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  VERTEBRATES. 
  87 
  

  

  to 
  the 
  posterior 
  angle 
  ; 
  postero-lateral 
  border,/ 
  basal 
  portion 
  chan- 
  

   neled 
  and 
  expanded 
  behind. 
  The 
  principal 
  lobe 
  occupies 
  something 
  

   less 
  than 
  half 
  the 
  transverse 
  diameter 
  of 
  the 
  crown, 
  presenting 
  a 
  

   nearly 
  plane 
  or 
  slightly 
  convex 
  slope 
  to 
  the 
  abrupt 
  anterior 
  edge, 
  

   along 
  which 
  should 
  lie 
  the 
  distinctly 
  defined 
  narrow, 
  plain 
  belt 
  did 
  

   these 
  teeth 
  belong 
  to 
  Deltoptychius, 
  and 
  where, 
  indeed, 
  in 
  some 
  

   specimens 
  a 
  faint 
  depressed 
  line 
  does 
  occur, 
  a 
  character 
  in 
  accord- 
  

   ance 
  with 
  the 
  homologous 
  form 
  of 
  the 
  latter 
  genus; 
  posteriorly 
  the 
  

   slope 
  more 
  abruptly 
  descends 
  into 
  the 
  wide 
  posterior 
  depression, 
  

   along 
  the 
  outer 
  border 
  of 
  which 
  lies 
  the 
  rather 
  suddenly 
  upraised 
  

   posterior 
  wing. 
  The 
  surface 
  presents 
  a 
  similar 
  punctate 
  appearance 
  

   noted 
  in 
  connection 
  with 
  the 
  before 
  mentioned 
  maxillary 
  teeth. 
  

   Breadth 
  across 
  inner 
  margin, 
  5mm; 
  length 
  of 
  antero-lateral 
  bor- 
  

   der 
  to 
  point 
  of 
  inrollment, 
  about 
  5 
  mm. 
  

  

  The 
  maxillary 
  teeth 
  above 
  noticed 
  present 
  an 
  approach 
  to 
  the 
  

   straight 
  postero-lateral 
  border 
  characteristic 
  of 
  Orthopleurodus 
  ; 
  but 
  

   here 
  the 
  resemblance 
  ceases, 
  for 
  the 
  border 
  is 
  gently 
  arched, 
  and 
  

   the 
  position 
  of 
  the 
  posterior 
  lobe, 
  or 
  rather 
  the 
  anterior 
  culmina- 
  

   tion 
  of 
  its 
  crest, 
  determines 
  its 
  relations 
  with 
  the 
  present 
  genus, 
  

   should 
  it 
  prove 
  not 
  to 
  be 
  referable 
  to 
  Deltoptychius. 
  Their 
  exist- 
  

   ence, 
  however, 
  is 
  extremely 
  suggestive 
  of 
  the 
  derivation 
  of 
  the 
  coal- 
  

   measure 
  genus, 
  although 
  there 
  seems 
  to 
  be 
  evidence 
  that 
  the 
  present 
  

   genus 
  suryived 
  the 
  introduction 
  of 
  Orthopleurodus, 
  along 
  with 
  which 
  

   its 
  supposed 
  representatives 
  are 
  found. 
  

  

  Compared 
  with 
  the 
  previously 
  described 
  forms 
  from 
  the 
  St. 
  Louis 
  

   limestone, 
  V. 
  simplex, 
  there 
  exists 
  the 
  most 
  intimate 
  relationship, 
  if 
  

   not 
  actual 
  specific 
  identity, 
  the 
  chief 
  distinction 
  consisting 
  in 
  the 
  

   apparent 
  greater 
  robustness 
  of 
  the 
  present 
  teeth. 
  

  

  With 
  the 
  above 
  observations, 
  until 
  more 
  complete 
  materials 
  shall 
  

   have 
  been 
  acquired, 
  the 
  determination 
  of 
  the 
  generic 
  identity 
  of 
  

   these 
  fragmentary 
  dental 
  remains 
  will 
  necessarily 
  remain 
  for 
  the 
  

   present 
  in 
  a 
  state 
  of 
  uncertainty. 
  So 
  intimately 
  are 
  the 
  species 
  of 
  

   Vaticinodus, 
  Stenopterodas 
  and 
  Deltoptychius 
  linked 
  together, 
  that 
  it 
  

   might 
  be 
  anticipated 
  the 
  more 
  differentiated 
  forms 
  of 
  Orthopleurodus 
  

   may 
  have 
  had 
  ancestors 
  in 
  direct 
  line 
  as 
  early 
  as 
  the 
  time 
  when 
  

   the 
  sediments 
  of 
  the 
  St. 
  Louis 
  and 
  Chester 
  formations 
  were 
  in 
  

   process 
  of 
  deposition. 
  

  

  Geological 
  position 
  and 
  locality 
  : 
  Chester 
  limestone, 
  upper 
  fish-bed 
  

   stratum; 
  Chester 
  and 
  Evansville, 
  Illinois. 
  

  

  