﻿246 
  PALEONTOLOGY 
  OF 
  [LL1NOIB. 
  

  

  (i. 
  a. 
  a 
  EUNEMACANTHUS, 
  1 
  Bt, 
  J. 
  and 
  W. 
  

  

  Pin-spines 
  arched 
  backward, 
  hit*.- 
  rally 
  compressed. 
  Dorsal 
  border 
  

   rounded 
  and 
  oeoapied 
  by 
  a 
  single 
  plain, 
  smooth 
  ridge 
  enameled 
  

   throughout, 
  and 
  perhaps 
  inbeveled 
  along 
  the 
  lateral 
  edges, 
  which 
  

   may 
  be 
  delicately 
  sculptured. 
  Lateral 
  Burfaoes 
  bearing 
  Longitudinal 
  

  

  costa- 
  interrupted 
  by 
  transverse 
  ridges. 
  Inter-costal 
  furrows 
  plain. 
  

   or 
  occupied 
  by 
  irregularly 
  dispersed 
  tubercles. 
  Posterior 
  face 
  trun- 
  

   cated, 
  the 
  lateral 
  angles 
  bearing 
  a 
  row 
  of 
  denticles 
  whose 
  apices 
  

   are 
  directed 
  upward. 
  Pulp 
  cavity 
  apparently 
  inclosed 
  throughout 
  

   the 
  greater 
  length 
  of 
  the 
  exposed 
  portion 
  of 
  the 
  spine. 
  

  

  The 
  typical 
  and 
  sob- 
  representative 
  of 
  the 
  genus 
  here 
  recogni 
  

   is 
  the 
  dorsal 
  spine, 
  originally 
  described 
  by 
  Messrs. 
  Newberry 
  aid 
  

   Worthen, 
  under 
  the 
  name 
  Ctenacantkuti 
  eottaUu, 
  from 
  the 
  Saint 
  

   Louis 
  limestone. 
  Allied 
  to 
  Ctenacantkus, 
  the 
  genus 
  is 
  especially 
  

   characterized 
  by 
  the 
  presence 
  of 
  the 
  plain 
  dorsal 
  ridge, 
  the 
  tul 
  

   ciliated 
  inter-costal 
  sulci, 
  and 
  the 
  upward 
  direction 
  of 
  the 
  denticles 
  

   arming 
  the 
  angles 
  of 
  the 
  posterior 
  face. 
  The 
  interrupted 
  oh 
  

   of 
  the 
  costie 
  of 
  the 
  lateral 
  surfaces 
  is 
  precisely 
  of 
  the 
  Bame 
  nature 
  

   so 
  prevalent 
  amongst 
  though 
  not 
  restricted 
  to 
  the 
  species 
  of 
  ('.-. 
  

   acanthus; 
  the 
  tuberoulose 
  inter-costal 
  grooves 
  and 
  the 
  direction 
  of 
  

   tin- 
  apices 
  of 
  the 
  posterior 
  denticles 
  recall 
  a 
  relationship 
  with 
  

   leroptychtuM, 
  agues., 
  from 
  which, 
  however, 
  it 
  is 
  distinguished 
  by 
  

   the 
  transversely 
  ridged 
  and 
  perhaps 
  tuberoulose 
  character 
  of 
  

  

  the 
  OOStffi. 
  

  

  El 
  mm 
  \, 
  \\ 
  i 
  in 
  - 
  ( 
  08TA1 
  '!■! 
  W. 
  • 
  . 
  

  

  pl 
  xxni. 
  Ha 
  

  

  '/nix? 
  ronlatur. 
  N 
  umii.i.y 
  ;.' 
  

  

  fl. 
  XII. 
  f. 
  :.. 
  

  

  spim 
  .»f 
  medium 
  Bize, 
  moderately 
  arohed 
  along 
  the 
  dorsal 
  • 
  

   and 
  Bomewhal 
  rapidly 
  tap. 
  in 
  rally 
  oompressed, 
  presenting 
  

  

  «i 
  dge 
  -hap. 
  .1 
  trai 
  on, 
  I 
  i 
  • 
  ant< 
  rior 
  or 
  dorsal 
  

  

  bed 
  transversely 
  in 
  about 
  i 
  quarter 
  of 
  i 
  circle, 
  lal 
  

   ml., 
  eli 
  i 
  and 
  mark.. 
  I 
  with 
  delicate, 
  -harp 
  raised 
  lines 
  directed 
  

   obliquelj 
  downward, 
  the 
  superior 
  surface 
  enveloped 
  in 
  s 
  polisl 
  

   snamel 
  layer, 
  occupying 
  s 
  Bpaoe 
  about 
  equal 
  to 
  two 
  of 
  I 
  

   The 
  laf 
  [ularly 
  diminish) 
  <1 
  in 
  - 
  

  

  ■ 
  

  

  