﻿154 
  PALAEONTOLOGY 
  OF 
  ILLINOIS. 
  

  

  round 
  the 
  base 
  of 
  the 
  coronal 
  ridge, 
  with 
  a 
  Blight 
  concavity 
  in 
  the 
  

   region 
  of 
  the 
  posterior 
  depression, 
  inferiorly 
  inheveled. 
  Coronal 
  ridge 
  

   relatively 
  narrow, 
  steeply 
  rising 
  from 
  a 
  faint 
  depressed 
  line 
  near 
  the 
  

   anterior 
  edge, 
  crest 
  rounded, 
  gradually 
  descending 
  into 
  the 
  posterior 
  

   wide 
  depression, 
  which 
  is 
  bordered 
  by 
  the 
  moderately 
  upr 
  

   alation. 
  Surface 
  nearly 
  smooth, 
  with 
  ohscure 
  transverse 
  lines 
  along 
  

   which 
  the 
  punotae 
  exhibit 
  elongated 
  or 
  otherwise 
  enlarged 
  openings, 
  

   rarely 
  anything 
  like 
  undulations 
  appearing. 
  Length 
  along 
  Ctt 
  

   coronal 
  ridge 
  to 
  inrolled 
  outer 
  margin 
  ahout 
  31 
  mm.; 
  length 
  of 
  

   anterior 
  articular 
  border 
  18 
  mm. 
  ; 
  greatest 
  breadth 
  across 
  inner 
  

   margin 
  21 
  mm. 
  

  

  The 
  collections 
  from 
  St. 
  Louis 
  horizons 
  afford 
  a 
  solitary 
  authentic 
  

   example 
  only 
  of 
  the 
  present 
  form, 
  the 
  nearly 
  perfect 
  tooth 
  above 
  

   described. 
  This 
  is 
  quite 
  distinct 
  from 
  the 
  earlier-occurring 
  bjm 
  

   and 
  offers 
  striking 
  resemblance 
  in 
  coronal 
  contour 
  with 
  the 
  bo 
  

   individuals 
  of 
  Deltodtu 
  cingidatus, 
  N. 
  and 
  W., 
  of 
  the 
  Chester 
  forma- 
  

   tion. 
  It 
  is, 
  perhaps, 
  distinguishable 
  from 
  the 
  latt. 
  by 
  the 
  

   somewhat 
  less 
  produced 
  posterior 
  angle, 
  less 
  elevated 
  coronal 
  ridge, 
  

   and 
  shorter 
  articular 
  edge 
  of 
  the 
  antero-lateral 
  border. 
  A 
  Biogle 
  

   other 
  imperfect 
  example 
  has 
  been 
  obtained 
  by 
  Mr. 
  Van 
  Borne 
  from 
  

   the 
  same 
  formation 
  at 
  Alton, 
  Illinois, 
  which 
  represents 
  the 
  mandi- 
  

   bular 
  posterior 
  form 
  possibly 
  of 
  the 
  same 
  species. 
  The 
  fragment 
  

   indicates 
  a 
  tooth 
  of 
  a 
  relatively 
  long, 
  narrow 
  outline 
  with 
  a 
  rather 
  

   oblique 
  inner 
  margin 
  in 
  front, 
  a 
  broad 
  gentle 
  elope 
  descending 
  from 
  

   the 
  crest 
  of 
  the 
  coronal 
  ridge 
  to 
  the 
  deep, 
  inheveled 
  coronal 
  fold 
  of 
  

   the 
  articular 
  border, 
  the 
  opposite 
  Bank' 
  abruptly 
  descending 
  into 
  the 
  

   posterior 
  depression; 
  the 
  alation, 
  as 
  also 
  the 
  outer 
  margin 
  of 
  the 
  

   tooth, 
  is 
  broken 
  away. 
  

  

  Geological 
  position 
  and 
  localitiea: 
  St. 
  Louis 
  formation; 
  Pella, 
  [owa, 
  

   Alton. 
  Illinois. 
  

  

  FODU8 
  Powki.i.ii. 
  St. 
  .1. 
  and 
  \Y. 
  

   PI. 
  X. 
  lie 
  1. 
  

  

  Peetfa 
  attain 
  medium 
  Bize. 
  Mandibular 
  terminal 
  form 
  subspatu- 
  

   late 
  in 
  outline, 
  moderately 
  arched 
  longitudinally 
  and 
  rather 
  

   inrolled 
  at 
  the 
  outer 
  margin, 
  antero-lateral 
  border 
  gently 
  incurved, 
  

   enamel 
  fold 
  inheveled 
  to 
  the 
  channeled 
  basal 
  rim; 
  

  

  .1 
  border 
  gradually 
  converges 
  toward 
  outer 
  inrolled 
  ma 
  

   making 
  an 
  angle 
  of 
  about 
  -j:j 
  with 
  the 
  anterior 
  border, 
  the 
  rounded, 
  

   inheveled 
  enamel 
  fold 
  well 
  defined 
  bj 
  a 
  narrow 
  buIous 
  from 
  the 
  

  

  