﻿VEBTEBRATES. 
  65 
  

  

  Not 
  infrequently 
  individuals 
  of 
  the 
  median 
  forms 
  are 
  met 
  with 
  

   which 
  show 
  one 
  or 
  more 
  partially 
  detached, 
  laterally 
  elongate 
  teeth 
  

   at 
  the 
  outer 
  extremity 
  of 
  the 
  plate, 
  which 
  is 
  always 
  abruptly 
  trun- 
  

   cated, 
  leading 
  to 
  the 
  inference 
  that 
  these 
  median 
  plates 
  themselves 
  

   were, 
  in 
  the 
  earlier 
  stages 
  of 
  development, 
  made 
  up 
  of 
  series 
  of 
  

   teeth. 
  On 
  the 
  other 
  hand, 
  certain 
  forms 
  occur 
  which 
  have 
  been 
  

   noticed 
  under 
  various 
  generic 
  designations 
  (Desmiodus, 
  Orodus), 
  

   which 
  are 
  composed 
  of 
  series 
  of 
  transverse 
  coronal 
  crests, 
  firmly 
  

   soldered 
  into 
  a 
  continuous 
  plate 
  at 
  the 
  impingement 
  of 
  their 
  bases, 
  

   in 
  which 
  condition 
  they 
  bear 
  a 
  remarkable 
  resemblance 
  in 
  outline 
  

   and 
  direction 
  of 
  inrollment, 
  to 
  the 
  above 
  mentioned 
  median 
  forms. 
  

   But 
  beyond 
  deducing 
  certain 
  permissible 
  conclusions 
  bearing 
  on 
  the 
  

   probable 
  affinities 
  of 
  Psephodus, 
  the 
  meagre 
  array 
  of 
  data 
  is 
  deemed 
  

   insufficient 
  to 
  warrant, 
  at 
  the 
  present 
  time, 
  the 
  critical 
  relegation 
  of 
  

   the 
  diverse 
  dental 
  materials 
  alluded 
  to 
  in 
  this 
  connection. 
  

   In 
  American 
  geological 
  history, 
  Psephodus 
  was 
  amongst 
  the 
  earli- 
  

   ye-/ 
  est 
  representatieis 
  of 
  its 
  order 
  in 
  the 
  Lower 
  Carboniferous 
  period, 
  

   / 
  its 
  first 
  occurrence 
  dating 
  from 
  the 
  Kinderhook 
  epoch. 
  Although 
  

   the 
  succeeding 
  Burlington, 
  Keokuk 
  and 
  Warsaw 
  formations 
  have 
  as 
  

   yet 
  afforded 
  no 
  evidence 
  of 
  the 
  presence 
  of 
  the 
  remains 
  of 
  the 
  genus 
  

   (if 
  we 
  except 
  the 
  occasional 
  appearance 
  of 
  Helodus 
  planus-like 
  forms), 
  

   its 
  forms 
  are 
  sparingly 
  met 
  with 
  in 
  the 
  St. 
  Louis 
  deposits, 
  and 
  in 
  

   the 
  super-adjacent 
  Chester 
  division 
  several 
  forms, 
  probably 
  pertain- 
  

   ing 
  to 
  a 
  single 
  species, 
  are 
  conspicuously 
  prevalent. 
  The 
  various 
  

   species 
  occurring 
  through 
  so 
  great 
  an 
  interval 
  of 
  time, 
  ranging 
  from 
  

   the 
  earliest 
  to 
  the 
  latest 
  epoch 
  of 
  the 
  period, 
  exhibit 
  an 
  extraordinary 
  

   persistency 
  of 
  the 
  type 
  without 
  parallel 
  amongst 
  the 
  Cochliodonts, 
  

   if 
  we 
  except 
  the 
  doubtful 
  relations 
  of 
  the 
  later 
  introduced 
  species 
  

   provisionally 
  identified 
  with 
  Vaticinodus. 
  From 
  this 
  point 
  of 
  view 
  

   the 
  genus 
  would 
  appear 
  to 
  be 
  prominently 
  isolated 
  so 
  far 
  as 
  relates 
  

   to 
  precursor 
  or 
  subsequent 
  derivatives. 
  Of 
  the 
  former 
  we 
  have 
  not 
  

   even 
  the 
  vaguest 
  intimation, 
  and 
  the 
  latter, 
  even 
  under 
  the 
  most 
  liberal 
  

   interpretations, 
  have 
  undergone 
  excessive 
  differentiation. 
  The 
  pres- 
  

   ence, 
  in 
  the 
  extreme 
  anterior 
  portion 
  of 
  the 
  jaw 
  of 
  Cochliodus, 
  of 
  

   transverse 
  coronal 
  ridges, 
  if 
  not 
  of 
  isolated 
  teeth, 
  in 
  series 
  corres- 
  

   ponding 
  to 
  those 
  abaft 
  and 
  forward 
  of 
  the 
  convolute 
  median 
  plate 
  

   in 
  Psephodus, 
  offer 
  tangible 
  evidence 
  of 
  their 
  probable 
  origin, 
  though 
  

   their 
  relations 
  are 
  less 
  intimate 
  than 
  might 
  be 
  expected 
  in 
  groups 
  

   almost 
  coeval 
  in 
  their 
  inception. 
  

  

  — 
  5 
  

  

  