﻿VEETEBKATES. 
  219 
  

  

  tooth 
  is 
  laterally 
  expanded, 
  dividing 
  the 
  articular 
  border 
  by 
  a 
  more 
  

   or 
  less 
  conspicuous 
  angulation 
  into 
  two 
  distinct 
  areas 
  somewhat 
  

   variable 
  in 
  extent. 
  The 
  basal 
  portion 
  of 
  the 
  tooth 
  is 
  very 
  deep, 
  the 
  

   outer 
  border 
  considerably 
  expanded 
  beyond 
  the 
  coronal 
  fold, 
  more 
  

   or 
  less 
  produced 
  at 
  the 
  posterior 
  angle, 
  and 
  inbeveled 
  below 
  to 
  the 
  

   relatively 
  reduced 
  inferior 
  surface 
  ; 
  inner 
  face 
  broadly 
  channeled 
  and 
  

   interrupted 
  by 
  the 
  before-mentioned 
  angulation; 
  anterior 
  face 
  also 
  

   excavated 
  vertically 
  and 
  somewhat 
  inbeveled 
  from 
  the 
  plane 
  of 
  the 
  

   coronal 
  surface 
  ; 
  posterior 
  margin 
  presenting 
  a 
  shallow 
  vertical 
  con- 
  

   cavity, 
  — 
  the 
  edges 
  all 
  round 
  making 
  an 
  angle, 
  except 
  along 
  the 
  outer 
  

   side, 
  with 
  the 
  inferior 
  surface, 
  which 
  latter 
  shows 
  a 
  faint 
  median 
  

   ridge. 
  The 
  coronal 
  surface 
  presents 
  a 
  minute 
  punctate 
  structure, 
  

   the 
  pores 
  somewhat 
  widely 
  spaced, 
  sometimes 
  confluent, 
  and 
  enclosed 
  

   in 
  the 
  usual 
  vertical 
  prisms. 
  Toward 
  the 
  posterior 
  margin 
  there 
  

   appear 
  indications 
  of 
  transverse 
  rugae. 
  The 
  basal 
  walls 
  are 
  verti- 
  

   cally 
  and 
  irregularly 
  plicated, 
  and 
  perforated 
  by 
  the 
  irregular 
  pores 
  

   £. 
  and 
  occasional 
  aly/olar 
  cavities 
  ; 
  the 
  abraded 
  inferior 
  surface 
  shows 
  

   delicate 
  vermiculose 
  markings, 
  but 
  in 
  the 
  perfect 
  state 
  it 
  presents 
  

   the 
  usual 
  dense, 
  smooth 
  layer, 
  which 
  also 
  envelopes 
  the 
  walls 
  of 
  

   the 
  base. 
  A 
  large-size 
  perfect 
  tooth 
  measures 
  across 
  the 
  posterior 
  

   margin 
  32 
  mm., 
  and 
  about 
  25 
  mm. 
  across 
  the 
  anterior 
  extremity 
  ; 
  

   length 
  along 
  inner 
  border 
  45 
  mm. 
  ; 
  depth 
  of 
  tooth 
  at 
  the 
  posterior 
  

   margin 
  21 
  mm., 
  and 
  20 
  mm. 
  at 
  the 
  opposite 
  extremity. 
  

  

  The 
  collections 
  afford 
  four 
  or 
  five 
  examples 
  of 
  a 
  form 
  of 
  teeth, 
  

   which, 
  in 
  their 
  coronal 
  contour 
  and 
  general 
  conformation, 
  also 
  in 
  

   the 
  details 
  of 
  their 
  superficial 
  structure, 
  show 
  unmistakable 
  relation- 
  

   ship 
  with 
  Psammoclus, 
  although 
  they 
  present 
  a 
  type 
  differing 
  in 
  cer- 
  

   tain 
  well-defined 
  characteristics 
  from 
  the 
  previously 
  herein 
  described 
  

   species 
  of 
  the 
  genus. 
  The 
  first 
  authentic 
  example 
  of 
  the 
  form 
  here 
  

   alluded 
  to 
  is 
  that 
  described 
  by 
  Professor 
  McCoy 
  1 
  under 
  the 
  name 
  P. 
  

   canaliculatus, 
  from 
  the 
  mountain 
  limestone 
  of 
  Ireland. 
  The 
  examples 
  

   before 
  us 
  all 
  belong 
  to 
  the 
  same 
  side, 
  while 
  that 
  figured 
  by 
  Professor 
  

   McCoy 
  represents 
  a 
  tooth 
  from 
  the 
  opposite 
  side. 
  As 
  in 
  the 
  pre- 
  

   viously 
  described 
  forms, 
  the 
  character 
  of 
  the 
  articular 
  walls 
  clearly 
  

   indicate 
  the 
  paired 
  arrangement 
  of 
  the 
  teeth, 
  — 
  indeed 
  in 
  the 
  instance 
  

   of 
  the 
  Upper 
  Burlington 
  species, 
  P. 
  Springeri, 
  the 
  examples 
  from 
  

   opposite 
  rows 
  are 
  about 
  equally 
  represented 
  in 
  the 
  collections, 
  and 
  

   the 
  same 
  may 
  be 
  said 
  of 
  the 
  European 
  species, 
  P. 
  porosus 
  and 
  P. 
  

   rugosus 
  (sp. 
  ?) 
  of 
  Agassiz. 
  While 
  this 
  statement 
  is 
  true 
  so 
  far 
  as 
  it 
  

   goes, 
  and 
  the 
  form 
  in 
  question 
  doubtless 
  occupied 
  a 
  lateral 
  position 
  

  

  1 
  British 
  Paleozoic 
  Fossils, 
  1855, 
  p. 
  613, 
  PI. 
  3 
  a, 
  f. 
  12. 
  

  

  