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  PALAEONTOLOGY 
  OF 
  ILLINo 
  

  

  ly— 
  generally, 
  however, 
  so 
  worn 
  down 
  over 
  the 
  outer 
  half 
  

   present 
  the 
  appearance 
  of 
  a 
  gradual 
  dope, 
  descending 
  into 
  the 
  

   shallow 
  median 
  depression, 
  which 
  is 
  bordered 
  on 
  the 
  opposite 
  Bide 
  

  

  by 
  the 
  narrow 
  wing-expansion. 
  A 
  mature 
  tooth 
  m< 
  

  

  along 
  the 
  anterior 
  lateral 
  border 
  18 
  mm.; 
  breadth 
  across 
  inner 
  

  

  margin, 
  9 
  mm.; 
  greatest 
  depth 
  of 
  antero-lateral 
  border, 
  2 
  mm. 
  

  

  Mandibular 
  posterior 
  teeth 
  triangular 
  in 
  outline, 
  rather 
  strongly 
  

   arched 
  and 
  inrolled 
  at 
  the 
  outer 
  extremity; 
  antero-lateral 
  border 
  

   steeply 
  beveled 
  from 
  the 
  prominent 
  angulation 
  of 
  the 
  coronal 
  border, 
  

   a 
  very 
  narrow 
  belt 
  of 
  enamel 
  enveloping 
  the 
  upper 
  edge, 
  beneath 
  

   which 
  the 
  deep 
  basal 
  belt 
  is 
  more 
  or 
  less 
  distinctly 
  tinted 
  in 
  the 
  

   direction 
  of 
  inrollment, 
  and 
  terminating 
  inferiorly 
  in 
  the 
  relatively 
  

   short, 
  attenuated 
  edge, 
  which 
  constitutes 
  about 
  halt 
  the 
  extent 
  of 
  

   this 
  side 
  of 
  the 
  tooth; 
  inner 
  margin 
  forming 
  nearly 
  a 
  right 
  angle 
  

   with 
  the 
  anterior 
  border, 
  which 
  it 
  joins 
  in 
  a 
  rounded 
  angle, 
  slightly 
  

   curved 
  sigmoidally 
  in 
  passing 
  to 
  the 
  produced 
  posterior 
  angle 
  of 
  

   the 
  alate 
  expansion, 
  in 
  breadth 
  about 
  equal 
  to 
  the 
  length 
  of 
  the 
  

   anterior 
  border 
  of 
  the 
  tooth; 
  postero-lateral 
  border 
  rapidly 
  converg- 
  

   ing 
  toward 
  the 
  outer 
  extremity 
  at 
  an 
  angle 
  of 
  60 
  with 
  the 
  anterior 
  

   border 
  of 
  the 
  crown, 
  or 
  about 
  60° 
  with 
  the 
  inferior 
  edge 
  of 
  th< 
  

   of 
  that 
  side, 
  slightly 
  anhed 
  and 
  bounded 
  along 
  the 
  upper 
  edge 
  by 
  

   the 
  narrow 
  enamel 
  fold, 
  the 
  nearly 
  vertical, 
  .slightly 
  channeled 
  basal 
  

   border 
  posteriorly 
  expanding 
  beyond 
  the 
  coronal 
  limits 
  and 
  produced 
  

   into 
  a 
  spur-like 
  process 
  forming 
  the 
  extreme 
  angle 
  of 
  the 
  pi 
  

   alation. 
  The 
  median 
  portion 
  of 
  the 
  crown 
  presents 
  a 
  broad 
  d< 
  

   smii, 
  denned 
  on 
  the 
  one 
  hand 
  by 
  the 
  abrupt 
  declivity 
  descending 
  

  

  from 
  the 
  elevated 
  angular 
  ridge 
  of 
  the 
  anterior 
  prominence, 
  and 
  on 
  

   the 
  Other 
  by 
  the 
  more 
  gentle 
  acclivity 
  rising 
  to 
  the 
  postero-lateral 
  

   border 
  of 
  the 
  broadly 
  expanded 
  wing, 
  presenting, 
  as 
  in 
  the 
  pre- 
  

   viously 
  described 
  form, 
  extreme 
  simplicity 
  in 
  coronal 
  contour. 
  1 
  

  

  along 
  the 
  anterior 
  border 
  of 
  the 
  crown 
  of 
  B 
  la 
  tuple, 
  16mm.; 
  

  

  greatest 
  depth 
  of 
  the 
  antero-lateral 
  slope, 
  5.6 
  mm.; 
  breadth 
  at 
  the 
  

   inner 
  margin, 
  18 
  mm. 
  

  

  The 
  surface 
  structure 
  of 
  both 
  of 
  the 
  form- 
  described 
  abo-, 
  

   essentially 
  identical. 
  The 
  punotffl 
  exhibit 
  in 
  a 
  marked 
  degree 
  the 
  

   transverse 
  arrangement 
  in 
  more 
  or 
  less 
  regularly 
  parallel 
  lim 
  

   inciding 
  with 
  the 
  curvature 
  of 
  the 
  inner 
  margin. 
  Ti 
  

  

  cially 
  noticeable 
  in 
  the 
  region 
  of 
  the 
  coronal 
  d< 
  d 
  and 
  the 
  

  

  loss 
  exposed 
  inner 
  portion 
  of 
  the 
  crown, 
  where, 
  in 
  specimens 
  which 
  

  

  from 
  trituration, 
  the 
  

   surface 
  | 
  fl 
  '" 
  m 
  '' 
  

  

  - 
  J0T- 
  

  

  