﻿60 
  PAL-EONTOLOGY. 
  

  

  Considering 
  the 
  more 
  typical 
  representatives 
  of 
  the 
  family, 
  the 
  

   earliest 
  to 
  appear 
  is 
  Chitonodus, 
  which 
  began 
  daring 
  the 
  Lower 
  Bur- 
  

   lington 
  epoch 
  and 
  is 
  last 
  met 
  with 
  in 
  the 
  St. 
  Louis, 
  it 
  is 
  clearly 
  

   the 
  forerunner 
  of 
  PaecUodus 
  and 
  Cochliodus, 
  which 
  appeared 
  at 
  later 
  

   epochs, 
  and 
  which 
  are 
  modifications 
  based 
  upon 
  the 
  same 
  plan, 
  

   and 
  which 
  might 
  readily 
  be 
  supposed 
  to 
  possess 
  features 
  that 
  

   entitle 
  them 
  to 
  at 
  least 
  sub-family 
  rank 
  in 
  contradistinction 
  to 
  the 
  

   before-mentioned 
  genera. 
  While 
  Cochliodus 
  was 
  apparently 
  of 
  com- 
  

   paratively 
  brief 
  duration, 
  since 
  its 
  remains 
  have 
  thus 
  far 
  only 
  been 
  

   met 
  with 
  in 
  the 
  St. 
  Louis 
  and 
  Chester 
  formations, 
  PaecUodus, 
  the 
  

   earlier 
  of 
  the 
  two 
  to 
  make 
  its 
  appearance, 
  dates 
  from 
  the 
  Warsaw. 
  

   the 
  last 
  representative 
  being 
  derived 
  from 
  the 
  upper 
  Coal 
  M 
  

   Throughout 
  this 
  vast 
  geological 
  range 
  its 
  species 
  manifest 
  the 
  

   tendency 
  toward 
  differentiation, 
  on 
  the 
  contrary 
  maintaining 
  the 
  

   distinctive 
  characteristics 
  of 
  the 
  genus 
  with 
  extraordinary 
  fidelity 
  to 
  

   the 
  type. 
  The 
  same 
  may 
  be 
  said 
  of 
  Deltodtu, 
  whose 
  inception 
  is 
  

   contemporary 
  with 
  Chitonodus, 
  with 
  which 
  its 
  relations 
  are. 
  perhaps, 
  

   the 
  most 
  intimate. 
  This 
  is 
  especially 
  pronounced 
  in 
  th< 
  if 
  the 
  

  

  homologous 
  terminal 
  forms 
  of 
  the 
  mandible 
  of 
  either 
  genus, 
  

   dus 
  maintaining 
  a 
  simple 
  convex 
  anterior 
  lobe 
  in 
  lieu 
  of 
  the 
  

   median 
  coronal 
  prominence 
  of 
  Chitonodus, 
  which 
  is 
  a 
  mere 
  modifi- 
  

   cation 
  of 
  the 
  former, 
  and 
  which 
  reached 
  its 
  extreme 
  differentiation 
  

   in 
  Cochliodus; 
  but 
  in 
  the 
  supposed 
  terminal 
  form 
  of 
  the 
  upper 
  jaw 
  

   we 
  are 
  again 
  presented 
  with 
  a 
  marked 
  departure 
  from 
  the 
  I 
  

   Uodns 
  type, 
  the 
  significance 
  of 
  which 
  is 
  Bought 
  in 
  the 
  previously 
  

   mentioned 
  genus 
  Sandalodus, 
  while 
  a 
  less 
  intimate 
  relationship 
  with 
  

   Deltoptychius 
  and 
  Orthopleurodus 
  is 
  noted, 
  but 
  sufficient 
  to 
  a 
  

   attention 
  with 
  suggestions 
  of 
  their 
  common 
  origin. 
  D 
  

   evidently 
  was 
  a 
  later 
  modification 
  of 
  DeUodus, 
  and. 
  although 
  it 
  

   well 
  denned 
  group, 
  it 
  is 
  not 
  clear 
  whether 
  it 
  should 
  he 
  deemi 
  

   iter 
  rank 
  than 
  Bubgenerio. 
  

  

  The 
  foregoing 
  observations 
  are 
  rather 
  what 
  the 
  comparatively 
  

  

  re 
  facts 
  serin 
  to 
  BUggest, 
  than 
  actual 
  demonstrations 
  of 
  a 
  com* 
  

  

  plete 
  Bystem 
  of 
  derivation 
  ai 
  tic 
  relations. 
  I; 
  fore 
  with 
  

  

  oi 
  trepidation 
  we 
  venture 
  to 
  exhibit 
  these 
  -up; 
  

   relationships 
  in 
  diagramatio 
  form; 
  bul 
  it 
  will 
  at 
  hast 
  subserve 
  the 
  

   purposes 
  of 
  illustrating 
  in 
  a 
  vivid 
  manner 
  the 
  Btratigraphical 
  i 
  

   of 
  the 
  varii 
  alluded 
  to. 
  

  

  