﻿848 
  PALEONTOLOGY 
  OP 
  ILLINOIS. 
  

  

  spine, 
  which 
  probably 
  attained 
  a 
  length 
  of 
  at 
  least 
  160 
  nun., 
  and 
  

   though 
  somewhat 
  distorted 
  by 
  pressure, 
  it 
  presents 
  in 
  ■ 
  \ 
  

  

  !>• 
  manner 
  the 
  details 
  of 
  the 
  surface 
  ornamentation 
  of 
  ■ 
  

   handsonu 
  fin-defense, 
  of 
  which 
  illustrations 
  are 
  given 
  in 
  the 
  

   plate 
  cited. 
  Toward 
  the 
  tip 
  in 
  the 
  older 
  portions 
  of 
  the 
  spine 
  the 
  

  

  lateral 
  oosteB 
  are 
  worn 
  nearly 
  smooth 
  along 
  their 
  srests, 
  though 
  re- 
  

   taining 
  vestiges 
  of 
  the 
  transverse 
  tuberculations 
  in 
  the 
  pectinated 
  

   lateral 
  borders. 
  Toward 
  the 
  base 
  the 
  costa- 
  show 
  uniformly 
  less 
  effects 
  

   from 
  wear, 
  where 
  the 
  transverse 
  ridges 
  are 
  beautifully 
  j 
  

   But 
  throughout 
  its 
  entire 
  length 
  the 
  dorsal 
  ridge 
  presents 
  the 
  same 
  

   uniform 
  plain, 
  smooth 
  surface, 
  an 
  effect 
  which 
  it 
  would 
  seern 
  in- 
  

   consistent 
  to 
  attribute 
  to 
  abrasion. 
  The 
  specimen 
  also 
  dearly 
  shows 
  

   the 
  form 
  of 
  the 
  posterior 
  denticles, 
  as 
  described, 
  and 
  which 
  offer 
  

   altogether 
  an 
  anomalous 
  feature 
  compared 
  with 
  the 
  downward- 
  

   hooked 
  denticles 
  occurring 
  in 
  the 
  borders 
  of 
  the 
  posterior 
  face 
  of 
  

   typical 
  Ctenacantkus. 
  

  

  Geological 
  position 
  and 
  Uicalitii: 
  St. 
  Louis 
  limestone 
  : 
  Alton. 
  Illi- 
  

   nois. 
  

  

  Gems 
  ASTKKOI'TYCIIirs. 
  

  

  A8TER01TVC1III 
  - 
  THHBLLUB, 
  St. 
  .1. 
  and 
  W. 
  

   PL 
  \\i. 
  I 
  be 
  *. 
  

  

  Pin 
  spine 
  small, 
  gradually 
  tapering 
  and 
  moderately 
  arched 
  along 
  

   the 
  anterior 
  edge, 
  much 
  oompressed 
  laterally, 
  giving 
  the 
  trans' 
  

   section 
  a 
  thin 
  wedge-shaped 
  outline, 
  posterior 
  face 
  unknown, 
  

   eral 
  surfaces 
  gently 
  OOnvei 
  tram 
  occupied 
  by 
  regular, 
  delicate 
  

  

  rounded 
  OOSte, 
  intercostal 
  furrows, 
  increasing 
  in 
  width 
  toward 
  the 
  

  

  anterior 
  edge, 
  the 
  posterior 
  furrows 
  oocupied 
  by 
  a 
  singb 
  

  

  punctate 
  line, 
  the 
  mid. 
  lie 
  and 
  anterior 
  ones 
  showing 
  two 
  or 
  more 
  

  

  such 
  lines 
  separated 
  by 
  thread 
  lik. 
  .anna, 
  the 
  wide 
  d. 
  pro-iei 
  

   side 
  the 
  anterior 
  Keel 
  bearing 
  Several 
  of 
  these 
  intercostal 
  stria 
  and 
  

  

  filiform 
  i 
  the 
  anterior 
  keel, 
  like 
  the 
  lateral 
  oosts), 
  is 
  snv< 
  

  

  in 
  a 
  smooth 
  enamel 
  layer. 
  The 
  pulp 
  oavity 
  occupies 
  about 
  half 
  the 
  

  

  diameter 
  "f 
  the 
  spine, 
  presenting 
  an 
  angular-ovate 
  mtion, 
  from 
  

  

  which 
  may 
  he 
  inf. 
  in 
  d 
  the 
  keeled 
  condition 
  of 
  the 
  posterior 
  f:, 
  

  

  The 
  abov< 
  rep 
  re 
  se 
  nted 
  by 
  a 
  fragment 
  apparentl] 
  

  

  to 
  the 
  middle 
  portion 
  of 
  a 
  unall 
  delicate 
  ipine, 
  whioh 
  shows 
  

  

  interior 
  keel 
  and 
  the 
  lateral 
  surface 
  n. 
  arly 
  to 
  th. 
  p. 
  iters! 
  

  

  '"it 
  the 
  , 
  f 
  wall- 
  are 
  br.-k. 
  n 
  away, 
  i 
  

  

  