﻿VEETEBRATES. 
  71 
  

  

  supposed 
  opposed 
  form 
  of 
  the 
  upper 
  jaw, 
  the 
  lateral 
  borders 
  more 
  

   gradually 
  converge 
  the 
  anterior 
  position 
  and 
  greater 
  prominence 
  of 
  

   the 
  coronal 
  ridge, 
  while 
  the 
  inner 
  margin 
  is 
  less 
  obliquely 
  rounded 
  

   than 
  obtains 
  in 
  the 
  corresponding 
  forms 
  of 
  the 
  last 
  described 
  species. 
  

   Geological 
  position 
  and 
  locality: 
  Upper 
  fish-bed 
  of 
  the 
  Kinderhook 
  

   formation 
  ; 
  Burlington, 
  Iowa. 
  

  

  Psephodus? 
  Symmetricus, 
  St. 
  J. 
  and 
  W. 
  

  

  PL 
  I, 
  Pig. 
  6, 
  7. 
  

  

  The 
  present 
  species 
  is 
  represented 
  by 
  a 
  single 
  form, 
  with 
  which 
  

   only 
  one 
  or 
  two 
  individuals, 
  of 
  doubtful 
  generic 
  relations, 
  have 
  thus 
  

   far 
  been 
  provisionally 
  identified. 
  The 
  form 
  corresponds 
  to 
  that 
  re- 
  

   ferred 
  to 
  the 
  median 
  position 
  on 
  the 
  lower 
  jaw. 
  The 
  typical 
  example 
  

   is 
  that 
  of 
  a 
  tooth 
  below 
  medium 
  size, 
  sub-trapezoidal 
  in 
  outline, 
  

   moderately 
  arched 
  from 
  within 
  outwards 
  and 
  somewhat 
  closely 
  in- 
  

   rolled, 
  giving 
  to 
  the 
  outline 
  a 
  relatively 
  great 
  breadth 
  as 
  compared 
  

   to 
  the 
  length 
  of 
  the 
  tooth. 
  The 
  outer 
  margin 
  and 
  coronal 
  region 
  is 
  

   worn 
  and 
  broken 
  away, 
  nor 
  are 
  the 
  postero 
  — 
  and 
  antero— 
  lateral 
  

   borders 
  sufficiently 
  preserved 
  to 
  show 
  more 
  than 
  the 
  projection 
  of 
  

   the 
  basal 
  rim 
  beyond 
  the 
  coronal 
  limits, 
  as 
  obtains 
  in 
  both 
  examples, 
  

   the 
  borders 
  gradually 
  converging, 
  coronal 
  limits 
  defined 
  from 
  the 
  

   basal 
  rim 
  by 
  a 
  shallow 
  sulcus 
  ; 
  the 
  inner 
  margin 
  is 
  broadly 
  rounded, 
  

   with 
  a 
  slight 
  sigmoidal 
  curvature 
  in 
  passing 
  from 
  the 
  obtuse 
  anterior 
  

   angle 
  to 
  the 
  slightly 
  produced 
  posterior 
  angle. 
  The 
  crown 
  surface 
  

   gently 
  rises 
  from 
  the 
  antero-lateral 
  border, 
  culminating 
  in 
  a 
  broad 
  

   low 
  crest 
  situate 
  in 
  the 
  posterior 
  half 
  of 
  the 
  coronal 
  area, 
  thence 
  

   more 
  steeply 
  sloping 
  into 
  the 
  narrow 
  shallow 
  concavity 
  parallel 
  

   with 
  the 
  posterior 
  border 
  ; 
  surface 
  minutely 
  punctate. 
  Greatest 
  

   diameter 
  across 
  the 
  inner 
  margin 
  .17 
  mm. 
  ; 
  length 
  along 
  the 
  antero- 
  

   lateral 
  border 
  7.5 
  mm. 
  

  

  The 
  type 
  specimen 
  shows 
  a 
  faint 
  longitudinal 
  depression 
  near 
  the 
  

   median 
  line, 
  which 
  does 
  not 
  appear 
  to 
  have 
  been 
  produced 
  by 
  wear 
  

   as 
  it 
  descends 
  to 
  the 
  unworn 
  enameled 
  inner 
  margin, 
  and 
  toward 
  

   the 
  antero-lateral 
  border 
  the 
  surface 
  is 
  marked 
  by 
  a 
  thread-like 
  

   groove, 
  defining 
  a 
  narrow 
  plain 
  belt 
  along 
  that 
  side, 
  which 
  is 
  appa- 
  

   rently 
  also 
  a 
  permanent 
  constituent 
  of 
  the 
  coronal 
  contour. 
  

  

  Another 
  and 
  smaller 
  tooth 
  belonging 
  to 
  the 
  left 
  ramus 
  of 
  the 
  

   lower 
  jaw, 
  shows 
  nearly 
  the 
  same 
  outline 
  as 
  the 
  specimen 
  described 
  

   above, 
  but 
  it 
  is 
  distinguished 
  by 
  the 
  greater 
  prominence 
  of 
  the 
  

  

  