﻿198 
  PALEONTOLOGY 
  OF 
  ILLINOIS 
  

  

  species 
  is 
  produced 
  by 
  the 
  wrinkling 
  of 
  the 
  enamel 
  or 
  external 
  lay 
  

   and 
  which 
  apparently 
  has 
  no 
  other 
  relation 
  to 
  the 
  medullary 
  tul 
  

   than 
  to 
  rudely 
  define 
  them 
  in 
  irregular 
  and 
  transverse 
  or 
  longi- 
  

   tudinal 
  rows, 
  the 
  punctse 
  rarely 
  confluent, 
  and 
  the 
  i 
  appear- 
  

   ance 
  becoming 
  obsolete 
  or 
  more 
  or 
  less 
  obscured 
  over 
  the 
  more 
  

   i 
  parts 
  of 
  the 
  triturating 
  surface. 
  The 
  impression 
  also 
  pro- 
  

   rails 
  that 
  the 
  tendency 
  to 
  rugosity 
  of 
  the 
  coronal 
  surface 
  in 
  

   with 
  age, 
  since 
  this 
  appearance 
  bo 
  far 
  as 
  obsei 
  ms 
  to 
  he 
  most 
  

   prevalent 
  and 
  conspicuous 
  in 
  large 
  individuals 
  belonging 
  to 
  the 
  

   series 
  which 
  have 
  received 
  several 
  accessions, 
  the 
  innermost 
  indi- 
  

   viduals 
  of 
  which 
  have 
  suffered 
  little 
  from 
  the 
  abrading 
  effects 
  of 
  

   trituration 
  while 
  in 
  use; 
  but 
  it 
  is 
  not 
  an 
  essential 
  characi 
  

   some 
  species 
  evidently 
  always 
  remained 
  quite 
  smooth 
  in 
  their 
  

   onal 
  areas. 
  The 
  inferior 
  surface 
  is 
  plane, 
  in 
  a 
  general 
  way 
  con- 
  

   forming 
  to 
  that 
  of 
  the 
  crown, 
  and 
  even 
  possessing 
  distinctive 
  

   characteristics 
  as 
  applied 
  to 
  species; 
  it 
  shows 
  in 
  the 
  perfect 
  Btate 
  

   a 
  rather 
  dense 
  thin 
  layer, 
  perhaps 
  in 
  degree 
  rather 
  than 
  structually 
  

   differing 
  from 
  the 
  more 
  cellulose 
  middle 
  layer 
  composing 
  the 
  hulk 
  

   of 
  the 
  hase, 
  and 
  usually 
  marked 
  by 
  more 
  or 
  leas 
  distinct 
  Longitudi- 
  

   nal 
  grooves, 
  or 
  smooth, 
  and 
  faintly 
  keeled 
  nearest 
  the 
  inner 
  articular 
  

   border. 
  

  

  In 
  drawing 
  up 
  the 
  foregoing 
  diagnostic 
  account 
  of 
  Paammodua, 
  the 
  

   authors 
  have 
  been 
  actuated 
  by 
  the 
  desire 
  to 
  place 
  before 
  the 
  student 
  

   of 
  palseichthyology 
  such 
  data 
  as 
  they 
  themselves 
  possessed 
  relating 
  

   to 
  the 
  genus, 
  and 
  which 
  they 
  owe 
  largely 
  to 
  the 
  kind 
  offices 
  of 
  their 
  

   oolaborators. 
  It 
  will 
  have 
  been 
  observed 
  that 
  the 
  genus 
  embra 
  

   a 
  variety 
  of 
  forms, 
  which, 
  while 
  their 
  generic 
  identity 
  is 
  unqu 
  

   tioned, 
  offer 
  so 
  diverse 
  appearances 
  as. 
  in 
  the 
  absence 
  of 
  other 
  than 
  

   detached 
  and 
  fragmentary 
  remains, 
  to 
  greatly 
  complicate 
  the 
  de- 
  

   termination 
  of 
  the 
  relative 
  position 
  the 
  forms 
  occupied 
  upon 
  the 
  

   jaws. 
  Their 
  congeneric 
  identity 
  is 
  proven 
  by 
  identity 
  of 
  outline 
  ami 
  

   contour 
  and 
  superficial 
  structure 
  common 
  alike 
  to 
  each 
  and 
  all 
  of 
  

  

  the 
  forms, 
  and 
  these 
  latter 
  reBolve 
  themselves 
  into 
  certain 
  well- 
  

   defined 
  groups; 
  so 
  that 
  specific 
  distinctions 
  may 
  b, 
  i 
  in 
  

   unique 
  I 
  samples 
  pertaining 
  to 
  on 
  or 
  the 
  other 
  form. 
  But 
  in 
  the 
  

   attempt 
  to 
  associate 
  these 
  forms 
  under 
  definite. 
  Bp 
  mhiua- 
  

  

  tions, 
  the 
  observer 
  is 
  necessarily 
  compelled 
  to 
  rely, 
  to 
  some 
  extent 
  

   ; 
  ,i 
  [east, 
  on 
  other 
  than 
  superficial 
  resemblances, 
  as. 
  r 
  pie, 
  

  

  the 
  association 
  of 
  the 
  individuals 
  m 
  the 
  bonXOCfl 
  whence 
  th. 
  

  

  derived. 
  The 
  process 
  is 
  often 
  further 
  complicated 
  by 
  th< 
  

  

  of 
  collecting, 
  for 
  it 
  not 
  unfrequently 
  happens 
  that 
  a 
  

  

  