﻿VEKTEBEATES. 
  201 
  

  

  with 
  the 
  associated 
  forms. 
  They 
  plainly 
  held 
  a 
  lateral 
  position, 
  and 
  

   from 
  the 
  character 
  of 
  the 
  articular 
  border, 
  especially 
  as 
  displayed 
  

   in 
  mature 
  individuals, 
  they 
  apparently 
  alternated 
  with 
  a 
  row 
  of 
  me- 
  

   dian 
  symmetrical 
  teeth, 
  of 
  whose 
  remains, 
  however, 
  not 
  a 
  vestige 
  

   has 
  been 
  detected 
  in 
  the 
  collections 
  accessible 
  to 
  us. 
  In 
  the 
  event 
  

   of 
  the 
  above 
  inference 
  proving 
  well 
  founded, 
  the 
  latter 
  form 
  would, 
  

   with 
  its 
  associates, 
  present 
  a 
  dental 
  formula 
  entirely 
  unique 
  and 
  dis- 
  

   tinct 
  from 
  what 
  obtains 
  in 
  typical 
  Psammodus. 
  There 
  is 
  marked 
  

   diversity 
  amongst 
  the 
  latter, 
  as 
  will 
  be 
  made 
  apparent 
  in 
  the 
  sub- 
  

   joined 
  descriptions 
  of 
  species. 
  

  

  Attention 
  is 
  also 
  directed 
  to 
  certain 
  resemblances 
  existing 
  between 
  

   the 
  Psammodi 
  and 
  Cochliodonts 
  . 
  As 
  is 
  well 
  known, 
  the 
  teeth 
  of 
  

   the 
  latter 
  were 
  disposed 
  on 
  distinct 
  rami 
  of 
  the 
  jaws, 
  instead 
  of 
  

   spanning 
  the 
  jaws 
  from 
  side 
  to 
  side, 
  as 
  is 
  the 
  case 
  with 
  the 
  

   Psammoclonts. 
  But 
  in 
  respect 
  to 
  the 
  outer 
  lateral 
  borders 
  of 
  certain 
  

   forms 
  of 
  teeth 
  representative 
  of 
  the 
  two 
  families, 
  characteristics 
  are 
  

   observed 
  which 
  hardly 
  are 
  to 
  be 
  relegated 
  to 
  the 
  category 
  of 
  mere 
  

   resemblances, 
  and 
  the 
  presence 
  of 
  which 
  suggests 
  interesting 
  homolo- 
  

   gies 
  between 
  the 
  respective 
  forms 
  belonging 
  to 
  either 
  group. 
  In 
  

   this 
  way 
  the 
  projection 
  of 
  tbe 
  basal 
  rim 
  beyond 
  the 
  coronal 
  border 
  

   in 
  the 
  external 
  border 
  and 
  its 
  prolongation 
  into 
  an 
  acute 
  spur 
  

   terminating 
  at 
  the 
  posterior 
  extremity, 
  are 
  unquestionably 
  the 
  same 
  

   in 
  the 
  exterior 
  and 
  posterior 
  terminal 
  forms 
  of 
  either 
  family, 
  — 
  the 
  

   posterior 
  teeth 
  of 
  Cochliodonts 
  homologically 
  corresponding 
  with 
  the 
  

   form 
  constituting 
  the 
  exterior 
  rows 
  of 
  teeth 
  in 
  the 
  Psammodonts, 
  

   even 
  the 
  direction 
  of 
  the 
  obliquity 
  of 
  this 
  side 
  adding 
  stress 
  to 
  the 
  

   likeness 
  in 
  which 
  the 
  forms 
  of 
  both 
  families 
  share. 
  The 
  succeeding 
  

   forms 
  of 
  the 
  Cochliodonts 
  find 
  their 
  representatives 
  in 
  the 
  inner 
  

   rows 
  (where 
  such 
  exist) 
  of 
  the 
  Myliodonts, 
  and 
  in 
  those 
  genera 
  of 
  

   the 
  latter 
  in 
  which 
  the 
  jaws 
  are 
  arched 
  by 
  a 
  single 
  row 
  of 
  dental 
  

   plates- 
  ^The 
  condition 
  is 
  such 
  as 
  would 
  be 
  produced 
  were 
  the 
  rami 
  

   of 
  the 
  Cochliodont 
  jaws 
  drawn 
  out 
  into 
  a 
  transverse 
  position 
  and 
  

   their 
  dental 
  armature 
  consolidated 
  into 
  single 
  plates 
  stretching 
  from 
  

   side 
  to 
  side 
  across 
  the 
  symphysis. 
  Indeed, 
  in 
  these 
  early 
  forms 
  a 
  

   much 
  more 
  intimate 
  relationship 
  exists 
  between 
  the 
  types 
  of 
  these 
  

   two 
  great 
  orders 
  of 
  Selachians 
  than 
  obtains 
  between 
  their 
  represen- 
  

   tatives 
  of 
  later 
  geological 
  times. 
  

  

  The 
  genus 
  is 
  strictly 
  of 
  Carboniferous 
  origin 
  and 
  duration, 
  indeed 
  

   beginning 
  nearly 
  at 
  the 
  dawn 
  of 
  the 
  earlier 
  period 
  and 
  ceasing 
  at 
  

   its 
  close. 
  The 
  Coal 
  Measures 
  thus 
  far 
  have 
  not 
  yielded 
  a 
  trace 
  of 
  

   the 
  remains 
  of 
  Psammodus. 
  

  

  