﻿70 
  PALEONTOLOGY 
  OF 
  ILLINOIS. 
  

  

  rounded 
  with 
  a 
  slight 
  sigmoidal 
  curvature 
  extending 
  into 
  the 
  obscure 
  

   posterior 
  elation, 
  the 
  vertical 
  plane 
  of 
  the 
  base 
  equal 
  in 
  depth 
  to 
  

   the 
  height 
  of 
  the 
  enamel 
  fold 
  from 
  which 
  it 
  is 
  well 
  defined; 
  outer 
  

   margin 
  obliquely 
  truncated 
  from 
  the 
  anterior 
  angle 
  downward 
  and 
  

   outward, 
  usually 
  much 
  worn 
  and 
  otherwise 
  mutilated. 
  The 
  anterior 
  

   and 
  posterior 
  borders 
  show 
  more 
  or 
  less 
  distinct 
  and 
  rather 
  wide 
  

   undulations. 
  Coronal 
  surface 
  uniformly 
  and 
  closely 
  punctate, 
  some- 
  

   what 
  strongly 
  arched 
  transversely 
  into 
  a 
  broad 
  round, 
  d 
  prominen 
  

   culminating 
  a 
  little 
  anterior 
  to 
  the 
  median 
  line, 
  posterior 
  slope 
  

   gently 
  concave. 
  Breadth 
  of 
  medium-sized 
  tooth 
  across 
  inner 
  mar- 
  

   gin, 
  23 
  mm.; 
  ditto, 
  outer 
  margin, 
  18 
  mm.; 
  length 
  along 
  antero- 
  

   lateral 
  border, 
  20 
  mm.; 
  about 
  equal 
  to 
  the 
  opposite 
  border; 
  depth 
  

   of 
  tooth 
  at 
  middle 
  of 
  anterior 
  border, 
  2»6 
  ram. 
  Second, 
  median 
  

   form 
  of 
  the 
  lower 
  (?) 
  jaw. 
  Teeth 
  corresponding 
  with 
  the 
  second 
  

   form, 
  described 
  under 
  P. 
  oUiqwu, 
  and 
  having 
  the 
  same 
  general 
  

   outline, 
  but 
  distinguishable 
  by 
  the 
  less 
  rapid 
  convergence 
  of 
  the 
  

   lateral 
  borders 
  and 
  the 
  considerable 
  gibbosity 
  of 
  the 
  coronal 
  region, 
  

   which 
  rises 
  into 
  a 
  low 
  broad 
  prominence, 
  traversing 
  the 
  crown 
  

   nearly 
  centrally, 
  or 
  a 
  little 
  abaft 
  the 
  median 
  line. 
  The 
  large 
  uniquCi 
  

   individual 
  of 
  this 
  particular 
  form 
  afforded 
  by 
  the 
  collections, 
  is 
  

   somewhat 
  mutilated, 
  the 
  borders 
  and 
  extremity 
  not 
  being 
  clearly 
  

   shown; 
  the 
  inner 
  margin 
  is 
  broadly 
  rounded, 
  with 
  a 
  slight 
  sigmoidal 
  

   oblique 
  course, 
  the 
  crown 
  surface 
  punctate 
  as 
  in 
  tin 
  other 
  forms. 
  

  

  Comparisons 
  of 
  the 
  first 
  above 
  mentioned 
  form 
  with 
  the 
  type 
  

   specimen 
  described 
  by 
  Messrs. 
  Newberry 
  and 
  Worthen, 
  under 
  the 
  

   name 
  Helodua 
  placenta, 
  leaves 
  no 
  doubt 
  as 
  to 
  their 
  specific 
  identity. 
  

   The 
  original 
  specimen 
  figured 
  in 
  the 
  report 
  of 
  the 
  Illinois 
  Geol, 
  

   Survey. 
  II. 
  PI. 
  V. 
  tig. 
  4, 
  la. 
  is 
  that 
  of 
  a 
  small 
  individual, 
  the 
  

  

  transverse 
  diameter 
  of 
  which 
  considerably 
  exceeds 
  the 
  length 
  bom 
  

  

  within 
  outwards, 
  the 
  inner 
  margin 
  and 
  base 
  being 
  in 
  an 
  extraor- 
  

   dinarily 
  peri. 
  ••! 
  state 
  of 
  preservation, 
  even 
  showing 
  the 
  delicate 
  oren- 
  

   Ulations 
  of 
  the 
  rather 
  heavy 
  coronal 
  fold 
  that 
  rises 
  from 
  and 
  prOJ< 
  

   som. 
  what 
  beyond 
  the 
  mural 
  basal 
  area. 
  The 
  outer 
  portion 
  of 
  the 
  

   tooth, 
  "ii 
  tin 
  other 
  hand, 
  is 
  muoh 
  worn, 
  though 
  the 
  entire 
  outline 
  

  

  is 
  still 
  preserved, 
  ite 
  characteristics 
  hem- 
  well 
  reproduced 
  in 
  one 
  of 
  

   the 
  figures 
  illustrating 
  the 
  i« 
  herein 
  interpreted. 
  

  

  pored 
  with 
  Piephodiu 
  obliquut, 
  the 
  forms 
  ..f 
  the 
  present 
  

  

  reveal 
  the 
  following 
  Balienl 
  distinctions 
  : 
  The 
  supposed 
  mandibular 
  

  

  median 
  teeth 
  latively 
  narrower, 
  lateral 
  borders 
  less 
  rapidly 
  

  

  . 
  I),,, 
  more 
  uearlj 
  central 
  position 
  of 
  th. 
  axis 
  of 
  the 
  

  

  ,,„,;,! 
  ridge, 
  and 
  th. 
  ogularly 
  rounded 
  inner 
  margin; 
  in 
  the 
  

  

  