﻿80 
  PALiEOXTOLOdY 
  OF 
  I 
  LUNCH- 
  

  

  logical 
  position 
  and 
  locality: 
  Chester 
  limestone, 
  Cheater, 
  Illi- 
  

   nois. 
  

  

  <h:m 
  8 
  YATICINonrS 
  . 
  Bt. 
  J. 
  and 
  W. 
  

  

  The 
  distribution 
  and 
  investigation 
  of 
  the 
  materials 
  Qlnatratiog 
  

   Deltoptychius 
  and 
  allied 
  genera, 
  have 
  bronghl 
  into 
  prominence 
  homol- 
  

   ogous 
  forms 
  of 
  teeth 
  which 
  range 
  themselves 
  under 
  a 
  group 
  appar- 
  

   ently 
  characterized 
  by 
  persistent 
  features, 
  in 
  contradistinction 
  to 
  

   Deltoptychius 
  on 
  the 
  one 
  hand, 
  and 
  StenopteroduM 
  on 
  the 
  other, 
  

   and 
  which, 
  if 
  we 
  are 
  correct 
  in 
  inferring 
  their 
  generic 
  distinetn. 
  

   offer 
  a 
  premonition 
  of 
  the 
  later 
  differentiated 
  Orthopleurodus, 
  i 
  

   Unfortunately, 
  however, 
  the 
  state 
  of 
  preservation 
  of 
  the 
  matt-rial 
  

   illustrating 
  these 
  forms 
  is 
  in 
  no 
  instance 
  such 
  as 
  leads 
  to 
  indubita- 
  

   ble 
  conclusion 
  in 
  regard 
  to 
  their 
  generic 
  distinctness 
  from 
  St, 
  

   terodus. 
  Hence 
  the 
  various 
  specific 
  categories 
  into 
  which 
  the 
  forms 
  

   from 
  the 
  several 
  formations 
  readily 
  resolve 
  themselves, 
  are 
  pro- 
  

   visionally 
  recognized 
  under 
  the 
  above 
  generic 
  designation/ 
  

  

  The 
  forms 
  above 
  referred 
  to 
  represent 
  the 
  posterior 
  teeth 
  of 
  the 
  

   upper 
  and 
  lower 
  jaws. 
  The 
  maxilliary 
  form 
  is 
  in 
  every 
  respeel 
  like 
  

   the 
  corresponding 
  teeth 
  of 
  Deltoptychius, 
  save 
  in 
  the 
  apparent 
  

   absence 
  of 
  the 
  least 
  vestige 
  of 
  the 
  presenci 
  of 
  I 
  ndary 
  l< 
  

  

  the 
  entire 
  anterior 
  portion 
  of 
  the 
  tooth 
  forward 
  of 
  the 
  posterior 
  

   prominence 
  presenting 
  a 
  plain 
  surface, 
  such 
  as 
  obtains 
  in 
  Stenop- 
  

   terodus. 
  The 
  anterior-lateral 
  border 
  is 
  probably 
  truncated 
  from 
  the 
  

   inner 
  angle 
  slightly 
  obliquely 
  outward 
  and 
  forward, 
  in 
  which 
  respeoi 
  

   it 
  differs 
  from 
  the 
  prevalent 
  condition 
  observed 
  in 
  typical 
  /></ 
  

   tychius 
  and 
  Stenopterodus. 
  The 
  mandibular 
  form 
  is 
  also 
  in 
  general 
  

  

  outline 
  and 
  contour 
  like 
  that 
  of 
  the 
  former 
  genU8; 
  hut 
  the 
  indi- 
  

   viduals 
  here 
  referred 
  to 
  do 
  not 
  preserve 
  a 
  trace 
  of 
  ih« 
  narrow 
  plain 
  

   bell 
  such 
  as 
  in 
  Deltoptychius, 
  distinctly 
  defines 
  the 
  median 
  lobe 
  from 
  

   the 
  edge 
  of 
  the 
  abrupt 
  anterolateral 
  border. 
  In 
  the 
  latter 
  particular 
  

   the 
  teeth 
  referred 
  to 
  I 
  semblance 
  to 
  the 
  mandibular 
  

  

  posterior 
  form 
  of 
  Orthoplsurodus, 
  As 
  all 
  of 
  the 
  few 
  examples 
  of 
  

  

  tin 
  form 
  are 
  represented 
  by 
  evidently 
  worn 
  individuals, 
  it 
  cannot 
  

  

  be 
  denied 
  thai 
  they 
  may 
  be 
  merely 
  abraded 
  teeth 
  of 
  DsUoptychi 
  

   With 
  on. 
  • 
  or 
  two 
  exceptions, 
  ill- 
  'I.-.-. 
  I 
  in 
  thi> 
  connection 
  

  

  unquestionably 
  distinct 
  from 
  those 
  hereinafter 
  described 
  under 
  

   the 
  bead 
  of 
  Deltoptychius. 
  These 
  possihl. 
  to 
  the 
  

  

  ■ih. 
  in 
  .1 
  

  

  