﻿VERTEBRATES. 
  241 
  

  

  tubercles 
  of 
  the 
  posterior 
  ribs 
  are 
  similarly 
  beautifully 
  ornamented 
  

   and 
  distinguishable 
  by 
  their 
  more 
  oval 
  transverse 
  outline, 
  relatively 
  

   smaller 
  size 
  and 
  acuminate 
  crest 
  which 
  rises 
  abruptly 
  over 
  the 
  

   superior 
  margins. 
  The 
  plain 
  surfaces 
  are 
  striato-punctate 
  longitudi- 
  

   nally. 
  The 
  type 
  specimen 
  attains 
  a 
  length 
  of 
  11.5 
  centimetres, 
  the 
  

   proportions 
  as 
  also 
  the 
  form 
  being 
  well 
  represented 
  in 
  the 
  illustra- 
  

   tions. 
  

  

  The 
  species 
  above 
  noticed 
  is 
  one 
  of 
  the 
  handsomest 
  of 
  the 
  genus. 
  

   The 
  specimen 
  figured 
  is 
  the 
  most 
  perfect, 
  but 
  other 
  fragments 
  have 
  

   been 
  obtained 
  indicating 
  larger-sized 
  spines 
  equally 
  strongly 
  marked 
  

   by 
  the 
  distinctive 
  features 
  of 
  the 
  species, 
  and 
  which 
  readily 
  dis- 
  

   tinguish 
  it 
  from 
  any 
  allied 
  form 
  known 
  to 
  us. 
  In 
  the 
  deflected 
  

   costse 
  along 
  the 
  posterior 
  margins 
  it 
  bears 
  some 
  resemblance 
  to 
  

   Ctenacanthus 
  deflexus 
  of 
  the 
  St. 
  Louis 
  limestone, 
  but 
  is 
  otherwise 
  

   markedly 
  distinct 
  from 
  that 
  species, 
  as 
  will 
  be 
  apparent 
  on 
  compar- 
  

   ing 
  the 
  illustrations 
  given 
  of 
  each 
  respectively. 
  This 
  is 
  one 
  of 
  the 
  

   many 
  interesting 
  discoveries 
  of 
  the 
  remains 
  of 
  fishes 
  in 
  our 
  Coal 
  

   Measure 
  strata 
  made 
  by 
  Mr. 
  Alexander 
  Butters, 
  in 
  whose 
  honor 
  the 
  

   specific 
  name 
  is 
  given. 
  

  

  Geological 
  position 
  and 
  locality: 
  Lower 
  Coal 
  Measures 
  (roof 
  shales 
  

   over 
  coal 
  No. 
  5, 
  Illinois 
  general 
  section) 
  ; 
  Carlinville, 
  Illinois. 
  

  

  Genus 
  ACOISTDYLACANTHUS, 
  St. 
  J. 
  and 
  W. 
  

  

  ACONDYLACANTHUS 
  RECTUS, 
  St. 
  J. 
  and 
  W. 
  

   PL. 
  XXVI. 
  Fig. 
  2. 
  

  

  Dorsal 
  ray 
  of 
  small 
  size, 
  slightly 
  arched 
  along 
  the 
  sharply-rounded 
  

   anterior 
  edge, 
  moderately 
  compressed 
  laterally, 
  and 
  very 
  gradually 
  

   tapering 
  throughout 
  its 
  length. 
  Posterior 
  face 
  deeply 
  grooved 
  and 
  

   traversed 
  by 
  a 
  faint 
  median 
  keel, 
  striato-punctate, 
  and 
  bordered 
  by 
  

   the 
  relatively 
  thick 
  walls 
  of 
  the 
  postero-lateral 
  angles, 
  which 
  latter 
  

   are 
  rounded 
  and 
  bear 
  close 
  along 
  their 
  inner 
  edge 
  a 
  row 
  of 
  compara- 
  

   tively 
  strong, 
  closely 
  approximate, 
  laterally 
  compressed, 
  downward- 
  

   hooked 
  denticles, 
  which 
  extend 
  apparently 
  from 
  the 
  apex 
  (where 
  

   they 
  are 
  worn 
  obsolete) 
  to 
  the 
  dorsal 
  line, 
  below 
  which 
  the 
  pulp 
  

   cavity 
  issues. 
  Lateral 
  surfaces 
  moderately 
  convex 
  transversely, 
  

   rounded 
  into 
  the 
  postero-lateral 
  angle, 
  and 
  occupied 
  by 
  eight 
  or 
  more 
  

   close, 
  rounded 
  costae 
  separated 
  by 
  narrow 
  intercostal 
  furrow, 
  those 
  *fi 
  

  

  —16 
  A 
  / 
  

  

  