﻿VERTEBEATES. 
  251 
  

  

  Genus 
  GYRACAOTHUS, 
  Agassiz. 
  

   Gyracanthus? 
  cordatus, 
  St. 
  J. 
  and 
  W. 
  

  

  PI. 
  XXVI, 
  Fie. 
  4. 
  

  

  Spine 
  of 
  small 
  size, 
  very 
  slightly 
  arched 
  along 
  the 
  anterior 
  angle 
  

   and 
  gradually 
  tapering, 
  transversely 
  triangular 
  or 
  cordate 
  in 
  section, 
  

   Lateral 
  surfaces 
  broadly 
  convex 
  transversely, 
  meeting 
  in 
  a 
  narrow 
  

   ridge 
  forming 
  an 
  acute 
  angle 
  at 
  the 
  anterior 
  edge 
  ; 
  postero-lateral 
  

   angles 
  rounded 
  and 
  merging 
  into 
  the 
  broad, 
  moderately 
  excavated 
  

   posterior 
  face. 
  Exposed 
  portion 
  of 
  the 
  lateral 
  surfaces 
  ornamented 
  

   with 
  stellate 
  tubercles 
  closely 
  set 
  longitudinally, 
  but 
  showing 
  a 
  

   more 
  or 
  less 
  distinct 
  arrangement 
  in 
  oblique 
  rows, 
  with 
  a 
  forward 
  

   and 
  upward 
  course. 
  Pulp 
  cavity 
  relatively 
  large, 
  nearly 
  circular 
  in 
  

   section, 
  with 
  a 
  thin 
  wall 
  in 
  the 
  axis 
  of 
  the 
  posterior 
  face 
  and 
  thick- 
  

   ened 
  at 
  the 
  lateral 
  angles, 
  opening 
  below 
  in 
  a 
  deep 
  furrow 
  extending 
  

   to 
  the 
  proximal 
  extremity. 
  

  

  The 
  description 
  is 
  based 
  upon 
  a 
  single 
  specimen, 
  which 
  shows 
  

   about 
  5.5 
  centimetres 
  of 
  the 
  length 
  of 
  a 
  small 
  spine. 
  Both 
  extrem- 
  

   ities 
  have 
  been 
  broken 
  away, 
  and 
  the 
  specimen 
  is 
  otherwise 
  disfig- 
  

   ured 
  by 
  abrasion, 
  so 
  much 
  so, 
  indeed, 
  as 
  to 
  render 
  the 
  surface 
  

   features 
  indistinct 
  over 
  the 
  greater 
  part 
  of 
  the 
  lateral 
  surfaces, 
  

   which 
  appears 
  to 
  be 
  closely 
  covered 
  with 
  minute 
  tubercles 
  ; 
  how- 
  

   ever, 
  these 
  latter 
  on 
  one 
  Bide 
  reveal 
  the 
  oblique 
  disposition 
  charac- 
  

   teristic, 
  or 
  at 
  least 
  prevalent, 
  in 
  Gyracanthus, 
  and 
  which, 
  together 
  

   with 
  the 
  contour 
  of 
  the 
  posterior 
  face, 
  seem 
  to 
  warrant 
  its 
  reference 
  

   to 
  that 
  genus. 
  In 
  the 
  more 
  abraded 
  surface 
  areas, 
  where 
  

   the 
  tubercules 
  are 
  worn 
  away 
  and 
  preserving 
  only 
  their 
  bases, 
  the 
  

   obliquity 
  is 
  even 
  more 
  readily 
  traced 
  in 
  exactly 
  opposite 
  direction 
  

   to 
  that 
  characteristic 
  of 
  Gyracanthus 
  ; 
  again, 
  they 
  have 
  the 
  appear- 
  

   ance 
  of 
  irregular 
  disposition, 
  which, 
  in 
  connection 
  with 
  the 
  rather 
  

   marked 
  keeled 
  condition 
  of 
  the 
  anterior 
  edge, 
  suggested 
  resemblance 
  

   to 
  Geisacanthus. 
  But 
  the 
  posterior 
  face 
  is 
  quite 
  unlike 
  that 
  of 
  the 
  

   latter 
  genus, 
  being 
  simply 
  excavated 
  or 
  channeled 
  longitudinally 
  and 
  

   without 
  the 
  median 
  angle 
  or 
  ridge 
  characteristic 
  of 
  both 
  Gyracan- 
  

   thus 
  and 
  Geisacanthus. 
  In 
  the 
  distal 
  half 
  of 
  the 
  best 
  preserved 
  

   side, 
  near 
  the 
  postero-lateral 
  angle, 
  the 
  tubercles 
  are 
  laterally 
  com- 
  

   pressed 
  and 
  closely 
  impacted 
  at 
  their 
  sides, 
  while 
  considerably 
  

   spaced^longitudinally, 
  with 
  a 
  nearly 
  transverse 
  disposition 
  ; 
  but 
  p.lse- 
  

  

  