﻿112 
  PALAEONTOLOGY 
  OF 
  [LLINOI 
  

  

  species 
  being 
  chiefly 
  distinguishable 
  by 
  differences 
  in 
  proportions. 
  

   The 
  absence 
  of 
  transverse 
  ruga 
  may 
  be 
  deemed 
  of 
  greater 
  than 
  

   specific 
  importance. 
  The 
  genus, 
  however, 
  presents 
  in 
  the 
  ensemble 
  

   of 
  its 
  specific 
  forms 
  and 
  the 
  within 
  bounds 
  somewhat 
  diverse 
  char- 
  

   acteristics, 
  features 
  which 
  indicate 
  for 
  it 
  an 
  important 
  position 
  at 
  

   the 
  head 
  of 
  this 
  particular 
  group 
  of 
  the 
  family, 
  and 
  from 
  which 
  

   Cocfdiodua 
  was 
  probably 
  an 
  off-shoot 
  at 
  a 
  later 
  date. 
  It 
  embraces 
  

   besides 
  the 
  Burlington 
  forms, 
  species 
  occurring 
  in 
  the 
  Keokuk 
  for- 
  

   mation 
  described 
  under 
  the 
  terms 
  Cochftochu 
  lotus, 
  Leidy, 
  {Cochtio- 
  

   dus 
  nobilis, 
  X. 
  and 
  W.) 
  Poecilodtu 
  rugonu, 
  N. 
  and 
  W., 
  (including 
  

   P. 
  ornatus, 
  N. 
  and 
  W.)j 
  etc. 
  We 
  are 
  not 
  aware 
  of 
  the 
  existence 
  of 
  

   later 
  representatives 
  than 
  those 
  above 
  enumerated 
  from 
  the 
  Keokuk 
  

   formation 
  and 
  a 
  single 
  species 
  from 
  the 
  St. 
  Louis, 
  the 
  Chi 
  

   species 
  belonging 
  to 
  typical 
  Cochliodus. 
  

  

  Were 
  we 
  in 
  possession 
  of 
  sufficiently 
  complete 
  materials 
  to 
  con- 
  

   clusively 
  demonstrate 
  the 
  relations 
  of 
  the 
  species 
  characterized 
  by 
  

   the 
  presence 
  of 
  the 
  transverse 
  imbrications 
  or 
  ruga', 
  and 
  thoc 
  

   which 
  the 
  coronal 
  surface 
  is 
  simply 
  marked 
  by 
  more 
  or 
  less 
  subor- 
  

   dinate 
  revolving 
  furrows 
  and 
  ridges, 
  it 
  might 
  be 
  found 
  t' 
  

   constitute 
  two 
  distinct 
  groups 
  more 
  or 
  less 
  well 
  defined 
  one 
  from 
  the 
  

   other. 
  In 
  that 
  event 
  Cochliodus 
  lotus 
  would 
  naturally 
  fall 
  into 
  the 
  

   group 
  represented 
  by 
  the 
  primitive 
  Burlington 
  Bpi 
  ties 
  Chitonodu$ 
  

   antiquus; 
  while 
  on 
  the 
  other 
  hand, 
  the 
  Keokuk 
  "Foctfodas 
  rug 
  

   would 
  remain 
  in 
  association 
  with 
  the 
  species 
  heir 
  first 
  described 
  

   under 
  Chitonodut 
  Springeri. 
  

  

  ChTTOMODUS 
  Si'kin'.kki. 
  St. 
  J. 
  and 
  W. 
  

   PL 
  VI. 
  Vie. 
  B 
  15. 
  

  

  Mandibular 
  posterior 
  teeth 
  b 
  lal 
  in 
  outline. 
  | 
  in- 
  

  

  rolled. 
  Anterolateral 
  border 
  with 
  a 
  slightly 
  oblique 
  course 
  outward 
  

   an 
  d 
  forward, 
  oi 
  moderate 
  height, 
  coronal 
  enamel 
  forming 
  a 
  narrow 
  

   i,nel 
  abruptly 
  folded 
  over 
  the 
  edge 
  and 
  inbeveled 
  to 
  the 
  channeled, 
  

   |y 
  pitted 
  basal 
  portion; 
  postero-lateral 
  b 
  

  

  ioint 
  <»f 
  inrollment 
  at 
  an 
  aide 
  of 
  about 
  60" 
  with 
  the 
  

   opposite 
  Bide, 
  coronal 
  enamel 
  forming 
  a 
  well-marked 
  rounded 
  fold 
  

   along 
  the 
  upper 
  edge 
  of 
  the 
  ohanneled 
  basal 
  portion; 
  inner 
  m 
  

  

  ;,t 
  angle 
  with 
  the 
  anterolateral 
  border, 
  gradually 
  pro- 
  

  

  I 
  backward 
  from 
  the 
  obtuse 
  anterii 
  i 
  sharply 
  curved 
  

  

  ,,„,,„! 
  ti, 
  of 
  the 
  coronal 
  promii 
  kh 
  a 
  i 
  oding 
  

  

  ,,tion 
  in 
  passing 
  thi 
  or 
  depression 
  into 
  the 
  Bubacnte 
  

  

  isually 
  inbevel< 
  d. 
  Superfli 
  i 
  tl 
  characters 
  pro- 
  

  

  