﻿284 
  PALiEONTOLndY 
  OF 
  ILLINOIS. 
  

  

  more 
  frequently 
  bifurc 
  renting 
  less 
  broad, 
  flat 
  surfaces 
  toward 
  

  

  the 
  base, 
  and 
  the 
  pulp 
  cavity 
  apparently 
  has 
  greater 
  extent 
  tow; 
  

   th< 
  !ty 
  than 
  is 
  the 
  case 
  in 
  C. 
  Mayi. 
  The 
  posterior 
  wall 
  of 
  

  

  the 
  spini 
  is 
  destroyed, 
  revealing 
  the 
  deep 
  furrow 
  of 
  the 
  pulp 
  cavity 
  

   the 
  entire 
  length 
  of 
  the 
  fragment 
  and 
  which 
  was 
  probably 
  closed 
  to 
  

   the 
  line 
  of 
  insertion. 
  In 
  <". 
  Mayi 
  the 
  posterior 
  face 
  was 
  simply 
  

   channeled 
  and 
  bordered 
  by 
  the 
  lateral 
  flanges, 
  which 
  -how 
  traces 
  of 
  

   the 
  posterior 
  denticle-, 
  also 
  that 
  the 
  lateral 
  surfaces 
  bore 
  longitud- 
  

   inal 
  costa 
  quite 
  to 
  the 
  posterior 
  sd 
  

  

  The 
  unique 
  example 
  described 
  was 
  discovered 
  by 
  Mr. 
  L. 
  A. 
  I 
  

   of 
  Keokuk, 
  in 
  whose 
  honor 
  the 
  species 
  is 
  dedicated, 
  

  

  Geological 
  position 
  and 
  locality: 
  Keokuk 
  limestone; 
  excaval 
  

  

  for 
  the 
  Government 
  canal 
  round 
  the 
  Keokuk 
  rapids, 
  near 
  Montr. 
  

   Iowa. 
  

  

  Ctenacantiics 
  Dbflxxub, 
  St. 
  J. 
  and 
  \Y. 
  

  

  plxxii, 
  ns. 
  i. 
  

  

  Fin 
  spine 
  of 
  medium 
  Bize, 
  laterally 
  compressed, 
  moderately 
  arc 
  

   along 
  the 
  anterior 
  edge, 
  the 
  posterior 
  face 
  relatively 
  - 
  

   cave, 
  the 
  spine 
  uniformly 
  expanding 
  presenting 
  a 
  greal 
  breadth 
  

   the 
  dorsal 
  line, 
  and 
  indicating 
  a 
  very 
  recumbent 
  position. 
  i 
  

   lateral 
  surfaces 
  very 
  gradually 
  converge, 
  with 
  slight 
  convexity, 
  from 
  

   the 
  postero-lateral 
  angles 
  forward,. 
  where 
  they 
  arc 
  regularly 
  rounded 
  

   into 
  the 
  anterior 
  edge; 
  the 
  posterior 
  face, 
  in 
  the 
  somewhat 
  abraded 
  

   specimen, 
  is 
  abruptly 
  truncated, 
  with 
  indication- 
  of 
  a 
  low 
  lot 
  

   tudinaJ 
  keel, 
  the 
  angular 
  lateral 
  edges 
  bearing 
  a 
  row 
  of 
  minute, 
  

   vertically 
  compressed 
  denticles 
  which 
  are 
  Bpaoed 
  by 
  about 
  their 
  own 
  

   diameter. 
  Th< 
  eostation 
  of 
  th. 
  lateral 
  surfaces 
  i- 
  quite 
  regular, 
  the 
  

   rid 
  darly 
  diminishing 
  in 
  size 
  posteriorly, 
  and 
  simply 
  c 
  

  

  separated 
  by 
  :i 
  narrow 
  impressed 
  line, 
  increasing 
  by 
  bifurcation 
  

   which 
  much 
  more 
  frequent!} 
  ately 
  along 
  th. 
  dorsal 
  

  

  line, 
  beyond 
  winch 
  the 
  attenuated 
  basal 
  walls 
  projeot 
  in 
  a 
  narrow 
  

  

  border 
  f» 
  i 
  point 
  ju>t 
  above 
  the 
  inferior 
  angle 
  of 
  tl, 
  

  

  face. 
  The 
  anieiio] 
  - 
  occupied 
  by 
  a 
  comparatively 
  wide 
  ridi 
  

  

  Which 
  at 
  intei 
  . 
  i- 
  off 
  lateral 
  r;d 
  I 
  -. 
  The 
  . 
  ntn 
  ir- 
  

  

  crowded 
  with 
  the 
  costal 
  tubercles, 
  which 
  form 
  im 
  

   ular 
  Buboonical 
  prominences 
  with 
  vertically 
  plicated 
  or 
  ,d 
  

  

  producing 
  Hate 
  appearance, 
  and 
  which 
  

  

  I 
  than 
  represented 
  in 
  the 
  full-size 
  illustration, 
  along 
  I 
  

  

  worn 
  ;> 
  ■ 
  . 
  win 
  r< 
  any 
  remain- 
  of 
  the 
  

  

  