﻿VERTEBRATES. 
  109 
  

  

  3.5 
  m 
  m., 
  and 
  length 
  along 
  the 
  straight 
  posterolateral 
  border 
  about 
  

   2 
  mm. 
  Trapezoidal 
  in 
  outline, 
  it 
  is 
  relatively 
  broader 
  than 
  the 
  

   Upper 
  Burlington 
  teeth 
  of 
  the 
  same 
  form 
  ; 
  the 
  postero-lateral 
  border 
  

   shows 
  a 
  slight 
  forward 
  obliquity 
  in 
  its 
  outward 
  course, 
  basal 
  portion 
  

   exceedingly 
  attenuated 
  ; 
  antero-lateral 
  border/ 
  converging 
  toward 
  

   point 
  of 
  inrollment 
  at 
  an 
  angle 
  of 
  about 
  30° 
  with 
  the 
  opposite 
  

   border, 
  too 
  imperfect 
  to 
  show 
  the 
  character 
  of 
  the 
  basal 
  rim 
  be- 
  

   neath 
  the 
  narrow, 
  enamel 
  fold 
  ; 
  inner 
  margin 
  very 
  gently 
  arched 
  

   from 
  the 
  anterior 
  angle, 
  and 
  abruptly 
  rounded 
  to 
  the 
  obscure, 
  pos- 
  

   tero-inner 
  angle. 
  Crown 
  presenting 
  a 
  broad, 
  faintly-depressed 
  slope, 
  

   culminating 
  in 
  a 
  low 
  ridge 
  closely 
  bordering 
  the 
  postero-lateral 
  side, 
  

   to 
  which 
  it 
  is 
  abruptly 
  sloped 
  and 
  merged 
  into 
  the 
  distinctly-defined, 
  

   narrow, 
  marginal 
  fold. 
  Coronal 
  punctation 
  relatively 
  coarse 
  and 
  closely 
  

   arranged, 
  very 
  similar 
  to 
  that 
  observed 
  in 
  the 
  mandibular 
  and 
  max- 
  

   illary 
  posterior 
  teeth. 
  

  

  The 
  specific 
  relations 
  of 
  the 
  above 
  described 
  form 
  are, 
  of 
  course, 
  

   only 
  inferentially 
  determined. 
  It 
  might 
  appear 
  to 
  be 
  an 
  anomalous 
  

   state 
  of 
  things, 
  especially 
  in 
  view 
  of 
  the 
  fact 
  that 
  the 
  mandibular 
  

   and 
  maxillary 
  posterior 
  forms 
  of 
  the 
  species 
  to 
  which 
  it 
  is 
  pro- 
  

   visionally 
  attributed 
  occur 
  in 
  comparative 
  numbers, 
  the 
  latter 
  form 
  

   should 
  be 
  represented 
  by 
  a 
  unique 
  example. 
  But 
  the 
  small 
  size 
  of 
  

   the 
  teeth, 
  and 
  their 
  thin, 
  scale-like 
  fragility, 
  may 
  in 
  part, 
  at 
  least, 
  

   account 
  for 
  their 
  rare 
  preservation 
  and 
  infrequency 
  in 
  collections. 
  

  

  Geological 
  position 
  and 
  localities 
  .- 
  St. 
  Louis 
  limestone 
  ; 
  Alton 
  and 
  

   Monroe 
  county, 
  (Illinois) 
  ; 
  St. 
  Louis, 
  (Missouri), 
  and 
  Pella, 
  (Iowa). 
  

  

  Genus 
  CHITOM)DUS, 
  St. 
  J. 
  and 
  W. 
  

  

  Teeth 
  representing 
  the 
  various 
  forms 
  characteristic 
  of 
  the 
  genera 
  

   of 
  Cochliodonts. 
  

  

  Mandibular 
  posterior 
  teeth 
  trapezoidal 
  in 
  outline, 
  strongly 
  to 
  mod- 
  

   erately 
  arched 
  in 
  the 
  direction 
  of 
  inrollment. 
  Antero-lateral 
  border 
  

   generally 
  but 
  slightly 
  oblique 
  to 
  a 
  line 
  projected 
  between 
  the 
  ante- 
  

   rior 
  and 
  posterior 
  angles 
  of 
  the 
  inner 
  margin, 
  basal 
  portion 
  chan- 
  

   neled, 
  continued 
  downward 
  and 
  outward 
  into 
  a 
  thin 
  marginal 
  rim, 
  

   bordered 
  above 
  by 
  the 
  narrow, 
  inbeveled 
  fold 
  of 
  coronal 
  enamel 
  ; 
  

   postero-lateral 
  border 
  converging 
  toward 
  outer 
  side 
  or 
  point 
  of 
  inroll- 
  

   ment 
  at 
  a 
  moderate 
  angle, 
  basal 
  portion 
  expanded 
  into 
  a 
  thin 
  rim 
  

   projecting 
  a 
  greater 
  or 
  less 
  distance 
  beyond 
  the 
  coronal 
  border 
  and 
  

   terminating 
  in 
  the 
  more 
  or 
  less 
  produced 
  spur 
  of 
  the 
  posterior 
  

   angle, 
  in 
  front 
  similarly 
  channeled 
  and 
  defined 
  above 
  by 
  the 
  rounded 
  

  

  