﻿VEETEBRATES. 
  211 
  

  

  direction, 
  making 
  an 
  obtuse 
  angle 
  with 
  the 
  anterior 
  margin 
  and 
  

   which 
  is 
  strongly 
  beveled 
  from 
  the 
  acuminate 
  culmination 
  of 
  the 
  

   outer 
  coronal 
  ridge 
  ; 
  the 
  crown 
  is 
  rather 
  deeply 
  depressed 
  trans 
  - 
  

   versely, 
  and 
  very 
  moderately 
  arched 
  longitudinally, 
  presenting 
  a 
  

   beautifully 
  sculptured 
  surface 
  which 
  bears 
  a 
  striking 
  resemblance 
  to 
  

   the 
  style 
  of 
  ornamentation 
  observed 
  in 
  the 
  typical 
  specimen 
  above 
  

   described. 
  The 
  specific 
  identity 
  of 
  the 
  present 
  example 
  with 
  that 
  

   form, 
  however, 
  is 
  merely 
  conjectural. 
  The 
  specimen 
  is 
  of 
  especial 
  

   interest 
  from 
  the 
  fact 
  that 
  it 
  belongs 
  to 
  an 
  opposite 
  row, 
  — 
  all 
  the 
  

   individuals 
  of 
  P. 
  crassidus 
  at 
  present 
  known 
  being 
  from 
  the 
  same 
  <? 
  : 
  

   side. 
  

  

  Geological 
  position 
  and 
  localities: 
  Upper 
  Burlington 
  limestone, 
  main 
  

   fish-bed 
  ; 
  Quincy 
  and 
  Cedar 
  Creek, 
  Henderson 
  county, 
  111. 
  

  

  Psammodus 
  grandis, 
  St. 
  J. 
  and 
  W. 
  

  

  PI. 
  XV, 
  Fig. 
  1-3. 
  

  

  Teeth 
  attain 
  large 
  size. 
  Maxillary 
  form 
  subrhomboiclal 
  in 
  out- 
  

   line, 
  moderately 
  arched 
  longitudinally, 
  gently 
  depressed 
  transversely. 
  

   Inner 
  articular 
  border 
  nearly 
  straight, 
  — 
  one 
  specimen 
  showing 
  a 
  

   slightly 
  truncated 
  posterior 
  angle 
  for 
  the 
  lesser 
  articular 
  facet, 
  

   while 
  a 
  smaller 
  individual 
  shows 
  a 
  faint 
  angulation 
  a 
  little 
  in 
  

   advance 
  of 
  the 
  middle, 
  but 
  so 
  disfigured 
  by 
  abrasion 
  as 
  not 
  to 
  

   / 
  clearly 
  show 
  its 
  character,—^Coronal 
  enamel 
  forming 
  a 
  narrow 
  fold 
  

  

  more 
  or 
  less 
  well 
  defined 
  from 
  the 
  deep 
  basal 
  portion; 
  outer 
  border 
  

   not 
  preserved 
  ; 
  anterior 
  margin 
  very 
  gently 
  curved 
  or 
  nearly 
  straight 
  ; 
  

   posterior 
  margin 
  gently 
  arched, 
  more 
  or 
  less 
  irregularly 
  undulated, 
  

   especially 
  in 
  mature 
  teeth. 
  Coronal 
  surface 
  in 
  young 
  examples 
  

   smooth 
  and 
  regularly 
  convex 
  and 
  concave 
  in 
  the 
  two 
  diameters, 
  

   maturer 
  individuals 
  being 
  traversed 
  by 
  more 
  or 
  less 
  conspicuous 
  

   transverse 
  undulations, 
  and 
  which 
  are 
  even 
  faintly 
  discernible 
  in 
  the 
  

   earlier-formed 
  smaller 
  teeth. 
  The 
  surface 
  is 
  elegantly 
  sculptured, 
  

   the 
  rugose 
  lines 
  showing 
  a 
  decided 
  transverse 
  arrangement, 
  though 
  

   sometimes 
  taking 
  an 
  oblique 
  course 
  over 
  limited 
  areas, 
  and 
  in 
  the 
  

   more 
  exposed 
  parts 
  presenting 
  a 
  reticulate 
  appearance 
  like 
  Arabesque 
  

   designs, 
  and 
  in 
  worn 
  surfaces 
  becoming 
  obsolete 
  where 
  the 
  ordinary 
  

   minute 
  punctae 
  appear. 
  A 
  small 
  tooth 
  presents 
  the 
  following 
  rela- 
  

   tive 
  proportions 
  : 
  Breadth 
  across 
  posterior 
  margin, 
  about 
  30 
  milli- 
  

   metres; 
  breadth 
  across 
  anterior 
  margin 
  24 
  mm; 
  length 
  along 
  inner 
  

   articular 
  border 
  about 
  £0 
  mm. 
  

  

  