﻿VERTEBEATES. 
  69 
  

  

  of 
  coalescing 
  of 
  transversely 
  elongate 
  teeth 
  of 
  the 
  younger 
  with 
  the 
  

   broad 
  long 
  plates 
  of 
  the 
  later 
  stages 
  of 
  growth. 
  The 
  latter 
  feature 
  

   assumes 
  various 
  phases, 
  from 
  a 
  shallow 
  undulation 
  in 
  the 
  border 
  to 
  

   a 
  deep 
  slit 
  extending 
  through 
  the 
  thickness 
  of 
  the 
  crown 
  and 
  base, 
  

   the 
  impinging 
  margins 
  being 
  crenulated 
  as 
  in 
  the 
  inner 
  and 
  outer 
  

   margin 
  of 
  entire 
  individuals. 
  None 
  of 
  the 
  Kinderhook 
  examples, 
  

   however, 
  show 
  this 
  latter 
  feature, 
  yet 
  it 
  may 
  be 
  expected 
  to 
  occur 
  

   in 
  them 
  as 
  well 
  as 
  in 
  the 
  Chester 
  species. 
  A 
  medium- 
  sized 
  tooth 
  

   of 
  the 
  Chester 
  species 
  affords 
  the 
  following 
  measurements 
  : 
  Breadth 
  

   across 
  inner 
  margin, 
  22 
  mm. 
  ; 
  ditto, 
  outer 
  margins, 
  13 
  mm. 
  ; 
  length 
  

   along 
  anterior 
  border, 
  13 
  mm. 
  ; 
  ditto, 
  posterior 
  border, 
  16 
  mm. 
  ; 
  

   thickness 
  of 
  tooth 
  at 
  middle 
  of 
  anterior 
  border, 
  3 
  mm. 
  ; 
  ditto, 
  pos- 
  

   terior 
  border, 
  4 
  mm. 
  

  

  The 
  teeth 
  of 
  the 
  present 
  species 
  differ 
  from 
  European 
  species 
  

   Psephodus 
  magnus, 
  Agass., 
  as 
  also 
  from 
  the 
  Chester 
  P. 
  crenulatus 
  

   (N. 
  and 
  W.) 
  in 
  their 
  more 
  symmetrical 
  outline 
  and 
  distinct 
  defini- 
  

   tion 
  of 
  the 
  angles. 
  The 
  Chester 
  species, 
  perhaps, 
  bears 
  more 
  inti- 
  

   mate 
  comparison, 
  but 
  it 
  is 
  not 
  known 
  to 
  have 
  attained 
  nearly 
  the 
  

   size 
  of 
  the 
  European 
  species. 
  The 
  Kinderhook 
  teeth 
  under 
  consid- 
  

   eration 
  are 
  further 
  distinguished 
  by 
  the 
  comparatively 
  distinct 
  an- 
  

   gulation 
  of 
  the 
  coronal 
  ridge 
  of 
  the 
  mandibular 
  form. 
  

  

  Psephodus 
  placenta, 
  (N. 
  and 
  W., 
  sp.) 
  ' 
  

  

  PI. 
  II, 
  Fig. 
  5-8. 
  

   Helodus 
  placenta, 
  Newbeeey 
  and 
  Woethen, 
  1866, 
  111. 
  Geol. 
  Surv., 
  II, 
  p. 
  80 
  PI. 
  V, 
  Pig. 
  4, 
  4a. 
  

  

  Associated 
  with 
  the 
  forms 
  last 
  described 
  under 
  the 
  name 
  Psephodus 
  

   obliquus, 
  the 
  collections 
  contain 
  a 
  few 
  examples 
  of 
  teeth, 
  which 
  so 
  

   far 
  as 
  relates 
  to 
  superficial 
  markings 
  and 
  general 
  appearances, 
  

   might 
  be 
  presumed 
  to 
  have 
  belonged 
  to 
  the 
  fishes 
  that 
  bore 
  those 
  

   teeth. 
  But 
  in 
  all 
  the 
  collections 
  from 
  Chester 
  localities, 
  where 
  the 
  

   forms 
  of 
  P. 
  crenulatus 
  abound, 
  there 
  is 
  not 
  a 
  single 
  representative 
  

   of 
  the 
  form 
  here 
  alluded 
  to. 
  Therefore, 
  in 
  recognition 
  of 
  its 
  dis- 
  

   tinctive 
  features, 
  it 
  is 
  provisionally 
  referred 
  to 
  a 
  distinct 
  species. 
  

  

  As 
  at 
  present 
  understood, 
  only 
  two 
  forms 
  of 
  teeth 
  have 
  been 
  

   recognized 
  as 
  probably 
  belonging 
  to 
  this 
  species. 
  First, 
  median 
  

   form 
  of 
  the 
  upper 
  jaw 
  (?)/ 
  Teeth 
  irregularly 
  quadrilateral 
  in 
  out- 
  

   line, 
  moderately 
  inrolled 
  spirally, 
  anterior 
  and 
  posterior 
  borders 
  

   differing 
  little 
  in 
  relative 
  length, 
  converging 
  at 
  a 
  slight 
  angle, 
  the 
  

   narrow 
  enamel 
  fold 
  defined 
  by 
  a 
  slight 
  groove 
  from 
  the 
  basal 
  por- 
  

   tion, 
  which 
  shows 
  coarse 
  verrucose 
  markings; 
  inner 
  margin 
  broadly 
  

  

  