﻿PAL 
  BONTOLOG! 
  OP 
  ILLINOIS. 
  

  

  Ctenacanthcs 
  Harrison 
  i, 
  St. 
  J. 
  and 
  \V. 
  

  

  PL 
  Will, 
  Hft 
  1. 
  

  

  collection 
  from 
  the 
  St. 
  Louis 
  formation 
  contain 
  an 
  example 
  •>*. 
  

   of 
  a 
  large 
  spine 
  winch 
  is 
  apparently 
  intimately 
  allied 
  to 
  the 
  1 
  

  

  — 
  . 
  Ctenaeanthtu 
  defiexiu, 
  l>ut 
  which 
  presents 
  certain 
  

   well-marked 
  peculiarities 
  at 
  variance 
  with 
  the 
  distinctive 
  features 
  of 
  

   thai 
  These 
  differences 
  do 
  not 
  appear 
  to 
  be 
  such 
  as 
  might 
  

  

  be 
  attributable 
  to 
  age, 
  and 
  on 
  the 
  whole 
  we 
  are 
  inclined 
  to 
  regard 
  

   them 
  as 
  being 
  of 
  Bpecific 
  importance. 
  

  

  The 
  specimen 
  under 
  consideration 
  shows 
  the 
  greater 
  part 
  of 
  a 
  

   -pine 
  which 
  probably 
  attained 
  a 
  length 
  of 
  bet* 
  - 
  I 
  and 
  25 
  centi- 
  

   metres, 
  and 
  it 
  apparently 
  belonged 
  to 
  the 
  broad-hased 
  type 
  of 
  which 
  

   the 
  preceding 
  species 
  constitutes 
  a 
  typical 
  representative. 
  Present- 
  

   ing 
  about 
  the 
  same 
  degree 
  of 
  curvature 
  along 
  the 
  dorsal 
  edge, 
  it 
  

   more 
  gradually 
  tapers, 
  and 
  the 
  transverse 
  section 
  is 
  less 
  com]. 
  

   laterally, 
  approaching 
  nearer 
  a 
  triangular 
  outline 
  than 
  is 
  th 
  

   in 
  the 
  former 
  species, 
  which, 
  however, 
  it 
  resembles 
  in 
  the 
  solid 
  tip 
  

   and 
  comparatively 
  limited 
  extent 
  of 
  the 
  closed 
  pulp 
  cavity. 
  The 
  

   timen 
  is 
  much 
  worn, 
  but 
  not 
  sufficient 
  to 
  obliterate 
  the 
  limits 
  of 
  

   the 
  dorsal 
  line, 
  the 
  position 
  of 
  which 
  is 
  indicated 
  hy 
  a 
  symmetrical 
  

   undulation 
  m 
  the 
  abraded 
  surface 
  extending 
  a( 
  a 
  sharp 
  angle 
  hack- 
  

   ward 
  and 
  upward 
  from 
  the 
  inferior 
  dorsal 
  angle 
  : 
  the 
  posterior 
  edges 
  

   of 
  the 
  lateral 
  walls 
  are 
  also 
  evidently 
  rounded 
  by 
  abrasion, 
  so 
  that 
  

   it 
  is 
  difficult 
  to 
  make 
  a 
  satisfactory 
  restoration 
  of 
  the 
  outline 
  of 
  

   these 
  edges 
  that 
  will 
  accord 
  with 
  the 
  known 
  form 
  of 
  the 
  correspond- 
  

   portion 
  of 
  the 
  Bpecies 
  with 
  which 
  the 
  present 
  spine 
  is 
  Uy 
  

  

  compared, 
  along 
  the 
  dorsal 
  edge, 
  which 
  is 
  broadly 
  rounded 
  into 
  

   the 
  moderately 
  convex 
  sides, 
  several 
  rows 
  of 
  densely 
  tnberculated 
  

   ire 
  still 
  retained 
  in 
  a 
  wry 
  perfect 
  state 
  of 
  preservation, 
  

   showing 
  the 
  transvi 
  ad 
  tubercles 
  with 
  abrupt 
  declivity 
  

  

  \e 
  and 
  steep 
  slope 
  helow. 
  delicately 
  sculptured 
  hy 
  Vertical 
  OT 
  

  

  radiating 
  oarinae, 
  the 
  tubercles 
  spaced 
  by 
  about 
  their 
  own 
  diamei 
  

   vertically 
  and 
  those 
  of 
  one 
  row 
  separated 
  from 
  the 
  contiguous 
  i 
  

  

  by 
  :i 
  mere 
  line; 
  over 
  the 
  remainder 
  of 
  the 
  surface 
  tin 
  

  

  is 
  quite 
  convex, 
  with 
  i 
  low. 
  rounded 
  

  

  and. 
  in 
  the 
  Specimen, 
  sharply 
  rounded 
  into 
  the 
  

  

  lateral 
  Burfaoes; 
  there 
  remain- 
  n 
  along 
  the 
  

  

  or 
  wall 
  of 
  the 
  pulp 
  ca\ity 
  i 
  

  

  been 
  broken 
  away, 
  -it 
  originally 
  extended 
  a 
  disi 
  I 
  7 
  

  

  