﻿VERTEBRATES. 
  177 
  

  

  The 
  genus 
  as 
  herein 
  denned 
  offers 
  intimate 
  relationship 
  with 
  the 
  

   genera 
  Deltoptychius 
  and 
  Orthopleurodus, 
  especially 
  in 
  the 
  conforma- 
  

   tion 
  and 
  the 
  distribution 
  of 
  the 
  coronal 
  enamel 
  of 
  the 
  mandibular 
  

   posterior 
  teeth 
  ; 
  but 
  in 
  other 
  and 
  essential 
  characteristics 
  it 
  presents 
  

   unmistakable 
  features 
  removing 
  its 
  forms 
  from 
  any 
  of 
  the 
  groups 
  of 
  

   teeth 
  of 
  contemporaneous 
  genera 
  with 
  which 
  they 
  are 
  associated. 
  

   At 
  the 
  same 
  time 
  we 
  are 
  cognizant 
  of 
  the 
  diversity 
  of 
  the 
  dental 
  

   armature 
  of 
  the 
  jaws 
  of 
  the 
  Cochliodonts, 
  and 
  the 
  "missing 
  links" 
  

   which 
  are 
  required 
  in 
  most 
  instances 
  before 
  the 
  restoration 
  of 
  the 
  

   dentition 
  of 
  these 
  genera 
  can 
  be 
  consummated. 
  But 
  the 
  teeth 
  at 
  

   present 
  uader 
  consideration 
  do 
  not 
  seem 
  to 
  fall 
  into 
  any 
  such 
  

   recognized 
  combination, 
  as 
  evidenced 
  not 
  only 
  by 
  their 
  peculiar 
  

   contour, 
  but 
  also 
  by 
  the 
  peculiar 
  superficial 
  structure 
  observed 
  in 
  

   the 
  typical 
  representatives 
  of 
  the 
  genus, 
  and 
  which 
  is 
  not 
  found 
  in 
  

   connection 
  with 
  the 
  teeth 
  of 
  other 
  genera 
  occurring 
  in 
  the 
  same 
  

   geological 
  horizons. 
  Hence, 
  notwithstanding 
  only 
  the 
  more 
  conspic- 
  

   uous 
  elements 
  of 
  the 
  dentition 
  of 
  this 
  genus 
  are, 
  at 
  present, 
  known, 
  

   these 
  are 
  marked 
  by 
  peculiarities 
  which 
  seem 
  to 
  warrant 
  the 
  recog- 
  

   nition 
  of 
  their 
  distinct 
  generic 
  standing. 
  

  

  While 
  we 
  have 
  not 
  the 
  data 
  necessary 
  for 
  carrying 
  the 
  compari- 
  

   sons 
  to 
  a 
  final 
  conclusion, 
  we 
  are 
  much 
  impressed 
  by 
  the 
  inti- 
  

   mate 
  resemblances 
  which 
  subsist 
  between 
  the 
  present 
  genus 
  and 
  

   Tomodus, 
  Agass. 
  A 
  series 
  of 
  careful 
  sketches 
  of 
  authentic 
  examples 
  

   of 
  Tomodus 
  convexus, 
  Agass., 
  from 
  the 
  Mountain 
  limestone 
  of 
  Bristol, 
  

   England, 
  and 
  which 
  were 
  kindly 
  submitted 
  to 
  us 
  by 
  Lord 
  Enniskillen, 
  

   show 
  this 
  relationship 
  in 
  a 
  striking 
  manner. 
  The 
  genus 
  Tomodus, 
  

   indeed, 
  appears 
  to 
  be 
  a 
  Xystrodus 
  in 
  all 
  save 
  the 
  arrangement 
  of 
  

   the 
  punctffl 
  in 
  parallel 
  transverse, 
  lines; 
  and 
  in 
  the 
  latter 
  respect 
  

   we 
  find 
  a 
  close, 
  though 
  not 
  absolute 
  agreement, 
  in 
  the 
  initial 
  species 
  

   occurring 
  in 
  the 
  American 
  Carboniferous 
  species 
  X. 
  simplex. 
  

  

  Thus 
  far 
  ascertained, 
  the 
  genus 
  Xystrodus 
  is 
  met 
  with 
  in 
  both 
  the 
  

   great 
  divisions 
  of 
  the 
  Carboniferous 
  period. 
  One 
  of 
  the 
  writers, 
  in 
  

   1877, 
  described 
  in 
  Dr. 
  Hayden's 
  final 
  report 
  on 
  the 
  U. 
  S. 
  geological 
  

   survey 
  of 
  Nebraska 
  (p. 
  244, 
  PI. 
  IV, 
  f. 
  18), 
  a 
  tooth 
  under 
  the 
  name 
  

   Xystrodus? 
  occidentalis. 
  The 
  examination 
  of 
  subsequently 
  acquired 
  

   and 
  more 
  complete 
  suites 
  of 
  the 
  fish- 
  remains 
  of 
  our 
  Coal-Measure 
  

   strata, 
  from 
  the 
  upper 
  division 
  of 
  which 
  the 
  latter 
  specimen 
  was 
  

   derived, 
  has 
  led 
  us 
  to 
  relegate 
  that 
  particular 
  specimen 
  to 
  the 
  genus 
  

   Orthopleurodus. 
  it 
  probably 
  being 
  a 
  very 
  worn 
  example, 
  showing 
  only 
  

   a 
  part 
  of 
  the 
  inner 
  portion 
  of 
  a 
  mandibular 
  posterior 
  tooth 
  of 
  the 
  

   species 
  0. 
  carbonarius, 
  ( 
  — 
  Deltodus 
  angularis, 
  N. 
  and 
  W.) 
  

  

  —12 
  

  

  