﻿VERTEBRATES. 
  179 
  

  

  the 
  maxillary 
  posterior 
  tooth 
  shows 
  obscure 
  indications 
  of 
  parallel 
  

   transverse 
  disposition 
  of 
  the 
  punctse, 
  though 
  it 
  is 
  not 
  nearly 
  to 
  the 
  

   same 
  degree 
  apparent 
  as 
  in 
  the 
  typical 
  species 
  of 
  the 
  genus. 
  

  

  The 
  present 
  species 
  bears 
  a 
  somewhat 
  striking 
  resemblance 
  to 
  

   To 
  modus 
  convexus, 
  Agass., 
  of 
  the 
  Mountain 
  limestone 
  of 
  Great 
  Britain. 
  

   As 
  has 
  been 
  already 
  intimated, 
  there 
  would 
  seem 
  to 
  be 
  very 
  inti- 
  

   mate 
  relationship 
  between 
  Xystrodus 
  and 
  Tomodus, 
  the 
  forms 
  of 
  both, 
  

   especially 
  the 
  mandibular 
  posterior 
  teeth, 
  showing 
  the 
  same 
  outline 
  

   and 
  contour 
  of 
  the 
  crown, 
  and 
  chiefly 
  distinguished 
  one 
  from 
  the 
  

   other 
  by 
  the 
  presence 
  or 
  absence 
  of 
  transversely 
  disposed 
  punctae. 
  

   In 
  the 
  present 
  species 
  the 
  very 
  obscure 
  indications 
  of 
  the 
  disposi- 
  

   tion 
  of 
  the 
  punctae 
  in 
  rows 
  parallel 
  with 
  the 
  inner 
  margin 
  of 
  the 
  

   teeth 
  might 
  be 
  regarded 
  as 
  a 
  transitional 
  feature 
  connecting 
  the 
  

   more 
  widely 
  differentiated 
  species 
  which 
  are 
  accepted 
  as 
  the 
  typical 
  

   representatives 
  of 
  one 
  or 
  other 
  of 
  the 
  above 
  named 
  genera. 
  

  

  Geological 
  position 
  and 
  localities: 
  Upper 
  Burlington 
  limestone, 
  fish- 
  

   bed; 
  Bufiington 
  creek, 
  Louisa 
  county; 
  Burlington 
  and 
  Augusta 
  

   (Iowa) 
  ; 
  Monmouth, 
  Illinois. 
  

  

  Xystrodus 
  inconditus, 
  St. 
  J. 
  and 
  W. 
  

  

  PI. 
  VIII, 
  Fig. 
  1. 
  

  

  Teeth 
  very 
  small, 
  or 
  below 
  medium 
  size, 
  somewhat 
  strongly 
  

   arched 
  from 
  within 
  outwards. 
  Antero-lateral 
  border 
  nearly 
  straight, 
  

   or 
  presenting 
  the 
  usual 
  slight 
  sigmoidal 
  curvature, 
  vertical, 
  meeting 
  

   in 
  a 
  sharp 
  angle 
  above, 
  over 
  which 
  the 
  enamel 
  folds 
  in 
  a 
  rather 
  

   deep 
  belt 
  well 
  defined 
  from 
  the 
  basal 
  portion, 
  which 
  inclines 
  slightly 
  

   outward 
  to 
  the 
  thin 
  inferior 
  edge 
  ; 
  inner 
  margin 
  broadly 
  rounded, 
  

   with 
  a 
  slight 
  concave 
  curvature 
  in 
  passing 
  the 
  coronal 
  depression, 
  

   the 
  course 
  slightly 
  oblique 
  from 
  the 
  anterior 
  to 
  the 
  posterior 
  angle 
  ; 
  

   postero-lateral 
  border 
  apparently 
  somewhat 
  rapidly 
  converging 
  towards 
  

   the 
  outer 
  extremity, 
  showing 
  the 
  usual 
  narrow 
  enamel 
  fold 
  along 
  

   the 
  upper 
  edge, 
  but 
  oftener 
  mutilated 
  from 
  abrasion. 
  Coronal 
  sur- 
  

   face 
  occupied 
  by 
  a 
  rather 
  wide, 
  shallow 
  depression, 
  the 
  moderately 
  

   upraised 
  posterior 
  wing 
  presenting 
  a 
  relatively 
  wide 
  transverse 
  

   diameter, 
  on 
  the 
  opposite 
  side 
  rising 
  into 
  the 
  gently 
  arched 
  or 
  lat- 
  

   erally 
  convex 
  prominence 
  along 
  the 
  antero-lateral 
  border. 
  '"[ 
  Triturat- 
  

   ing 
  surface 
  in 
  almost 
  every 
  case 
  exhibiting 
  indications 
  of 
  excessive 
  

   wear, 
  in 
  which 
  even 
  the 
  punctate 
  structure 
  is 
  obscure 
  ; 
  along 
  the 
  

   inner 
  margin, 
  however, 
  the 
  teeth 
  often 
  preserve 
  the 
  smooth, 
  dense, 
  

   polished 
  enamel 
  coating, 
  but 
  in 
  the 
  middle 
  portion 
  of 
  the 
  crown, 
  

  

  