﻿VEBTEBRATES. 
  221 
  

  

  sentatives 
  of 
  the 
  present 
  form 
  occurring 
  in 
  the 
  St. 
  Louis 
  limestone, 
  

   vary 
  considerably 
  one 
  from 
  the 
  other. 
  The 
  large, 
  normally 
  entire 
  

   tooth 
  in 
  the 
  collection 
  of 
  Mr. 
  Van 
  Home 
  is 
  supplemented 
  from 
  the 
  

   same 
  locality 
  by 
  a 
  proportionately 
  shorter 
  tooth, 
  which 
  has 
  the 
  

   angulation 
  of 
  the 
  inner 
  articular 
  border 
  situated 
  nearer 
  the 
  middle 
  

   of 
  the 
  tooth 
  instead 
  of 
  one-fourth 
  the 
  distance 
  from 
  the 
  anterior 
  

   extremity 
  as 
  in 
  the 
  specimen 
  just 
  mentioned, 
  with 
  which 
  it 
  exactly 
  

   agrees 
  in 
  other 
  particulars. 
  A 
  third 
  example 
  from 
  the 
  same 
  locality, 
  

   at 
  Alton, 
  offers 
  still 
  other 
  differences, 
  as 
  in 
  the 
  obliquity 
  of 
  the 
  

   anterior 
  margin, 
  the 
  angulation 
  of 
  the 
  inner 
  border 
  being 
  crowded 
  

   forward 
  and 
  merged 
  with 
  the 
  inner 
  lateral 
  angle, 
  the 
  stronger 
  develope- 
  

   ment 
  of 
  the 
  outer 
  basal 
  rim, 
  and 
  the 
  prominent 
  transverse 
  rugae 
  

   that 
  envelope 
  the 
  posterior 
  two-thirds 
  of 
  the 
  coronal 
  surface, 
  the 
  

   anterior 
  slope 
  and 
  the 
  ridge 
  parallel 
  with 
  the 
  inner 
  border 
  being 
  

   worn 
  smooth, 
  revealing 
  the 
  ordinary 
  punctate 
  structure 
  noted 
  in 
  the 
  

   above 
  mentioned 
  specimens. 
  A 
  smaller 
  tooth 
  derived 
  from 
  the 
  same 
  

   formation, 
  at 
  Pella, 
  differs 
  from 
  the 
  above 
  in 
  the 
  very 
  slight 
  

   developement 
  of 
  the 
  angulation 
  in 
  the 
  articular 
  border, 
  which 
  is 
  

   situated 
  about 
  one-third 
  the 
  distance 
  from 
  the 
  anterior 
  angle 
  ; 
  the 
  

   coronal 
  fold 
  also 
  shows 
  a 
  sharp 
  angulation 
  along 
  the 
  vertical 
  inner 
  

   border, 
  but 
  the 
  opposite 
  border 
  is 
  worn 
  away, 
  destroying 
  every 
  

   vestige 
  of 
  the 
  prominent 
  basal 
  rim 
  usually 
  occurring 
  along 
  that 
  side 
  

   of 
  the 
  tooth, 
  though 
  the 
  basal 
  region 
  is 
  otherwise 
  very 
  like 
  that 
  

   described 
  from 
  normally 
  preserved 
  specimens. 
  The 
  coronal 
  surface, 
  

   however, 
  is 
  worn 
  quite 
  smooth, 
  showing 
  the 
  ordinary 
  punctse. 
  We 
  

   are 
  inclined 
  to 
  regard 
  all 
  of 
  the 
  above 
  mentioned 
  specimens 
  as 
  

   probably 
  pertaining 
  to 
  one 
  and 
  the 
  same 
  species, 
  the 
  variable 
  con- 
  

   dition 
  of 
  the 
  coronal 
  surface 
  being 
  due 
  to 
  the 
  effects 
  of 
  trituration, 
  

   and 
  the 
  other 
  peculiarities, 
  as 
  the 
  position 
  of 
  the 
  angulation 
  in 
  the 
  

   inner 
  border, 
  such 
  as 
  may 
  be 
  attributed 
  to 
  individual 
  variation. 
  

   This 
  inference, 
  however, 
  may 
  appear 
  inconsistent 
  at 
  first 
  sight, 
  but 
  

   we 
  have 
  already 
  observed 
  marked 
  variations 
  in 
  the 
  same 
  parts, 
  as 
  

   noted 
  in 
  individuals 
  described 
  under 
  Psammodus 
  plenus, 
  of 
  whose 
  

   specific 
  identity 
  scarcely 
  a 
  doubt 
  can 
  arise. 
  

  

  The 
  specimen 
  figured 
  in 
  PI. 
  XI, 
  f. 
  3, 
  3a, 
  of 
  Vol. 
  II., 
  111. 
  Geol. 
  

   Survey, 
  where 
  the 
  form 
  is 
  referred 
  by 
  Messrs. 
  Newberry 
  and 
  Wor- 
  

   then 
  to 
  Psammodus 
  rugosus, 
  Agass., 
  also 
  represents 
  a 
  short 
  tooth, 
  

   in 
  an 
  almost 
  perfect 
  state 
  of 
  preservation, 
  save 
  the" 
  wearing 
  away 
  

   of 
  a 
  patch 
  of 
  the 
  crown 
  at 
  the 
  outer 
  anterior 
  angle. 
  It 
  presents 
  a 
  

   rhomboidal 
  outline, 
  the 
  anterior 
  margin 
  gently 
  curved 
  forward, 
  the 
  

   deep 
  basal 
  portion 
  inbeveled, 
  the 
  posterior 
  margin 
  correspondingly 
  

  

  