﻿204 
  PALEONTOLOGY 
  OF 
  ILLINOIS 
  

  

  The 
  above 
  not 
  iced 
  forms 
  are 
  about 
  equally 
  represented 
  in 
  num- 
  

   bers 
  in 
  the 
  collections 
  from 
  the 
  Upper 
  Burlington 
  "fish-bed," 
  where 
  

   they 
  ore 
  Dot 
  uncommon 
  fossils. 
  Tin- 
  truncated 
  articular 
  facets 
  vary 
  

   somewhat 
  in 
  extent 
  as 
  they 
  do 
  also 
  in 
  position, 
  occurring 
  in 
  one 
  

   individual 
  at 
  cue 
  angle 
  and 
  in 
  another 
  at 
  the 
  opposite 
  angle; 
  and 
  

   judging 
  from 
  the 
  character 
  of 
  the 
  articular 
  border, 
  it 
  seems 
  most 
  

   probable 
  that 
  the 
  teeth 
  were 
  ranged 
  in 
  double 
  rows, 
  the 
  individuals 
  

   of 
  one 
  or 
  the 
  other 
  range 
  slightly 
  in 
  advance 
  of 
  the 
  corresponding 
  

   teeth 
  of 
  the 
  opposite 
  range, 
  the 
  long 
  articular 
  face 
  and 
  short 
  facet 
  

   of 
  one 
  tooth 
  joining 
  the 
  same 
  parts 
  of 
  the 
  opposite 
  contiguous 
  tooth. 
  

   This 
  presumed 
  disposition 
  of 
  the 
  teeth 
  produces 
  the 
  alternating 
  ap- 
  

   pearance 
  shown 
  in 
  the 
  outline 
  diagrams 
  given 
  in 
  connection 
  with 
  

   the 
  illustrations 
  of 
  the 
  several 
  sorts 
  of 
  teeth 
  hen 
  specifically 
  

   ciated. 
  The 
  individuals 
  of 
  the 
  two 
  forms 
  show 
  precisely 
  the 
  same 
  

   variability 
  in 
  the 
  truncated 
  angle 
  of 
  the 
  inner 
  articular 
  border, 
  though 
  

   the 
  asymmetry 
  is 
  much 
  less 
  pronounced 
  in 
  the 
  Bupposed 
  maxillary 
  form 
  

   than 
  it 
  is 
  in 
  the 
  shorter 
  form 
  referred 
  to 
  the 
  mandible 
  of 
  the 
  same 
  

   species. 
  Indeed, 
  it 
  is 
  the 
  absence 
  of 
  symmetry 
  in 
  these 
  teeth 
  that 
  

   offers 
  one 
  of 
  the 
  strongest 
  contrasts 
  with 
  the 
  living 
  representatrt 
  

   the 
  family. 
  Yet 
  it 
  should 
  be 
  remembered 
  thai 
  the 
  existing 
  Ifyliodonts 
  

   ar< 
  subject 
  to 
  abnormal 
  variations, 
  such 
  as 
  the 
  interpolation 
  of 
  an 
  

   extra 
  row 
  of 
  lateral 
  dental 
  plates, 
  while 
  the 
  individuals 
  of 
  th< 
  

   eral 
  ranges 
  are 
  placed 
  in 
  alternating 
  order. 
  The 
  collections 
  have 
  

   been 
  ransacked, 
  but 
  without 
  result 
  in 
  revealing 
  the 
  vestige 
  ofs 
  sym- 
  

   metrical 
  median 
  dental 
  plate, 
  such 
  an 
  one 
  as 
  would 
  restore 
  to 
  the 
  

  

  rows 
  of 
  teeth 
  a 
  perfectly 
  symmetrical 
  arrangement, 
  like 
  that 
  illus- 
  

   trated 
  in 
  the 
  hypothetical 
  diagram 
  annexed, 
  the 
  lateral 
  forms 
  of 
  which 
  

  

  will 
  readily 
  be 
  recognized 
  in 
  the 
  

   illustrations 
  of 
  actual 
  Bpeoimi 
  

   We 
  can. 
  tlh 
  refore, 
  only 
  eonj< 
  oture 
  

   the 
  possible 
  existence 
  of 
  median 
  

  

  h 
  similar 
  in 
  shape 
  to 
  the 
  BUp- 
  

  

  posed 
  form 
  shown 
  in 
  the 
  diagram, 
  

  

  Hypothetical 
  iltaarnm 
  showing 
  median 
  „„ 
  i 
  ,i 
  ■ 
  .1 
  ., 
  ,1 
  

  

  ,,iut,..,.r 
  r,„„.. 
  s, 
  .j, 
  , 
  IH..I..I1 
  1 
  ■[.- 
  . 
  and 
  their 
  al 
  the 
  

  

  BCOre 
  and 
  a 
  half 
  of 
  examples 
  Of 
  the 
  lateral 
  forms 
  contain, 
  d 
  in 
  the 
  

  

  collections, 
  would 
  seem 
  to 
  afford 
  ^li^ht 
  grounds 
  for 
  the 
  supposition 
  

   of 
  the 
  existence 
  of 
  intermediate 
  symmetrical 
  teeth 
  in 
  the 
  dental 
  

   formula 
  of 
  the 
  genus 
  to 
  which 
  the 
  species 
  beta 
  

  

  