﻿VEBTEBKATES. 
  175 
  

  

  Genus 
  XYSTKODUS, 
  Agassiz. 
  

  

  Xystrodus, 
  Agassiz, 
  Mss., 
  1859 
  (Cochliodus 
  striatus, 
  Ag. 
  Poios. 
  

   Foss. 
  Ill, 
  p. 
  174; 
  Xystrodus 
  angustus, 
  Ag. 
  Mss., 
  1859. 
  

  

  The 
  genus 
  Xystrodus 
  was 
  recognized 
  by 
  Professor 
  Agassiz 
  in 
  1859, 
  

   to 
  which 
  he 
  transferred 
  the 
  species 
  previously 
  described 
  under 
  the 
  

   name 
  Cochliodus 
  striatus, 
  Ag., 
  from 
  the 
  Carboniferous 
  limestone 
  of 
  

   Ireland. 
  The 
  later 
  term 
  was 
  accepted 
  by 
  European 
  palseichthyolo- 
  

   gists, 
  and 
  at 
  a 
  later 
  date. 
  1869, 
  was 
  published 
  in 
  the 
  list 
  of 
  type 
  

   specimens 
  of 
  the 
  magnificent 
  collection 
  at 
  Florence 
  Court, 
  belonging 
  

   to 
  Lord 
  Enniskillen, 
  as 
  also 
  in 
  the 
  lists 
  of 
  the 
  British 
  Museum 
  and 
  

   and 
  other 
  institutions, 
  and 
  incorporated 
  in 
  the 
  great 
  work 
  by 
  Dr. 
  

   Bigsby, 
  the 
  "Thesaurus 
  Devonico-Carboniferous." 
  

  

  In 
  accordance 
  with 
  the 
  presumptive 
  determinations 
  and 
  nomencla- 
  

   ture 
  adopted 
  in 
  the 
  present 
  work, 
  the 
  original 
  or 
  type 
  of 
  the 
  genus 
  

   Xystrodus 
  striatus 
  is 
  referable 
  to 
  the 
  maxillary 
  posterior 
  form, 
  and 
  not- 
  

   withstanding 
  the 
  opposed 
  mandibular 
  teeth 
  have 
  been 
  obtained 
  from 
  

   the 
  same 
  horizons 
  and 
  localities 
  affording 
  the 
  former, 
  we 
  are 
  not 
  

   aware 
  of 
  their 
  having 
  been 
  specifically 
  identified 
  and 
  described. 
  

   The 
  latter 
  teeth 
  are 
  recognizable 
  by 
  the 
  extreme 
  alation 
  of 
  the 
  

   postero-lateral 
  border, 
  giving 
  to 
  the 
  general 
  figure 
  excessive 
  breadth 
  

   compared 
  to 
  the 
  length 
  in 
  the 
  direction 
  of 
  inrollment. 
  

  

  The 
  above-mentioned 
  teeth 
  agree 
  in 
  all 
  essential 
  features 
  with 
  the 
  

   forms 
  occurring 
  in 
  the 
  various 
  American 
  Lower 
  Carboniferous 
  forma- 
  

   tions. 
  Especially 
  is 
  this 
  notable 
  in 
  the 
  case 
  of 
  the 
  Chester 
  teeth 
  

   Xystrodus 
  verus, 
  which 
  indeed 
  constitutes 
  a 
  typical 
  representative 
  Of 
  

   the 
  genus. 
  Therefore, 
  from 
  such 
  materials 
  as 
  we 
  possess, 
  the 
  fol- 
  

   lowing 
  diagnoses 
  of 
  the 
  opposed 
  terminal 
  teeth 
  of 
  the 
  upper 
  and 
  

   lower 
  jaws 
  are 
  prepared; 
  of 
  the 
  other 
  dental 
  forms 
  with 
  Avhich 
  they 
  

   were 
  associated 
  during 
  the 
  life 
  of 
  the 
  fish 
  to 
  which 
  they 
  belonged, 
  

   as 
  yet 
  we 
  have 
  failed 
  to 
  arrive 
  at 
  even 
  a 
  surmise 
  as 
  to 
  their 
  char- 
  

   acter. 
  

  

  Mandibular 
  posterior 
  teeth 
  distinguished 
  by 
  their 
  triangular 
  out- 
  

   line 
  and 
  great 
  transverse 
  breadth 
  of 
  the 
  inner 
  margin 
  as 
  compared 
  

   with 
  the 
  longitudinal 
  diameter. 
  Coronal 
  surface 
  occupying 
  the 
  entire 
  

   superior 
  area 
  of 
  the 
  tooth, 
  comparatively 
  plain, 
  more 
  or 
  less 
  con- 
  

   cave, 
  or 
  with 
  a 
  longitudinal 
  depression, 
  the 
  coronal 
  prominence 
  

   crowded 
  to 
  the 
  anterior 
  side, 
  nearly 
  fiat 
  or 
  but 
  gently 
  convex 
  trans- 
  

   versely, 
  at 
  the 
  opposite 
  border 
  terminating 
  in 
  the 
  expanded, 
  upraised 
  

  

  