﻿VERTEBRATES. 
  

  

  207 
  

  

  bordered 
  exteriorly 
  by 
  a 
  nearly 
  straight, 
  vertical 
  face 
  similarly 
  

   marked 
  to 
  the 
  other 
  vertical 
  sides 
  by 
  vertical 
  beaded 
  rugosities. 
  

   This 
  specimen, 
  if 
  it 
  is 
  correctly 
  deciphered, 
  has 
  a 
  nearly 
  rectangu- 
  

   lar 
  outline 
  so 
  far 
  as 
  relates 
  to 
  the 
  external 
  border 
  and 
  the 
  anterior 
  

   and 
  posterior 
  margins, 
  the 
  angulation 
  of 
  the 
  inner 
  border 
  giving 
  the 
  

   tooth 
  a 
  pentangular 
  figure. 
  Taking 
  into 
  consideration 
  the 
  vertical 
  

   condition 
  of 
  the 
  straight 
  exterior 
  border, 
  which 
  shows 
  features 
  con- 
  

   sonant 
  with 
  an 
  articular 
  face, 
  this 
  tooth 
  certainly 
  suggests 
  a 
  form 
  

   not 
  hitherto 
  recognized, 
  and 
  which 
  is 
  at 
  the 
  same 
  time 
  a 
  true 
  

   Psammodus. 
  It 
  is 
  possible 
  that 
  the 
  specimen 
  represents 
  one 
  side 
  of 
  

   a 
  median 
  series 
  of 
  teeth 
  comprising 
  two 
  ranges, 
  the 
  inner 
  articular 
  

   borders 
  alternating 
  and 
  the 
  exterior 
  borders 
  joining 
  the 
  inner 
  artic- 
  

   ular 
  borders 
  of 
  teeth 
  of 
  the 
  form 
  of 
  those 
  described 
  under 
  P. 
  

   Spi'ingeri, 
  &c. 
  ? 
  The 
  specimen 
  is 
  strongly 
  suggestive 
  of 
  such 
  a 
  com- 
  

   bination 
  of 
  dental 
  elements, 
  after 
  the 
  fashion 
  sketched 
  in 
  the 
  an- 
  

   nexed 
  cut, 
  Fig. 
  a. 
  The 
  specific 
  

   identity 
  of 
  the 
  " 
  division 
  bed 
  " 
  

   tooth 
  and 
  that 
  last 
  referred 
  to 
  

   from 
  the 
  main 
  fish-bed 
  of 
  the 
  

   Upper 
  Burlington, 
  is 
  only 
  conjec- 
  

   turally 
  inferred. 
  The 
  partially 
  

   shaded 
  pentagonal 
  plate 
  on 
  the 
  

   right 
  of 
  the 
  double 
  median 
  series 
  

   gives 
  a 
  reduced 
  outline 
  of 
  the 
  

   s 
  ^r 
  theticaldiagramofPsrtmwod 
  " 
  s< 
  " 
  r 
  " 
  specimen 
  here 
  referred 
  to. 
  

  

  ■ 
  Geological 
  position 
  and 
  locality: 
  Fish-bed 
  of 
  the 
  Upper 
  Burlington- 
  

   Keokuk 
  division 
  beds 
  ; 
  DesMoines 
  Co., 
  Iowa. 
  

  

  Psammodus 
  Lovlanus, 
  St. 
  J. 
  and 
  W. 
  

  

  PL 
  XIV, 
  Fig. 
  7-9. 
  

  

  Teeth 
  attain 
  medium 
  size. 
  Maxillary 
  form 
  subrhomboidal 
  in 
  out- 
  

   line, 
  rather 
  strongly 
  arched 
  from 
  within 
  outward. 
  The 
  margins 
  

   gently 
  arched 
  forward 
  and 
  parallel, 
  the 
  anterior 
  margin 
  apparently 
  

   somewhat 
  strongly 
  inbeveled, 
  though 
  really 
  vertical 
  to 
  the 
  edge 
  of 
  

   the 
  crown 
  surface, 
  posterior 
  margin 
  showing 
  a 
  belt 
  of 
  enamel 
  ex- 
  

   tending 
  perhaps 
  one-fourth 
  its 
  depth, 
  and 
  well 
  defined 
  from 
  the 
  

   slightly 
  excavated 
  base. 
  Inner 
  border 
  making 
  nearly 
  a 
  right-angle 
  

   with 
  the 
  margins, 
  the 
  truncated 
  posterior 
  angle 
  reaching 
  about 
  one- 
  

   fourth 
  the 
  distance 
  of 
  the 
  border, 
  and 
  making 
  a 
  rather 
  strongly 
  

   marked 
  angulation 
  with 
  the 
  anterior 
  portion 
  of 
  the 
  articular 
  face. 
  

  

  