﻿206 
  PALEONTOLOGY 
  OF 
  ILLINOl 
  

  

  prove 
  them 
  to 
  be 
  specifically 
  identical. 
  The 
  individuals 
  exhibit 
  p 
  

   cisely 
  the 
  same 
  variableness 
  as 
  regards 
  tin- 
  position 
  of 
  the 
  hum 
  

   angle 
  of 
  the 
  articular 
  border, 
  though, 
  as 
  has 
  been 
  already 
  remark) 
  

   the 
  facet 
  is 
  of 
  greater 
  extent; 
  indeed, 
  in 
  some 
  instai 
  upying 
  

  

  Dearly 
  half 
  the 
  length 
  of 
  the 
  border. 
  

  

  Geological 
  position 
  and 
  localities; 
  Upper 
  Burlington 
  lime- 
  - 
  

   chiefly 
  from 
  the 
  main 
  "fish-bed," 
  a 
  single 
  example 
  from 
  the 
  upper 
  

   fish-bed 
  horizon; 
  Buffington 
  Creek, 
  Pleasant 
  Grove, 
  Augusta, 
  Iowa, 
  

   and 
  Quincy, 
  Illinois. 
  

  

  Psammodus 
  turgidi 
  m, 
  St. 
  J. 
  and 
  W. 
  

  

  PL 
  XV. 
  Fig. 
  4. 
  

  

  A 
  unique 
  example 
  of 
  the 
  maxillary 
  tooth, 
  derived 
  from 
  the 
  fish- 
  

   bed 
  of 
  the 
  Burlington-Keokuk 
  "division 
  beds," 
  shows 
  a 
  still 
  wider 
  

   divergence 
  from 
  Psammodus 
  Springeri, 
  and 
  which 
  we 
  have 
  provision- 
  

   ally 
  recognized 
  under 
  the 
  above 
  specific 
  designation. 
  In 
  outline 
  the 
  

   tooth 
  resembles 
  the 
  same 
  form 
  of 
  /'. 
  Springeri 
  and 
  P. 
  tumidus, 
  

   relationship 
  with 
  the 
  latter 
  being 
  most 
  intimate. 
  The 
  inner 
  border 
  

   is 
  nearly 
  equally 
  divided 
  into 
  two 
  long 
  articular 
  facets 
  separated 
  by 
  

   an 
  angulation, 
  the 
  truncate 
  facet 
  being 
  at 
  the 
  posterior 
  angle, 
  the 
  

   coronal 
  belt 
  very 
  deep 
  and 
  well 
  defined 
  from 
  the 
  slightly 
  channeled 
  

   basal 
  portion. 
  The 
  inner 
  coronal 
  ridge 
  is 
  very 
  prominent, 
  occupy- 
  

   ing 
  fully 
  half 
  the 
  transverse 
  diameter 
  of 
  the 
  crown, 
  broadly 
  arched 
  

   in 
  both 
  directions 
  and 
  defined 
  from 
  the 
  median 
  depression 
  by 
  a 
  

   sharp 
  impressed 
  line; 
  outer 
  lateral 
  border 
  not 
  known. 
  The 
  Burffl 
  

   shows 
  a 
  minute 
  punctate 
  structure 
  through 
  the 
  dense 
  enamel 
  la] 
  

   enveloping 
  the 
  crown; 
  along 
  the 
  posterior 
  margin 
  the 
  surface 
  pre- 
  

   sents 
  a 
  beautiful 
  rugose 
  appearance 
  from 
  the 
  beaded 
  Longitudinal 
  

  

  plica-, 
  which 
  extend 
  some 
  distance 
  from 
  the 
  margin, 
  and 
  the 
  artic- 
  

   ular 
  holder 
  as 
  also 
  the 
  preserved 
  portions 
  of 
  the 
  anterior 
  and 
  ] 
  

  

  terior 
  margins 
  are 
  similarly 
  ornamented 
  with 
  vertical 
  rugosil 
  

   The 
  specimen 
  measure-- 
  between 
  the 
  angles 
  of 
  the 
  inner 
  border 
  

  

  16.6 
  mm. 
  probably 
  ADOUt 
  the 
  same 
  as 
  th> 
  diam- 
  

  

  eter 
  at 
  the 
  posterior 
  margin. 
  

  

  A 
  solitary 
  and 
  BOmewhal 
  mutilated 
  specimen, 
  obtained 
  hy 
  Mr. 
  

  

  Bpringer 
  from 
  the 
  main 
  fish-bed 
  of 
  the 
  upper 
  Burlington 
  Limestone, 
  

  

  ,,]•, 
  actly 
  tin- 
  same 
  character 
  of 
  inner 
  articular 
  hor.hr 
  that 
  

  

  tishes 
  the 
  above 
  described 
  tooth. 
  But 
  the 
  coronal 
  prominence 
  

   the 
  latter 
  example, 
  although 
  very 
  broad, 
  u 
  comparatively 
  low 
  and 
  

  

  in. 
  to 
  the 
  shallow 
  median 
  depression, 
  which 
  was 
  apparently 
  

  

  