﻿VERTEBRATES. 
  205 
  

  

  Geological 
  position 
  and 
  localities 
  : 
  Upper 
  Burlington 
  limestone, 
  "fish- 
  

   bed;" 
  Buffington 
  creek, 
  Louisa 
  county, 
  Pleasant 
  Grove, 
  Augusta, 
  

   and 
  Burlington, 
  Iowa 
  ; 
  Cedar 
  creek, 
  Henderson 
  county, 
  and 
  Quincy, 
  

   Illinois. 
  

  

  Psammodus 
  tumidus, 
  St. 
  J. 
  and 
  W. 
  

  

  PL 
  XIY, 
  Pig. 
  1-4. 
  

  

  The 
  collections 
  contain 
  several 
  examples 
  representing 
  two 
  forms 
  

   corresponding 
  with 
  the 
  supposed 
  mandibular 
  and 
  maxillary 
  teeth 
  of 
  

   the 
  foregoing 
  species, 
  Psammodus 
  Springeri, 
  of 
  the 
  same 
  deposits, 
  and 
  

   which, 
  presenting 
  apparently 
  persistent 
  characters 
  by 
  which 
  they 
  

   may 
  be 
  distinguished, 
  we 
  are 
  led 
  to 
  provisionally 
  indicate 
  under 
  a 
  

   distinct 
  specific 
  appellation. 
  Presenting 
  the 
  same 
  general 
  outline, 
  

   they 
  are 
  distinguished 
  from 
  P. 
  Springeri 
  principally 
  by 
  the 
  greater 
  

   extent 
  of 
  the 
  truncated 
  angle 
  of 
  the 
  inner 
  articular 
  border 
  and 
  the 
  

   distinct 
  longitudinal 
  rugosities 
  that 
  make 
  a 
  highly 
  wrought 
  orna- 
  

   mental 
  belt 
  usually 
  extending 
  a 
  short 
  distance 
  forward 
  from 
  the 
  

   posterior 
  margin, 
  both 
  of 
  which 
  characters 
  are 
  common 
  to 
  the 
  two 
  

   forms. 
  The 
  short, 
  broad 
  mandibular 
  teeth 
  do 
  not 
  differ 
  otherwise 
  

   to 
  any 
  marked 
  extent 
  from 
  the 
  same 
  form 
  of 
  the 
  above-mentioned 
  

   species, 
  unless 
  the 
  crown 
  shows 
  a 
  somewhat 
  greater 
  transverse 
  con- 
  

   cavity 
  and 
  more 
  strongly 
  arched 
  contour 
  from 
  within 
  outward; 
  

   however, 
  the 
  prominence 
  along 
  the 
  inner 
  lateral 
  border 
  is 
  somewhat 
  

   more 
  distinctly 
  defined 
  by 
  longitudinal 
  furrows 
  than 
  is 
  observed 
  in 
  

   the 
  corresponding 
  teeth 
  of 
  P. 
  Springeri. 
  But 
  in 
  respect 
  to 
  the 
  max- 
  

   illary 
  form, 
  the 
  most 
  striking 
  contrast 
  is 
  noticeable 
  in 
  the 
  latter 
  

   particular. 
  The 
  inner 
  coronal 
  prominence 
  becomes 
  strongly 
  devel- 
  

   oped, 
  presenting 
  a 
  distinctly 
  defined 
  lobe, 
  laterally 
  rounded 
  with 
  

   steep 
  declivity 
  falling 
  to 
  the 
  depressed 
  median 
  area, 
  which 
  in 
  some 
  

   instances 
  is 
  quite 
  strongly 
  plicated 
  longitudinally 
  in 
  addition 
  to 
  tbe 
  

   rugose 
  belt 
  ornamenting 
  the 
  crown 
  surface 
  immediately 
  along 
  the 
  

   posterior 
  margin 
  ; 
  in 
  worn 
  examples 
  the 
  rugose 
  sculpturing 
  is 
  obso- 
  

   lete. 
  In 
  the 
  majority 
  of 
  instances 
  the 
  limit 
  of 
  the 
  inner 
  coronal 
  

   ridge 
  is 
  defined 
  by 
  a 
  narrow 
  impressed 
  line. 
  The 
  lesser 
  coronal 
  

   ridge 
  along 
  the 
  exterior 
  lateral 
  border 
  bears 
  intimate 
  resemblance 
  to 
  

   P. 
  Springeri, 
  and 
  the 
  same 
  observation 
  may 
  be 
  applied 
  to 
  the 
  super- 
  

   ficial 
  structure 
  of 
  the 
  teeth. 
  

  

  At 
  the 
  outset 
  we 
  were 
  inclined 
  to 
  regard 
  the 
  teeth 
  above 
  referred 
  

   to 
  as 
  merely 
  variations 
  from 
  the 
  normal 
  condition 
  of 
  Psammodus 
  

   Springeri. 
  Indeed 
  a 
  more 
  extensive 
  suite 
  of 
  material 
  may 
  possibly 
  

  

  