﻿VERTEBRATES. 
  

  

  257 
  

  

  with 
  the 
  latter 
  interpretation 
  of 
  their 
  relative 
  position, 
  or 
  in 
  pairs 
  

   at 
  the 
  lateral 
  line. 
  We 
  are, 
  however, 
  not 
  sufficiently 
  familiar 
  with 
  

   the 
  species 
  of 
  the 
  genus 
  to 
  be 
  able 
  to 
  decide 
  to 
  what 
  extent 
  this 
  

   asymmetrical 
  character 
  may 
  be 
  relied 
  upon 
  or 
  whether 
  it 
  is 
  per- 
  

   sistent 
  alike 
  in 
  all 
  representatives 
  of 
  the 
  genus. 
  Oracanthus 
  has 
  

   already 
  been 
  compared 
  with 
  the 
  dermal 
  scutes 
  of 
  the 
  Rays, 
  and 
  a 
  

   rather 
  heterogeneous 
  variety 
  of 
  forms 
  have 
  been 
  generically 
  identi- 
  

   fied. 
  In 
  some 
  particulars, 
  as 
  the 
  oblique 
  course 
  of 
  the 
  tubercula- 
  

   tum, 
  they 
  recall 
  a 
  resemblance 
  to 
  the 
  Gyracanthi, 
  but 
  no 
  vestige 
  

   exists 
  of 
  the 
  strong 
  implanted 
  base 
  accompanying 
  Gyracanthus, 
  while 
  

   their 
  general 
  proportions 
  and 
  outline 
  are 
  widely 
  different. 
  Neither 
  

   are 
  we 
  prepared 
  to 
  affirm 
  relationship 
  with 
  forms 
  of 
  teeth 
  occurring 
  

   in 
  the 
  same 
  deposits, 
  although 
  their 
  supposed 
  affinities 
  naturally 
  

   suggest 
  possible 
  relations 
  with 
  Psammodus. 
  

  

  Geological 
  position 
  and 
  localities 
  : 
  St. 
  Louis 
  limestone 
  ; 
  St. 
  Louis, 
  

   Mo., 
  Alton, 
  111., 
  Pella, 
  Iowa, 
  &c. 
  

  

  Oracanthus 
  rectus, 
  St. 
  J. 
  and 
  W. 
  

  

  PI. 
  XXV, 
  Fig. 
  3. 
  

  

  Dr. 
  Hamback 
  has 
  obtained 
  from 
  the 
  Chester 
  formation 
  a 
  mere 
  

   fragment 
  of 
  a 
  spine, 
  showing 
  a 
  narrow 
  section 
  of 
  what 
  appears 
  to 
  

   have 
  formed 
  part 
  of 
  the 
  dorsal 
  or 
  anterior 
  edge, 
  which 
  certainly 
  

   possesses 
  congeneric 
  relations 
  with 
  the 
  fossils 
  from 
  the 
  Keokuk 
  lime- 
  

   stone, 
  to 
  which 
  we 
  gave 
  the 
  name 
  Oracanthus 
  ? 
  obliquus 
  (111. 
  Geol. 
  

   Surv., 
  VI, 
  p. 
  477, 
  PI. 
  12, 
  f. 
  16), 
  although 
  they 
  are 
  respectively 
  dis- 
  

   tinguished 
  by 
  well-marked 
  specific 
  characters. 
  

  

  The 
  Chester 
  specimen 
  shows 
  a 
  spine 
  with 
  a 
  very 
  obtusely 
  angular, 
  

   nearly 
  straight 
  anterior 
  ( 
  '?) 
  edge, 
  the 
  lateral 
  surfaces 
  diverging 
  at 
  a 
  

   moderate 
  angle 
  therefrom, 
  and 
  studded 
  with 
  rudely 
  sculptured, 
  

   irregular-shaped 
  conical 
  tubercles, 
  ranged 
  in 
  closely 
  approximated 
  

   vertical 
  lines 
  and 
  more 
  or 
  less 
  regular 
  oblique 
  rows, 
  on 
  the 
  left 
  side 
  

   rising 
  from 
  left 
  to 
  right; 
  along 
  the 
  anterior 
  ridge 
  the 
  tubercles 
  be- 
  

   come 
  transversely 
  elongated, 
  showing 
  the 
  same 
  strong 
  radiating 
  

   plicae 
  and 
  wide 
  vertical 
  spacing; 
  the 
  right 
  side 
  preserves 
  a 
  few 
  

   small, 
  scattered 
  tubercles, 
  belonging 
  to 
  two 
  or 
  three 
  rows, 
  and 
  which 
  

   appear 
  to 
  form 
  oblique 
  rows 
  ascending 
  in 
  the 
  opposite 
  direction 
  to 
  

   that 
  observed 
  in 
  the 
  left 
  side. 
  The 
  tubercles 
  present 
  a 
  more 
  abrupt 
  

   slope 
  above, 
  and 
  at 
  one 
  part 
  of 
  the 
  fragment 
  they 
  appear 
  to 
  be- 
  

   come 
  confluent 
  along 
  one 
  of 
  the 
  lateral 
  costae, 
  forming 
  an 
  irregular, 
  

  

  —17 
  

  

  