﻿102 
  PALEONTOLOGY 
  OF 
  ILLINOIS. 
  

  

  sumed 
  to 
  identify 
  with 
  the 
  mandibular 
  median 
  form 
  of 
  the 
  present 
  

   genus. 
  The 
  latter 
  teeth 
  are 
  subrhombic 
  or 
  trapezoidal 
  in 
  outline, 
  

   with 
  a 
  nearly 
  straight 
  postero-lateral 
  border, 
  and 
  longer 
  and 
  more 
  

   obliquely 
  converging 
  opposite 
  border, 
  defined 
  by 
  the 
  sain, 
  style 
  of 
  

   enamel 
  fold 
  and 
  basal 
  rim, 
  characteristic 
  of 
  the 
  previously 
  men- 
  

   tioned 
  forms; 
  the 
  coronal 
  region 
  presents 
  a 
  broad, 
  nearly 
  plane 
  

   surface, 
  gradually 
  culminating 
  near 
  the 
  postero-lateral 
  border/ 
  in 
  a 
  

   depressed, 
  obscurely 
  defined 
  ridge, 
  where 
  the 
  slope 
  to 
  the 
  bonier 
  on 
  

   the 
  one 
  hand 
  is 
  abrupt, 
  and 
  on 
  the 
  other 
  very 
  gentle, 
  the 
  inner 
  

   margin 
  gently 
  arched 
  and 
  somewhat 
  rapidly 
  deflected 
  to 
  the 
  obi 
  

   posterior 
  angle. 
  

  

  The 
  genus 
  as 
  now 
  understood 
  is 
  represented 
  by 
  species 
  beginning 
  

   in 
  the 
  Upper 
  Burlington 
  limestone, 
  and 
  in 
  each 
  succeeding 
  forma- 
  

   tion 
  of 
  the 
  Lower 
  Carboniferous 
  period, 
  the 
  latest 
  authentic 
  occur- 
  

   rence 
  being 
  in 
  the 
  St. 
  Louis 
  limestone. 
  

  

  STENorrERonrs 
  planus, 
  St. 
  J. 
  and 
  W. 
  

   n. 
  iv, 
  pig. 
  ;» 
  14. 
  

  

  Represented 
  by 
  two 
  forms 
  of 
  teeth 
  below 
  medium 
  size, 
  presumably 
  

   occupying 
  corresponding 
  positions 
  on 
  opposite 
  ja 
  

  

  Posterior 
  teeth 
  of 
  the 
  upper 
  jaw 
  Bub-rhomboidal 
  in 
  outline, 
  rather 
  

   strongly 
  inrolled 
  and 
  arched 
  longitudinally. 
  Antero-hiteial 
  border 
  

   slightly 
  oblique 
  in 
  its 
  forward 
  and 
  outward 
  course, 
  basal 
  rim 
  ap- 
  

   parently 
  shallow, 
  and 
  defined 
  from 
  the 
  narrow 
  enamel 
  fold 
  by 
  a 
  

   shallow 
  groove; 
  postero-lateral 
  border 
  converging 
  at 
  an 
  angle 
  of 
  

   about 
  2"> 
  with 
  the 
  opposite 
  border, 
  the 
  basal 
  rim 
  somewhat 
  thick- 
  

   ened 
  and 
  produced 
  beyond 
  the 
  limits 
  of 
  the 
  narrow, 
  inrolled 
  enamel 
  

   fold, 
  inner 
  margin 
  broadly 
  arched 
  with 
  a 
  Blighl 
  angulation 
  near 
  the 
  

   middle 
  in 
  passing 
  the 
  base 
  of 
  the 
  coronal 
  prominence, 
  in 
  front 
  of 
  

   which 
  a 
  sharp, 
  slight 
  constriotion 
  occurs, 
  the 
  inferior 
  edge 
  in 
  beveled 
  

   in 
  the 
  worn 
  condition; 
  posterior 
  extremity 
  forming 
  a 
  sub-acute 
  

   angle, 
  the 
  margin 
  making 
  an 
  obtuse 
  angle 
  with 
  the 
  anterior 
  bonier. 
  

   Coronal 
  prominence 
  comprising 
  more 
  than 
  half 
  the 
  lateral 
  ana 
  of 
  

   tlie 
  surface, 
  culminating 
  in 
  a 
  low. 
  rounded 
  ores! 
  near 
  the 
  median 
  

   lme, 
  with 
  an 
  abrupt 
  slope 
  to 
  the 
  anterior 
  neck, 
  defined 
  by 
  an 
  in- 
  

   tervening 
  shallow 
  groove, 
  the 
  I 
  stops 
  I 
  tlj 
  and 
  

   faintly 
  de] 
  rsely; 
  the 
  anterior 
  neck 
  somewhat 
  rapidly 
  

   narrows 
  to 
  the 
  anterolateral 
  bord< 
  atly 
  convei 
  transvi 
  rsely 
  and 
  

   marked 
  by 
  fainy 
  Longitudinal 
  plica. 
  The 
  who], 
  outer 
  third 
  or 
  m 
  

  

  of 
  the 
  crown 
  suit 
  ii. 
  triturating 
  an 
  a. 
  the 
  

  

  result 
  of 
  wear, 
  rie* 
  the 
  minute, 
  regular 
  punota, 
  which 
  

  

  