﻿INTKODUCTOBY. 
  59 
  

  

  In 
  the 
  majority 
  of 
  eases 
  the 
  remains 
  of 
  fishes 
  are 
  found 
  in 
  definite, 
  

   readily 
  identifiable 
  horizons, 
  indeed 
  chiefly 
  in 
  a 
  limited 
  stratum, 
  

   where 
  they 
  are 
  crowded 
  together 
  forming 
  veritable 
  bone-beds, 
  though 
  

   by 
  no 
  means 
  thus 
  restricted, 
  as 
  their 
  remains 
  are 
  found 
  more 
  or 
  

   less 
  scattered 
  in 
  many 
  formations. 
  Yet 
  the 
  latter 
  occurrences 
  are, 
  

   as 
  might 
  be 
  expected, 
  comparatively 
  rare. 
  

  

  The 
  earliest 
  genus, 
  Psephodus, 
  whose 
  relations 
  with 
  the 
  more 
  

   typical 
  representatives 
  of 
  the 
  family 
  may 
  be 
  a 
  matter 
  of 
  reasonable 
  

   doubt, 
  had 
  its 
  beginning 
  in 
  the 
  oldest 
  or 
  Kinderhook 
  formation, 
  

   only 
  ceasiug 
  in 
  the 
  latest 
  or 
  Chester 
  epoch 
  of 
  the 
  Lower 
  Carbonifer- 
  

   ous 
  period. 
  Near 
  the 
  close 
  of 
  the 
  Kinderhook 
  epoch 
  Vaticinodus 
  

   was 
  introduced, 
  authentic 
  representatives 
  of 
  which 
  continued 
  to 
  exist 
  

   during 
  the 
  Upper 
  Burlington 
  epoch. 
  Tceniodus, 
  whose 
  relations 
  

   seem 
  to 
  be 
  nearest 
  Psephodus 
  of 
  all 
  the 
  true 
  Cochliodonts, 
  is 
  first 
  

   met 
  with 
  in 
  the 
  Keokuk 
  formation, 
  continuing 
  thence 
  into 
  the 
  

   Chester, 
  where 
  it 
  is 
  last 
  seen. 
  During 
  the 
  Upper 
  Burlington 
  epoch 
  

   Deltoptychius 
  was 
  introduced, 
  which 
  appears 
  to 
  be 
  a 
  modification 
  of 
  

   Vaticinodus, 
  as 
  is 
  also 
  Stenopterodus, 
  which 
  originated 
  at 
  the 
  same 
  

   time, 
  and 
  whose 
  remains 
  occur 
  in 
  each 
  succeeding 
  formation 
  as 
  late 
  

   as 
  the 
  St. 
  Louis, 
  while 
  Deltoptychius 
  continued 
  to 
  exist 
  during 
  the 
  

   deposition 
  of 
  the 
  Chester 
  limestone. 
  In 
  point 
  of 
  time, 
  also, 
  Sanda- 
  

   lodus 
  began 
  contemporaneously 
  with 
  the 
  latter 
  genera, 
  with 
  which 
  its 
  

   affinities 
  appear 
  to 
  be 
  most 
  intimate, 
  although 
  it 
  presents 
  a 
  more 
  

   marked 
  modification 
  of 
  generic 
  characteristics 
  than 
  those 
  distin- 
  

   guishing 
  the 
  three 
  last 
  named 
  genera, 
  one 
  from 
  the 
  other. 
  In 
  

   Orthopleurodus 
  these 
  modifications 
  are 
  carried 
  to 
  an 
  extreme, 
  as 
  

   especially 
  notable 
  in 
  the 
  maxillary 
  terminal 
  form, 
  though 
  its 
  man- 
  

   dibular 
  terminal 
  teeth 
  are 
  very 
  similar 
  to 
  the 
  homologous 
  form 
  of 
  

   Deltoptychius, 
  while 
  the 
  median 
  form 
  of 
  the 
  same 
  jaw 
  is 
  intimately 
  

   allied 
  to 
  that 
  of 
  Sandalodus. 
  Pertaining 
  to 
  the 
  same 
  category 
  are 
  

   the 
  forms 
  of 
  Xystrodus, 
  and, 
  perhaps, 
  Tomodus, 
  which 
  apparently 
  

   was 
  the 
  contemporary 
  of 
  the 
  above 
  mentioned 
  genera, 
  its 
  remains 
  

   being 
  first 
  encountered 
  in 
  the 
  Upper 
  Burlington, 
  and 
  only 
  ceasing 
  

   in 
  the 
  lower 
  Coal 
  Measures. 
  It 
  presents 
  the 
  simplest 
  form 
  of 
  the 
  

   group 
  to 
  which 
  it 
  especially 
  pertains, 
  and 
  throughout 
  its 
  extensive 
  

   vertical 
  range 
  or 
  distribution 
  in 
  time, 
  it 
  maintains 
  its 
  distinctive 
  

   peculiarities 
  with 
  remarkable 
  persistency. 
  

  

  