﻿170 
  PALEONTOLOGY 
  OF 
  ILLINOIS. 
  

  

  separated 
  by 
  a 
  low 
  median 
  ridge. 
  Surface 
  marked 
  by 
  more 
  or 
  less 
  

   distinct 
  lines 
  of 
  growth 
  parallel 
  with 
  the 
  inner 
  margin, 
  and 
  coarse, 
  

   irregular 
  and 
  irregularly 
  elongate 
  or 
  confluent 
  panels. 
  A 
  medium- 
  

   size 
  tooth 
  measures 
  across 
  the 
  inner 
  margin 
  6 
  mm. 
  ; 
  greatest 
  length 
  

   along 
  the 
  obligue 
  border 
  to 
  point 
  of 
  inrollment, 
  11 
  mm. 
  ; 
  vertical 
  

   depth 
  at 
  middle, 
  4-5 
  mm. 
  

  

  The 
  foregoing 
  diagnosis 
  presents 
  the 
  normal 
  condition 
  of 
  a 
  pecu- 
  

   liar 
  form 
  of 
  teeth 
  which 
  is 
  represented 
  by 
  half 
  a 
  dozen, 
  more 
  or 
  

   less/ 
  perfect 
  examples 
  in 
  the 
  collections 
  from 
  the 
  Upper 
  Burlington 
  

   fish-bed. 
  In 
  two 
  of 
  these 
  examples 
  the 
  inner 
  half 
  of 
  the 
  crown 
  

   shows 
  a 
  simple 
  shallow 
  depressed 
  area 
  defined 
  by 
  the 
  slightly 
  

   raised 
  border 
  folds, 
  while 
  a 
  third 
  specimen 
  shows 
  throughout 
  a 
  

   simple 
  transverse 
  convexity 
  of 
  the 
  coronal 
  surface. 
  The 
  latter 
  

   variety 
  also 
  possesses 
  a 
  narrower 
  basal 
  support, 
  and 
  in 
  both 
  f 
  its 
  

   peculiarities 
  it 
  agrees 
  with 
  a 
  solitary 
  example 
  in 
  Mr. 
  Bpringer's 
  col- 
  

   lection 
  from 
  the 
  upper 
  or 
  beds 
  of 
  passage 
  fish-bed 
  of 
  the 
  same 
  form- 
  

   ation. 
  The 
  latter 
  horizon 
  has 
  also 
  afforded 
  Mr. 
  Springer 
  a 
  second 
  

   specimen 
  which 
  is 
  undistin^uishable 
  from 
  the 
  variety 
  described 
  

   above. 
  

  

  Only 
  two 
  or 
  three 
  examples 
  of 
  this 
  form 
  have 
  thus 
  far 
  been 
  de- 
  

   tected 
  in 
  the 
  Keokuk 
  limestone, 
  above 
  which 
  it 
  has 
  not 
  been 
  i. 
  

   nized. 
  The 
  latter 
  teeth 
  are 
  very 
  like 
  the 
  typical 
  specimens 
  described 
  

   above, 
  showing 
  a 
  similarly 
  double-furrowed 
  coronal 
  surface, 
  the 
  only 
  

   marked 
  distinction 
  consisting 
  in 
  the 
  greater 
  size 
  and 
  more 
  robust 
  

   figure 
  of 
  the 
  Keokuk 
  teeth. 
  

  

  In 
  consequence 
  of 
  the 
  meagre 
  data 
  relating 
  to 
  this 
  form 
  it 
  is 
  dif- 
  

   ficult 
  to 
  determine 
  the 
  character 
  of 
  the 
  variations 
  alluded 
  to 
  — 
  

   wlntlicr 
  tiny 
  are 
  of 
  specific 
  or 
  merely 
  varietal 
  importance. 
  In 
  the 
  

   accompanying 
  description 
  of 
  the 
  figures 
  illustrating 
  the 
  BeveraJ 
  vari- 
  

   eties 
  the 
  following 
  provisional 
  terms 
  are 
  employed 
  to 
  distinguish 
  

   them: 
  Deltodop8i$t 
  bialveatut, 
  ]>..' 
  convexus, 
  /'. 
  Keokuk. 
  

  

  In 
  the 
  attempt 
  to 
  assign 
  the 
  present 
  form 
  to 
  its 
  proper 
  

   relations 
  we 
  are 
  met 
  at 
  the 
  outset 
  by 
  the 
  difficulties 
  incident 
  to 
  the 
  

   meagerness 
  of 
  the 
  material 
  for 
  the 
  purposes 
  of 
  comparative 
  exam* 
  

   inat 
  ion. 
  The 
  further 
  characterization 
  of 
  the 
  form 
  disci 
  -light 
  

  

  spiral 
  inrollment, 
  the 
  direction 
  of 
  which 
  indicates 
  that 
  the 
  oblique 
  

  

  border 
  is 
  apparently 
  homologous 
  with 
  the 
  postero-lateral 
  bord< 
  

  

  typical 
  CoohliodontS, 
  and 
  with 
  this 
  character 
  is 
  also 
  associated 
  the 
  

  

  obliquity 
  of 
  the 
  inner 
  margin 
  from 
  the 
  posterior 
  angle 
  to 
  the 
  obtuse 
  

   nut. 
  rior 
  ad 
  the 
  somewhat 
  marked 
  Baring 
  of 
  the 
  in: 
  

  

  basal 
  run 
  on 
  the 
  Mile 
  corresponding 
  to 
  the 
  antero-lateral 
  bolder; 
  

  

  