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  VEETEBKATES. 
  193 
  

  

  outer 
  inrolled 
  extremity 
  14 
  mm. 
  ; 
  greatest 
  breadth 
  between 
  angles 
  of 
  

   inner 
  margin 
  9.5 
  mm. 
  ; 
  breadth 
  of 
  coronal 
  prominence 
  at 
  inner 
  

   margin 
  5 
  mm. 
  ; 
  length 
  along 
  antero-lateral 
  border 
  11 
  mm. 
  Man- 
  

   dibular 
  median 
  tooth, 
  length 
  along 
  posterior 
  articular 
  border 
  8 
  mm., 
  

   breadth 
  along 
  inner 
  margin 
  6.5 
  mm. 
  

   trf^** 
  r&tv 
  The 
  long 
  posterior 
  teeth 
  of 
  the 
  - 
  mandible 
  , 
  which 
  in 
  well-preserved 
  

   examples 
  show 
  the 
  coronal 
  ridge 
  defined 
  on 
  either 
  side 
  by 
  a 
  slight 
  

   impressed 
  angulation, 
  owing 
  to 
  the 
  extreme 
  thinness 
  of 
  the 
  outer 
  

   portion 
  are 
  invariably 
  mutilated 
  by 
  having 
  the 
  outer 
  extremity 
  

   broken 
  away. 
  This 
  attenuation 
  is 
  probably 
  due 
  in 
  part, 
  at 
  least, 
  

   to 
  the 
  excessive 
  wear 
  this 
  part 
  of 
  the 
  coronal 
  surface 
  has 
  been 
  sub- 
  

   jected 
  to 
  while 
  in 
  use, 
  and 
  the 
  vicissitudes 
  to 
  which 
  they 
  have 
  been 
  

   exposed 
  subsequent 
  to 
  their 
  disengagement 
  from 
  the 
  maxillary 
  sup- 
  

   ports 
  has 
  resulted 
  in 
  disfiguring 
  this 
  part 
  of 
  the 
  teeth 
  so 
  that 
  the 
  

   form 
  of 
  the 
  extreme 
  antero-lateral 
  border 
  and 
  outer 
  extremity 
  can 
  

   only 
  be 
  conjectured. 
  However, 
  it 
  probably 
  partook 
  of 
  the 
  general 
  

   characteristics 
  of 
  the 
  corresponding 
  tooth 
  of 
  Stenopterodus 
  and 
  Del- 
  

   toptychius 
  in 
  truncation 
  and 
  definition 
  of 
  coronal 
  and 
  basal 
  regions. 
  

   The 
  far 
  more 
  uniformly 
  massive 
  posterior 
  teeth 
  of 
  the 
  mandible, 
  

   however, 
  are 
  often 
  found 
  quite 
  entire. 
  The 
  position 
  and 
  consequent 
  

   severe 
  usage 
  suffered 
  by 
  the 
  median 
  teeth 
  of 
  the 
  mandible 
  by 
  which 
  

   they 
  have 
  been 
  defaced, 
  as 
  also 
  their 
  extremely 
  simple 
  form, 
  may 
  

   have 
  led 
  to 
  their 
  being 
  overlooked 
  in 
  many 
  instances. 
  

  

  Geological 
  position 
  and 
  localities: 
  The 
  stratigraphical 
  distribution 
  

   of 
  the 
  above 
  species, 
  as 
  it 
  is 
  at 
  present 
  understood, 
  is 
  known 
  to 
  ex- 
  

   tend 
  from 
  the 
  lower 
  part 
  of 
  the 
  Lower 
  Coal 
  Measures, 
  well 
  up 
  into 
  

   the 
  Upper 
  division 
  ; 
  the 
  vertical 
  range 
  in 
  Kansas 
  is 
  known 
  to 
  include 
  

   above 
  1,500 
  feet 
  of 
  exclusively 
  Upper 
  Coal 
  Measure 
  strata 
  ; 
  LaSalle, 
  

   Springfield, 
  Carlinville, 
  Staunton, 
  Belleville, 
  Bloomington, 
  Danville, 
  

   also, 
  localities 
  in 
  Fulton 
  and 
  Vermilion 
  counties, 
  Illinois 
  ; 
  Posey 
  

   county, 
  Indiana; 
  several 
  localities 
  in 
  the 
  Upper 
  Coal 
  Measures 
  of 
  

   Iowa, 
  Nebraska, 
  Kansas 
  and 
  Missouri. 
  

  

  Orthopleurodus 
  convexus, 
  St. 
  J. 
  and 
  W. 
  

  

  PI. 
  XIII, 
  Fig. 
  4, 
  5. 
  

  

  Posterior 
  teeth 
  of 
  mandible 
  agreeing 
  closely 
  in 
  outline 
  with 
  the 
  

   corresponding 
  form 
  of 
  Orthopleurodus 
  carbonarius, 
  moderately 
  arched 
  

   from 
  within 
  outwards 
  and 
  revolute; 
  antero-lateral 
  border 
  moderately 
  

   curved 
  sigmoidally, 
  and 
  occupied 
  by 
  a 
  rather 
  deep 
  enamel 
  belt 
  be- 
  

   neath 
  the 
  sharp, 
  anterior 
  coronal 
  angulation; 
  inner 
  margin 
  broadly 
  

  

  —13 
  

  

  