﻿VERTEBRATES. 
  73 
  

  

  an 
  angle 
  of 
  55° 
  with 
  the 
  opposite 
  border, 
  with 
  which 
  it 
  agrees 
  in 
  

   other 
  respects, 
  although 
  the 
  basal 
  portion 
  is 
  thicker 
  and 
  expanded 
  

   posteriorly 
  beyond 
  the 
  limits 
  of 
  the 
  coronal 
  fold; 
  inner 
  margin 
  at 
  

   right 
  angles 
  to 
  the 
  antero-lateral 
  border, 
  broadly 
  arched 
  from 
  the 
  

   obtuse 
  anterior 
  angle 
  round 
  the 
  base 
  of 
  the 
  coronal 
  prominence, 
  

   behind 
  which 
  it 
  makes 
  a 
  slight 
  concavity 
  on 
  the 
  way 
  to 
  the 
  sub- 
  

   acute 
  posterior 
  extremity. 
  Coronal 
  surface 
  gradually 
  rising 
  into 
  the 
  

   rounded 
  crest 
  which 
  lies 
  a 
  little 
  behind 
  the" 
  median 
  line, 
  the 
  poste- 
  

   rior 
  slope 
  slightly 
  concave, 
  the 
  anterior 
  slope 
  even 
  Jess 
  so, 
  alate 
  

   expansion 
  obscurely 
  defined. 
  Surface 
  marked 
  by 
  minute, 
  wide- 
  

   spaced 
  punctae. 
  Greatest 
  lateral 
  diameter 
  across 
  the 
  inner 
  margin 
  

   13 
  mm. 
  ; 
  length 
  along 
  antero-lateral 
  border 
  9. 
  mm. 
  

  

  The 
  above 
  described 
  form 
  is 
  represented 
  by 
  a 
  couple 
  of 
  nearly 
  

   entire 
  teeth, 
  in 
  the 
  collection 
  of 
  Mr. 
  Van 
  Home. 
  They 
  are 
  closely 
  

   related 
  to 
  the 
  corresponding 
  form 
  of 
  the 
  representative 
  species 
  oc- 
  

   curring 
  in 
  the 
  Chester 
  formation, 
  which 
  latter, 
  however, 
  is 
  distin- 
  

   guished 
  by 
  the 
  finer 
  punctation, 
  more 
  oblique 
  backward 
  course 
  of 
  

   the 
  inner 
  margin, 
  and 
  less 
  central 
  position 
  of 
  the 
  coronal 
  ridge. 
  

  

  The 
  St. 
  Louis 
  collections 
  also 
  afford 
  many 
  examples 
  of 
  the 
  diverse 
  

   forms 
  of 
  teeth 
  which 
  have 
  usually 
  been 
  identified 
  with 
  the 
  genus 
  

   Helodus, 
  but 
  which, 
  possessing 
  certain 
  characteristics 
  in 
  common 
  

   with 
  unquestionable 
  forms 
  of 
  Psephodus, 
  seem 
  to 
  possess 
  intimate 
  

   generic 
  relations 
  with 
  the 
  present 
  form. 
  The 
  latter 
  teeth 
  may 
  be 
  

   distinguished 
  in 
  a 
  general 
  way 
  by 
  their 
  eccentric 
  coronal 
  prominence, 
  

   crenulated 
  lateral 
  borders 
  and 
  margin 
  of 
  the 
  abrupt 
  outer 
  coronal 
  

   slope, 
  presenting 
  extreme 
  variation 
  in 
  outline 
  and 
  coronal 
  contour. 
  

   Not 
  infrequent 
  examples 
  are 
  met 
  with 
  showing 
  the 
  downward 
  and 
  

   outward 
  produced 
  base 
  characteristic 
  of 
  Popliodus, 
  as 
  obtains 
  in 
  

   L. 
  (Helodus) 
  didymus, 
  Ag. 
  ; 
  but 
  more 
  frequently 
  the 
  posterior 
  basal 
  

   portion 
  is 
  inbeveled, 
  indicating 
  that 
  the 
  individual 
  may 
  have 
  con- 
  

   stituted 
  one 
  of 
  a 
  series 
  of 
  teeth 
  more 
  or 
  less 
  perfectly 
  joined 
  at 
  

   their 
  bases, 
  forming 
  a 
  continuous 
  plate 
  the 
  same 
  as 
  is 
  known 
  to 
  

   occur 
  in 
  individuals 
  of 
  Psephodus. 
  Varying 
  from 
  the 
  asymetrical 
  

   contorted 
  forms 
  to 
  broad 
  surfaced 
  sublunate 
  symmetrical 
  teeth, 
  

   almost 
  every 
  conceivable 
  variation 
  attributed 
  to 
  this 
  class 
  of 
  teeth 
  

   is 
  observed 
  amongst 
  the 
  St. 
  Louis 
  material, 
  the 
  same 
  as 
  occurs 
  in 
  

   the 
  Kinderhook 
  and 
  Chester 
  collections. 
  The 
  specific 
  identity 
  of 
  

   these 
  diverse 
  forms 
  with 
  the 
  teeth 
  above 
  described 
  is 
  in 
  many 
  in- 
  

   stances 
  as 
  clearly 
  indicated 
  as 
  may 
  be 
  under 
  the 
  circumstances 
  ; 
  

   but 
  that 
  all 
  of 
  them 
  belong 
  to 
  a 
  single 
  species, 
  we 
  would 
  not 
  under- 
  

   take 
  to 
  affirm 
  with 
  equal 
  confidence. 
  

  

  