﻿VEKTEBBATES. 
  259 
  

  

  Genus 
  PNIGE 
  ACANTHUS, 
  St. 
  J. 
  and 
  W. 
  

  

  Pnigeacanthus 
  trigonalis, 
  St. 
  J. 
  and 
  W. 
  

  

  PI. 
  XXIV, 
  Fig. 
  4. 
  

  

  Dormal 
  defense 
  as 
  seen 
  from 
  the 
  side 
  trigonal 
  in 
  outline, 
  laterally 
  

   much 
  compressed, 
  walls 
  thin, 
  terminating 
  in 
  an 
  obtuse 
  apex 
  pro- 
  

   jecting 
  backward 
  about 
  10° 
  past 
  a 
  point 
  vertical 
  to 
  the 
  postero- 
  

   inferior 
  angle, 
  exceedingly 
  attenuated 
  inferiorly 
  with 
  signs 
  of 
  a 
  plain 
  

   marginal 
  border 
  that 
  was 
  buried 
  in 
  the 
  integument. 
  Anterior 
  edge 
  

   gently 
  arched, 
  slightly 
  curved 
  toward 
  the 
  inferior 
  angle, 
  rounded 
  

   into 
  the 
  gently 
  convex 
  lateral 
  surfaces 
  ; 
  posterior 
  border 
  apparently 
  

   a 
  little 
  more 
  compressed, 
  gently 
  concave 
  vertically, 
  and 
  studded 
  

   with 
  relatively 
  strong, 
  irregular, 
  rudely 
  sculptured 
  tubercles. 
  The 
  

   anterior 
  border 
  is 
  armed 
  "with 
  large 
  irregular 
  vertically 
  elongate 
  

   depressed 
  tubercles, 
  which 
  seem 
  to 
  lie 
  immediately 
  on 
  the 
  left 
  flank 
  

   of 
  the 
  border, 
  forming 
  a 
  nearly 
  continuous 
  undulating 
  ridge. 
  The 
  

   tubercles, 
  so 
  far 
  as 
  may 
  be 
  determined 
  by 
  the 
  specimen, 
  are 
  mark- 
  

   edly 
  asymmetrical, 
  the 
  slope 
  facing 
  the 
  anterior 
  edge 
  being 
  abrupt 
  

   and 
  obscurely 
  vertically 
  sculptured, 
  the 
  opposite 
  side 
  presenting 
  a 
  

   wider 
  gentle 
  declivity. 
  The 
  lateral 
  surfaces 
  show 
  more 
  or 
  less 
  dis- 
  

   tinct 
  longitudinal 
  costas, 
  irregularly 
  striato-punctate, 
  and 
  studded 
  

   with 
  irregular 
  stellate 
  tubercles 
  whose 
  apices 
  are 
  directed 
  upward 
  ; 
  

   the 
  tubercles, 
  as 
  seen 
  in 
  the 
  left 
  side, 
  vary 
  in 
  size 
  and 
  present 
  a 
  

   rude 
  sort 
  of 
  transverse 
  disposition, 
  especially 
  toward 
  the 
  anterior 
  

   border 
  where 
  occasional 
  large 
  tubercles 
  occur, 
  but 
  irregularly 
  dis- 
  

   persed 
  as 
  relates 
  to 
  the 
  longitudinal 
  costae. 
  The 
  inner 
  surface 
  of 
  

   the 
  lateral 
  walls 
  is 
  smooth 
  with 
  a 
  fine 
  wavy 
  striato-punctate 
  struc- 
  

   ture. 
  The 
  spine 
  may 
  not 
  show 
  the 
  perfect 
  outline 
  of 
  the 
  inferior 
  

   margin, 
  but 
  the 
  preserved 
  portion 
  in 
  the 
  flattened 
  condition 
  of 
  the 
  

   specimen 
  measures 
  about 
  25 
  millimetres 
  from 
  the 
  antero-to 
  the 
  

   postero-inferior 
  angle 
  ; 
  height 
  of 
  posterior 
  border, 
  20 
  mm. 
  ; 
  ditto 
  of 
  

   anterior 
  border 
  at 
  least 
  33 
  mm. 
  

  

  The 
  collection 
  of 
  Mr. 
  Alexander 
  Butters 
  contains 
  the 
  unique 
  and 
  

   nearly 
  entire 
  example 
  upon 
  which 
  the 
  above 
  description 
  is 
  based. 
  

   The 
  specimen 
  is 
  embedded 
  in 
  a 
  limestone 
  matrix 
  in 
  such 
  manner 
  

   as 
  to 
  reveal 
  only 
  the 
  one 
  side, 
  and 
  part 
  of 
  this 
  is 
  broken 
  away 
  

   exposing 
  a 
  corresponding 
  area 
  of 
  the 
  inner 
  side 
  of 
  the 
  opposite 
  wall, 
  

   the 
  thin 
  inferior 
  portions 
  of 
  the 
  walls 
  being 
  brought 
  into 
  close 
  con- 
  

   tact 
  by 
  pressure. 
  The 
  specimen 
  is 
  unquestionably 
  closely 
  allied 
  to 
  

  

  