 
  

  

  or 
  8 
  centimetres 
  from 
  the 
  tip 
  before 
  it 
  opened 
  out 
  in 
  the 
  deep, 
  

   trough-like 
  excavation 
  that 
  extends 
  thence 
  to 
  the 
  proximal 
  ex- 
  

   tremity. 
  

  

  As 
  has 
  already 
  been 
  remarked, 
  the 
  specimen 
  described 
  may 
  prove 
  

   to 
  be 
  specifically 
  identical 
  with 
  that 
  noticed 
  under 
  the 
  head 
  of 
  

   Ctenacanthus 
  deflexm. 
  Bat 
  this 
  relationship 
  can 
  be 
  satisfactorily 
  

   determined 
  only 
  from 
  the 
  examination 
  of 
  additional 
  materials, 
  both 
  

   forms 
  being 
  at 
  the 
  present 
  time 
  known 
  from 
  unique 
  and 
  imperfect 
  

   examples. 
  The 
  specimen 
  here 
  referred 
  to 
  was 
  obtained 
  by 
  Mr. 
  

   George 
  B. 
  Harrison, 
  of 
  Bloomington, 
  to 
  whom 
  we 
  are 
  also 
  indebted 
  

   for 
  other 
  interesting 
  contributions. 
  

  

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

   nois. 
  

  

  Ctenacanthus 
  Pellensis, 
  St. 
  J. 
  and 
  W. 
  

  

  PI. 
  XXI, 
  Pig. 
  2 
  

  

  The 
  collection 
  of 
  Mr. 
  Van 
  Home 
  contains 
  a 
  fragment 
  of 
  a 
  medium- 
  

   sized 
  spine 
  of 
  a 
  species 
  not 
  hitherto 
  noticed. 
  The 
  specimen 
  pre- 
  

   serves 
  a 
  length 
  of 
  about 
  15 
  millimetres 
  apparently 
  from 
  near 
  the 
  

   dorsal 
  line, 
  and 
  from 
  the 
  high 
  opening 
  of 
  the 
  pulp 
  cavity 
  and 
  other 
  

   characters 
  to 
  be 
  noticed 
  further 
  on, 
  it 
  would 
  seem 
  to 
  belong 
  to 
  the 
  

   section 
  of 
  Ctenacanthi 
  characterized 
  by 
  the 
  great 
  extent 
  of 
  the 
  dorsal 
  

   line. 
  The 
  fragment 
  indicates 
  a 
  thick, 
  gradually 
  tapering 
  form, 
  obtuse 
  

   wedge-shaped 
  in 
  cross 
  section, 
  the 
  posterior 
  side 
  deeply 
  excavated 
  

   by 
  the 
  open 
  trough 
  of 
  the 
  pulp 
  cavity, 
  which 
  is 
  defined 
  by 
  the 
  bev- 
  

   eled 
  edges 
  of 
  the 
  postero-lateral 
  angles. 
  Lateral 
  surfaces 
  gently 
  

   convex, 
  somewhat 
  sharply 
  arched 
  into 
  the 
  rounded 
  or 
  subangular 
  

   anterior 
  edge. 
  The 
  surface 
  ornamentation 
  is 
  very 
  intricate 
  and 
  ele- 
  

   gant, 
  consisting 
  of 
  numerous 
  delicate 
  longitudinal 
  costse 
  more 
  or 
  

   less 
  regularly 
  diminishing 
  in 
  size 
  from 
  the 
  anterior 
  edge, 
  more 
  fre- 
  

   quently 
  bifurcated 
  and 
  deflected 
  on 
  nearing 
  the 
  postero-lateral 
  angles 
  

   where 
  they 
  cease 
  at 
  the 
  exterior 
  beveled 
  edge 
  ; 
  in 
  front 
  the 
  costse 
  

   present 
  plain, 
  rounded 
  enameled 
  crests 
  spaced 
  by 
  narrower 
  inter- 
  

   vening 
  sulci, 
  their 
  lateral 
  edges 
  studded 
  with 
  delicate 
  downward 
  

   curved 
  transverse 
  carinse 
  or 
  tubercles 
  ; 
  the 
  third 
  rib 
  from 
  the 
  dorsal 
  

   edge 
  shows 
  more 
  or 
  less 
  distinct 
  undulations, 
  and 
  the 
  fifth 
  rib 
  is 
  

   surmounted 
  by 
  small 
  stellate 
  tubercles 
  more 
  or 
  less 
  variable 
  in 
  the 
  

   details 
  of 
  surface 
  sculpture 
  and 
  disposition, 
  their 
  apices 
  directed 
  

   upward, 
  and 
  which 
  apparently 
  extend 
  over 
  the 
  entire 
  posterior 
  flank 
  

   of 
  the 
  lateral 
  surfaces. 
  

  

  