﻿VEBTEBBATES. 
  233 
  

  

  ICHTHYODORULITES. 
  

  

  Genus 
  CTESTACANTHUS, 
  Agassiz. 
  

   Ctenacanthus 
  Coxianus, 
  St. 
  J. 
  and 
  W. 
  

  

  PL 
  XXI, 
  Fig. 
  1. 
  

  

  Dorsal 
  spines 
  attain 
  large 
  size, 
  somewhat 
  rapidly 
  tapering, 
  later- 
  

   ally 
  compressed, 
  presenting 
  a 
  wedge-shaped 
  transverse 
  section, 
  mod- 
  

   erately 
  arched, 
  and 
  apparently 
  but 
  moderately 
  inclined 
  backward 
  

   judging 
  from 
  the 
  slight 
  obliquity 
  of 
  the 
  line 
  of 
  insertion 
  as 
  shown 
  

   in 
  the 
  example 
  before 
  us. 
  Anterior 
  edge 
  sharply 
  rounded 
  and 
  occu- 
  

   pied 
  by 
  a 
  single 
  carina 
  interrupted 
  by 
  strong 
  transverse 
  ridges; 
  

   lateral 
  surfaces 
  slightly 
  convex, 
  occupied 
  by 
  prominent 
  costae, 
  flat- 
  

   tened 
  along 
  their 
  crests, 
  and 
  separated 
  by 
  deep, 
  narrow 
  sulci, 
  in- 
  

   creasing 
  by 
  occasional 
  bifurcation, 
  and 
  ornamented 
  by 
  irregular 
  

   transverse 
  ridges, 
  which 
  present 
  an 
  abrupt 
  declivity 
  above 
  and 
  a 
  

   more 
  gentle 
  convexity 
  below, 
  apparently 
  smooth. 
  Posterior 
  face 
  

   unknown. 
  Pulp 
  cavity 
  apparently 
  occupying 
  less 
  than 
  half 
  the 
  

   antero-posterior 
  diameter, 
  and 
  situated 
  entirely 
  within 
  the 
  posterior 
  

   half 
  of 
  the 
  spine. 
  The 
  specimen 
  described 
  attained 
  a 
  length 
  of 
  at 
  

   least 
  150 
  millimetres 
  ; 
  antero-posterior 
  diameter 
  at 
  line 
  of 
  insertion 
  

   probably 
  45 
  mm. 
  ; 
  lateral 
  diameter 
  at 
  same 
  point 
  14 
  mm. 
  

  

  The 
  above 
  described 
  species 
  is 
  recognized 
  from 
  a 
  fragment 
  of 
  a 
  

   large 
  spine, 
  which 
  shows 
  a 
  length 
  of 
  90 
  millimetres, 
  including 
  part 
  

   of 
  the 
  base 
  showing 
  the 
  line 
  of 
  insertion 
  from 
  which 
  the 
  compara- 
  

   tively 
  erect 
  position 
  in 
  the 
  fish's 
  back 
  is 
  inferred. 
  Its 
  relations 
  are 
  

   most 
  intimate 
  with 
  Ctenacanthus 
  Mayi, 
  N. 
  and 
  W., 
  of 
  the 
  Upper 
  

   Burlington 
  limestone, 
  and 
  the 
  fish 
  to 
  which 
  it 
  belonged 
  was 
  a 
  repre- 
  

   sentative 
  species 
  of 
  that 
  to 
  which 
  C. 
  Mayi 
  pertained. 
  These 
  two 
  

   spines 
  are 
  almost 
  exactly 
  of 
  the 
  same 
  proportions 
  and 
  form. 
  The 
  

   Keokuk 
  specimen 
  is, 
  however, 
  somewhat 
  more 
  robust, 
  and 
  is 
  further 
  

   distinguishable 
  by 
  the 
  greater 
  width 
  and 
  depth 
  of 
  the 
  intercostal 
  

   sulci 
  and 
  relatively 
  narrower 
  costae; 
  the 
  transverse 
  carinas 
  or 
  nodes 
  

   along 
  the 
  anterior 
  ridge 
  are 
  also 
  narrower 
  and 
  less 
  tumid, 
  the 
  cost® 
  

  

  