﻿VEETEBBATES. 
  

  

  245 
  

  

  even 
  depends 
  the 
  determination 
  of 
  the 
  generic 
  relation 
  of 
  the 
  spine. 
  

   Specifically, 
  however, 
  the 
  specimen 
  is 
  clearly 
  distinct 
  from 
  any 
  form 
  

   heretofore 
  obtained 
  from 
  the 
  same 
  formation, 
  and, 
  judging 
  from 
  

   such 
  of 
  the 
  superficial 
  characters 
  as 
  are 
  actually 
  discernible, 
  the 
  

   spine 
  might 
  be 
  provisionally 
  identified 
  with 
  Acondylacanthus. 
  

  

  The 
  spine 
  gradually 
  tapers, 
  and 
  is 
  moderately 
  arched 
  along 
  the 
  

   anterior 
  edge, 
  the 
  inclination 
  making 
  an 
  angle 
  of 
  about 
  45° 
  with 
  

   line 
  of 
  insertion, 
  lateral 
  surfaces 
  gently 
  convex 
  transversely, 
  and 
  

   very 
  gradually 
  converging 
  toward 
  the 
  obtusely-rounded 
  anterior 
  edge, 
  

   slightly 
  compressed 
  to 
  the 
  postero-lateral 
  angles, 
  the 
  posterior 
  wall 
  

   broken 
  away, 
  exposing 
  the 
  oval-shaped 
  pulp 
  cavity 
  lying 
  within 
  the 
  

   posterior 
  half 
  of 
  the 
  spine. 
  The 
  exposed 
  lateral 
  surfaces 
  of 
  the 
  

   spine 
  are 
  quite 
  uniformly 
  fluted 
  longitudinally, 
  the 
  costae 
  present- 
  

   ing 
  comparatively 
  narrow 
  rounded 
  ridges, 
  rarely 
  bifurcating 
  or 
  

   implanted, 
  and 
  spaced 
  by 
  about 
  their 
  own 
  width 
  by 
  the 
  shallow 
  

   intercostal 
  grooves, 
  the 
  anterior 
  edge 
  apparently 
  occupied 
  by 
  a 
  

   wider 
  ridge, 
  vestiges 
  of 
  the 
  usual 
  enamel 
  layer 
  are 
  discernible 
  but 
  

   not 
  accompanied 
  by 
  visible 
  nodose 
  ornamentation. 
  The 
  base 
  was 
  

   deeply 
  embedded 
  in 
  the 
  integument, 
  and 
  cbanneled 
  behind 
  by 
  the 
  

   continuation 
  of 
  the 
  pulp 
  cavity. 
  The 
  spine 
  preserves 
  a 
  length 
  of 
  

   14 
  centimetres 
  ; 
  the 
  antero-posterior 
  diameter, 
  just 
  above 
  the 
  dorsal 
  

   line, 
  2.5 
  centim., 
  which, 
  however, 
  does 
  not 
  represent 
  the 
  actual 
  

   diameter 
  on 
  account 
  of 
  the 
  imperfect 
  condition 
  of 
  the 
  posterior 
  

   face 
  ; 
  lateral 
  diameter 
  at 
  same 
  point, 
  9 
  millimetres. 
  

  

  The 
  typical 
  forms 
  of 
  Acondylacanthus 
  are 
  more 
  slender, 
  and 
  pro- 
  

   portionately 
  narrower, 
  than 
  the 
  above 
  described 
  form, 
  and 
  should 
  

   the 
  latter 
  prove 
  to 
  possess 
  nodose 
  costae, 
  it 
  should 
  be 
  transferred 
  

   to 
  the 
  genus 
  Ctenacanthus. 
  But 
  as 
  has 
  been 
  already 
  remarked, 
  in 
  

   the 
  absence 
  of 
  any 
  such 
  ornamentation 
  and 
  the 
  apparent 
  smooth 
  

   plain 
  costae, 
  its 
  affinities 
  are 
  clearly 
  with 
  the 
  above 
  genus. 
  

  

  Geological 
  position 
  and 
  locality: 
  Keokuk 
  limestone 
  ; 
  government 
  

   canal 
  excavations 
  at 
  Keokuk 
  rapids, 
  near 
  Keokuk, 
  Iowa. 
  

  

  