﻿202 
  P. 
  E. 
  Raymond 
  — 
  Beecher's 
  Classification 
  of 
  Trilobites. 
  

  

  trilobites 
  there 
  are 
  two 
  types 
  of 
  head 
  structure 
  : 
  one 
  in 
  which 
  

   the 
  facial 
  sutures 
  are 
  confluent 
  around 
  the 
  frontal 
  margin, 
  and 
  

   the 
  free 
  cheeks 
  do 
  not 
  meet 
  but 
  are 
  separated 
  by 
  an 
  epistoma 
  ; 
  

   another 
  in 
  which 
  the 
  free 
  cheeks 
  meet 
  in 
  a 
  vertical 
  suture 
  and 
  

   an 
  epistoma 
  is 
  apparently 
  absent. 
  Now 
  the 
  first 
  condition 
  is 
  

   very 
  much 
  more 
  common 
  than 
  the 
  second, 
  and 
  where 
  the 
  

   second 
  condition 
  is 
  best 
  shown, 
  in 
  the 
  Asaphidse, 
  there 
  is 
  con- 
  

   siderable 
  evidence 
  that 
  proves 
  that 
  the 
  epistoma 
  is 
  present, 
  but 
  

   that 
  the 
  epistomal 
  sutures 
  are 
  obliterated 
  and 
  that 
  the 
  vertical 
  

   suture 
  is 
  secondary. 
  If, 
  then, 
  Trinucleus 
  is 
  a 
  degenerate 
  

   Opisthoparian, 
  the 
  plate 
  in 
  front 
  of 
  the 
  hypostoma 
  must 
  be 
  the 
  

   epistoma, 
  and 
  the 
  chances 
  are 
  that 
  the 
  part 
  of 
  the 
  marginal 
  

   suture 
  directly 
  at 
  the 
  front 
  is 
  a 
  part 
  of 
  the 
  true 
  facial 
  suture. 
  

   If 
  that 
  be 
  the 
  case 
  then 
  the 
  facial 
  suture 
  is 
  not 
  entirely 
  obliter- 
  

   ated, 
  and 
  we 
  ought 
  in 
  the 
  thousands 
  and 
  thousands 
  of 
  speci- 
  

   mens 
  which 
  have 
  been 
  examined, 
  to 
  find 
  at 
  least 
  one 
  or 
  two 
  

   which 
  show 
  traces 
  of 
  the 
  remainder 
  of 
  the 
  facial 
  suture 
  or 
  of 
  

   the 
  epistomal 
  sutures. 
  In 
  no 
  case 
  among 
  the 
  Opisthoparian 
  

   or 
  Proparian 
  trilobites 
  where 
  the 
  cheeks 
  are 
  fused 
  have 
  the 
  

   sutures 
  become 
  entirely 
  obliterated. 
  

  

  It 
  is 
  probably 
  permissible 
  to 
  say, 
  without 
  being 
  accused 
  of 
  

   sectional 
  pride, 
  that 
  the 
  North 
  American 
  specimens 
  of 
  " 
  Trinu- 
  

   cleus" 
  are 
  more 
  numerous 
  and 
  better 
  preserved 
  than 
  those 
  of 
  

   any 
  other 
  region. 
  We 
  do 
  not 
  find 
  the 
  abundant 
  entire 
  speci- 
  

   mens 
  such 
  as 
  occur 
  in 
  Bohemia, 
  but 
  in 
  our 
  specimens 
  the 
  test 
  

   is 
  well 
  preserved 
  and 
  the 
  specimens 
  are 
  not 
  flattened. 
  And 
  

   anyone 
  who 
  collects 
  in 
  our 
  rocks 
  readily 
  becomes 
  convinced 
  

   that 
  the 
  ventral 
  plate 
  of 
  " 
  Trinucleus 
  " 
  is 
  readily 
  separable 
  

   from 
  the 
  cranidium. 
  One 
  of 
  these 
  plates, 
  naturally 
  separated, 
  

   when 
  viewed 
  from 
  the 
  inside, 
  shows 
  the 
  first 
  two 
  rows 
  of 
  

   " 
  hour-glass 
  structures 
  " 
  broken 
  across 
  at 
  the 
  constriction, 
  but 
  

   the 
  posterior 
  rows 
  of 
  pits 
  were 
  not 
  deep 
  enough 
  to 
  connect 
  

   across, 
  so 
  that 
  they 
  show 
  from 
  the 
  inside 
  as 
  hemisphseric 
  

   mounds. 
  The 
  angles 
  of 
  this 
  ventral 
  plate 
  carry 
  the 
  genal 
  

   spines, 
  and 
  where 
  the 
  cranidium 
  is 
  found 
  without 
  the 
  ventral 
  

   plate, 
  the 
  angles 
  are 
  smoothly 
  rounded, 
  showing 
  that 
  the 
  suture 
  

   becomes 
  dorsal 
  in 
  position 
  at 
  least 
  at 
  those 
  points. 
  The 
  fact 
  

   that 
  this 
  ventral 
  plate 
  is 
  entirely 
  separable 
  along 
  well-defined 
  

   sutures, 
  and 
  that 
  it 
  bears 
  the 
  genal 
  spines, 
  seems 
  rather 
  conclu- 
  

   sive 
  evidence 
  that 
  it 
  is 
  a 
  primarily 
  independent 
  plate, 
  and 
  that 
  

   the 
  suture 
  is 
  not 
  secondary. 
  Reed 
  has 
  suggested 
  a 
  comparison 
  

   with 
  Tim-ulus, 
  but 
  in 
  that 
  genus 
  the 
  crack 
  extends 
  only 
  

   around 
  the 
  front 
  and 
  sides 
  of 
  the 
  cephalothorax, 
  does 
  not 
  

   extend 
  to 
  the 
  angles, 
  and 
  does 
  not 
  cut 
  off 
  a 
  separate 
  plate. 
  If 
  

   the 
  marginal 
  suture 
  in 
  the 
  Trinucleidse 
  were 
  a 
  similar 
  make- 
  

   shift 
  cracking, 
  there 
  seems 
  no 
  reason 
  why 
  it 
  should 
  not 
  be 
  

   similarly 
  incomplete. 
  The 
  marginal 
  suture 
  of 
  Trinucleus 
  

  

  