﻿•200 
  PALEONTOLOGY 
  OF 
  ILLINOI- 
  

  

  of 
  the 
  two 
  contiguous 
  teeth 
  of 
  the 
  opposite 
  parallel 
  row, 
  thus 
  pro- 
  

   ducing 
  an 
  asymmetrical 
  alternating 
  arrangement 
  only 
  less 
  symmet- 
  

   rical 
  than 
  ohtains 
  in 
  the 
  modern 
  MyUobaies. 
  That 
  their 
  real 
  affini- 
  

   ties 
  are 
  with 
  the 
  Myliodonts 
  there 
  can 
  be 
  no 
  doubt 
  ; 
  but 
  whether 
  

   they 
  should 
  be 
  regarded 
  as 
  entitled 
  to 
  a 
  specific 
  family 
  rank 
  distinct 
  

   from 
  their 
  living 
  allies, 
  we 
  do 
  not 
  deem 
  ourselves 
  prepared 
  at 
  the 
  

   present 
  moment 
  to 
  affirm. 
  As 
  far 
  back 
  as 
  18G2, 
  Professor 
  Agas 
  

   as 
  is 
  well 
  known 
  to 
  his 
  students 
  who 
  were 
  at 
  that 
  date 
  and 
  subse- 
  

   quently 
  matriculated 
  at 
  Cambridge, 
  had 
  recognized 
  the 
  ordinal 
  rela- 
  

   tionship 
  of 
  the 
  Psammodi, 
  and 
  besides 
  the 
  typical 
  Ptammodvs 
  he 
  

   also 
  included 
  the 
  several 
  allied 
  genera 
  of 
  the 
  Aetobates 
  type 
  antici- 
  

   pated 
  thus 
  early 
  in 
  the 
  earth's 
  history 
  by 
  the 
  genera 
  Copodus, 
  I'lmro- 
  

   gomphus, 
  Pinnacodus, 
  Labodus, 
  Dimyleus, 
  etc., 
  which 
  were 
  recogni. 
  

   from 
  mountain 
  limestone 
  species 
  occurring 
  in 
  the 
  British 
  Islands. 
  

  

  Notwithstanding 
  the 
  incompleteness 
  of 
  our 
  materials 
  in 
  certain 
  

   important 
  particulars, 
  we 
  deem 
  it 
  something 
  more 
  than 
  mere 
  con- 
  

   jecture 
  the 
  assignment 
  of 
  the 
  forms 
  to 
  definite 
  positions 
  upon 
  the 
  

   jaws. 
  As 
  is 
  well 
  known, 
  the 
  maxillary 
  dentaries 
  of 
  Myliobatet 
  present 
  

   a 
  gently 
  longitudinally 
  arched 
  condition 
  so 
  far 
  as 
  relates 
  to 
  the 
  grind- 
  

   ing 
  surface 
  of 
  the 
  series 
  of 
  teeth 
  with 
  which 
  the 
  jaw 
  was 
  paved 
  

   across; 
  and 
  that 
  the 
  mandible 
  presents 
  in 
  comparison 
  a 
  triturating 
  

   surface 
  strongly 
  rolled 
  in 
  the 
  same 
  direction. 
  The 
  same 
  state 
  of 
  

   things 
  may 
  readily 
  be 
  appreciated 
  in 
  a 
  suite 
  of 
  examples 
  of 
  Psdin- 
  

   modiu: 
  The 
  nearly 
  plane 
  or 
  longitudinally 
  slightly 
  arched 
  and 
  trans- 
  

   ursely 
  more 
  or 
  less 
  concave 
  coronal 
  contour 
  of 
  the 
  teeth, 
  may 
  with 
  

   reason 
  be 
  regarded 
  as 
  having 
  belonged 
  to 
  the 
  upper 
  jaw: 
  and 
  tl 
  

   teeth 
  which 
  exhibit 
  a 
  contour 
  almost 
  the 
  reverse 
  of 
  that 
  just 
  noticed, 
  

   being 
  perceptibly 
  more 
  strongly 
  arched 
  longitudinally, 
  also 
  trans- 
  

   versely 
  convex, 
  with 
  a 
  more 
  rapid 
  convergence 
  of 
  lines 
  projected 
  

   vertically 
  to 
  the 
  coronal 
  Borface 
  downward 
  from 
  either 
  margin, 
  indi- 
  

   cate 
  for 
  a 
  series 
  of 
  teeth 
  a 
  more 
  Btrongly 
  rolled 
  Borface 
  area 
  than 
  

   in 
  the 
  above 
  mentioned 
  form, 
  and 
  corresponding 
  in 
  this 
  essential 
  to 
  

   the 
  teeth 
  composing 
  the 
  series 
  of 
  the 
  mandible 
  in 
  the 
  Myliodoi 
  

   Bui 
  the 
  collections, 
  both 
  European 
  and 
  American, 
  afford 
  exam] 
  

   of 
  a 
  form 
  which, 
  possessing 
  essentially 
  the 
  outline 
  and 
  contour 
  of 
  

  

  the 
  previously 
  mentioned 
  forms, 
  differ 
  from 
  them 
  in 
  their 
  obl< 
  

   shape, 
  being 
  relatively 
  longer 
  than 
  wide, 
  and 
  the 
  depressed 
  median 
  

  

  region 
  and 
  rounded 
  condition 
  of 
  the 
  coronal 
  folds 
  along 
  the 
  lateral 
  

  

  borders. 
  Ml 
  of 
  the 
  examples 
  of 
  the 
  latter 
  form 
  which 
  have 
  oom< 
  

  

  OUT 
  notice 
  are 
  further 
  distinguished 
  by 
  the 
  relatr 
  :it 
  depth 
  of 
  

  

  the 
  basal 
  portion, 
  whioh, 
  however, 
  in 
  other 
  respeote 
  intimately 
  agn 
  

  

  