﻿VEBTEBBATES. 
  107 
  

  

  Posterior 
  tooth 
  of 
  the 
  lower 
  jaw 
  known 
  only 
  from 
  very 
  imperfectly 
  

   preserved 
  examples, 
  which, 
  however, 
  shows 
  a 
  rather 
  prominent 
  and 
  

   regularly 
  arched 
  median 
  lobe, 
  situate 
  nearly 
  centrally, 
  with 
  appar- 
  

   ently 
  well-developed 
  posterior 
  wing, 
  nearly 
  equalling 
  in 
  breadth 
  the 
  

   plane 
  anterior 
  area. 
  

  

  The 
  above 
  described 
  species 
  is 
  represented 
  in 
  the 
  collections 
  by 
  

   only 
  a 
  few 
  examples, 
  and 
  in 
  only 
  two 
  exceptions 
  are 
  the 
  complete 
  

   outline 
  and 
  coronal 
  contour 
  shown. 
  From 
  these 
  we 
  are 
  led 
  to 
  

   recognize 
  distinctive 
  peculiarities 
  that 
  readily 
  distinguish 
  the 
  teeth 
  

   from 
  congeneric 
  forms 
  previously 
  noticed, 
  with 
  the 
  exception 
  in 
  

   favor 
  of 
  the 
  possible 
  identity 
  of 
  the 
  Keokuk 
  forms 
  with 
  the 
  present 
  

   species. 
  

  

  Geological 
  position 
  and 
  locality: 
  Warsaw 
  limestone 
  ; 
  Golden 
  Bluffs, 
  

   near 
  Warsaw, 
  Illinois. 
  

  

  Stenopterodus 
  parvultjs, 
  (N. 
  and 
  W. 
  sp.) 
  

  

  PL 
  IV, 
  Fig. 
  4-8. 
  

  

  Sandalodus 
  paroulus 
  (in 
  part), 
  Newberry 
  and 
  Worthen, 
  1866, 
  111. 
  Geol. 
  Surv., 
  II, 
  p. 
  102, 
  

  

  PI. 
  X.f.l. 
  

  

  The 
  recognition 
  of 
  the 
  above 
  cited 
  species 
  was 
  based 
  upon 
  a 
  

   unique, 
  nearly 
  perfect 
  tooth, 
  representing 
  the 
  posterior 
  form 
  per- 
  

   taining 
  to 
  the 
  upper 
  jaw. 
  The 
  specimen 
  is 
  mutilated 
  along 
  the 
  

   outer 
  margin 
  in 
  front, 
  the 
  inrolled 
  extremity 
  being 
  broken 
  away. 
  

   The 
  figure, 
  however, 
  conveys 
  a 
  very 
  inadequate 
  impression 
  of 
  the 
  

   coronal 
  contour, 
  the 
  engraving 
  exaggerating 
  the 
  accidental 
  features 
  

   traceable 
  to 
  attrition 
  at 
  the 
  expense 
  of 
  the 
  natural 
  contour 
  surfaces. 
  

   The 
  specimen 
  exhibits 
  the 
  posterior 
  lobe 
  of 
  the 
  crown, 
  gradually 
  

   culminating 
  anteriorly, 
  where 
  it 
  is 
  suddenly 
  broken 
  down 
  and 
  defined 
  

   by 
  a 
  slight 
  depression 
  from 
  the 
  plane 
  anterior 
  area 
  with 
  which 
  it 
  

   merges 
  in 
  the 
  middle 
  outer 
  region 
  of 
  the 
  coronal 
  surface, 
  where 
  the 
  

   tooth 
  has 
  been 
  subjected 
  to 
  excessive 
  attrition 
  while 
  in 
  use. 
  In 
  the 
  

   outward 
  and 
  forward 
  obliquity 
  of 
  the 
  antero-lateral 
  border, 
  also 
  in 
  the 
  

   distinct 
  definition 
  of 
  the 
  shallow 
  enamel 
  fold 
  along 
  the 
  still 
  more 
  

   oblique 
  and 
  gently 
  arched 
  postero-lateral 
  border, 
  where 
  the 
  basal 
  

   portion 
  forms 
  a 
  prominent 
  rim, 
  reaching 
  to 
  the 
  point 
  of 
  inrollment, 
  

   the 
  original 
  figure 
  above 
  cited 
  conveys 
  a 
  more 
  accurate 
  idea 
  of 
  the 
  

   form. 
  

  

  The 
  acquisitions 
  of 
  Mr. 
  Van 
  Home 
  include 
  a 
  beautiful 
  series 
  of 
  

   the 
  above 
  mentioned 
  teeth, 
  with 
  which 
  are 
  associated, 
  equally 
  nu- 
  

   merous, 
  another 
  form, 
  which 
  we 
  have 
  come 
  to 
  regard 
  with 
  a 
  strong 
  

  

  