﻿64 
  PALEONTOLOGY 
  OF 
  ILLINOIS. 
  

  

  First, 
  teeth 
  presenting 
  a 
  trapezoidal 
  outline, 
  the 
  crown 
  traversed 
  

   Longitudinally 
  by 
  a 
  low, 
  ohtuse-crested 
  ridge, 
  culminating 
  posterior 
  

   of 
  the 
  median 
  line, 
  the 
  inner 
  margin 
  somewhat 
  angularly 
  rounded, 
  

   outer 
  extremity 
  always 
  truncate 
  with 
  channeled, 
  crenulate 
  articular 
  

   surface, 
  the 
  lateral 
  borders 
  undulated 
  as 
  though 
  for 
  odadaptation 
  with 
  

   the 
  lateral 
  articular 
  extremities 
  of 
  narrow 
  serial 
  teeth. 
  

  

  Second, 
  teeth 
  generally 
  more 
  ohliquely 
  trapezoidal 
  in 
  outline, 
  in 
  

   which 
  the 
  coronal 
  ridge 
  is 
  relatively 
  inconspicuous, 
  and 
  the 
  inner 
  

   margin 
  gently 
  arched, 
  from 
  which 
  the 
  lateral 
  borders 
  less 
  rapidly 
  

   converge 
  towards 
  the 
  outer 
  extremity, 
  their 
  articular 
  surfaces 
  also 
  

   undulated. 
  Coronal 
  surface 
  minutely 
  punctate. 
  The 
  above 
  forms 
  

   are 
  provisionally 
  referred 
  to 
  opposed 
  positions 
  on 
  the 
  lower 
  and 
  upper 
  

   jaws 
  respectively, 
  chiefly 
  on 
  account 
  of 
  the 
  somewhat 
  stronger 
  re- 
  

   semblance 
  of 
  the 
  first 
  mentioned 
  form 
  with 
  the 
  contour 
  of 
  the 
  man- 
  

   dibular 
  teeth 
  of 
  typical 
  Cochliodonts. 
  With 
  the 
  above 
  remain.- 
  are 
  

   a-sociated 
  narrow 
  or 
  transversely 
  elongate 
  teeth, 
  whose 
  crenulated 
  

   edges 
  and 
  coronal 
  contour 
  clearly 
  point 
  to 
  their 
  intimate 
  kinship 
  

   with 
  the 
  preceding 
  forms. 
  The 
  latter 
  include 
  the 
  European 
  form 
  

   originally 
  described 
  under 
  the 
  term 
  Hebdua 
  planut, 
  Agass.. 
  which 
  

   was 
  by 
  ('apt. 
  Jones 
  regarded 
  as 
  belonging 
  to 
  Ptephodua 
  m 
  ognm. 
  

   There 
  are 
  other 
  similar 
  forms 
  more 
  or 
  less 
  numerously 
  represent 
  

   in 
  the 
  collections, 
  which 
  may 
  also 
  prove 
  to 
  have 
  been 
  associated 
  

   with 
  those 
  above 
  noticed. 
  

  

  In 
  reference 
  to 
  the 
  dentition 
  of 
  Ptephodua 
  our 
  information 
  is 
  still 
  

   meagre, 
  although 
  the 
  identity 
  of 
  a 
  variety 
  of 
  seemingly 
  widely 
  di- 
  

   verse 
  forms 
  may 
  be 
  asserted 
  with 
  a 
  degree 
  of 
  confidence. 
  White 
  

   certain 
  conspicuous 
  forms 
  belonging 
  to 
  the 
  genus 
  may 
  be 
  compared 
  

   with 
  the 
  dental 
  elements 
  met 
  with 
  in 
  CocldiodlU, 
  there 
  was 
  unqi 
  

   tionably 
  a 
  marked 
  contrast 
  in 
  the 
  character 
  of 
  the 
  combined 
  dental 
  

  

  armament 
  of 
  the 
  jaws 
  of 
  those 
  genera. 
  Imbed, 
  the 
  resemblan 
  

  

  between 
  them 
  are 
  but 
  little 
  more 
  intimate 
  than 
  obtains 
  in 
  the 
  in- 
  

   stance 
  of 
  Cessation 
  and 
  Cochliodtu. 
  While 
  the 
  median 
  portion 
  of 
  

   the 
  rami 
  of 
  the 
  jaws 
  of 
  Psephodu* 
  was 
  enveloped 
  by 
  a 
  moderately 
  

   contorted 
  dental 
  plate, 
  constituting 
  its 
  chief 
  point 
  of 
  resemblance 
  

   with 
  Cochliodua, 
  this 
  plate 
  was 
  flanked 
  on 
  either 
  Bide 
  by 
  series 
  of 
  

  

  teeth 
  disposed 
  in 
  rows 
  from 
  within 
  outward 
  similar 
  to 
  the 
  occur- 
  

   rence 
  of 
  the 
  teeth 
  upon 
  the 
  jaws 
  of 
  I 
  I 
  >. 
  Therefore, 
  the 
  solid 
  

   triturating 
  plates 
  of 
  Paephodua 
  are 
  not 
  strictly 
  homologous 
  with 
  the 
  

  

  j 
  ;il 
  Mi 
  of 
  (■•.rhli'xius, 
  but 
  they 
  are 
  more 
  properly 
  d 
  

  

  •median" 
  1e. 
  tb 
  of 
  the 
  rami 
  of 
  the 
  jaWB. 
  

  

  