﻿184 
  Beeclxer 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  especially 
  where 
  they 
  are 
  continuous 
  around 
  the 
  front 
  of 
  the 
  

   cephalon, 
  as 
  in 
  Trinucleus 
  and 
  Ampyx. 
  The 
  presence 
  of 
  a 
  

   hypostoma 
  in 
  Agnostus 
  was 
  also 
  determined. 
  Even 
  in 
  the 
  

   higher 
  genera 
  of 
  this 
  order, 
  the 
  suture 
  is 
  frequently 
  unnoticed 
  

   in 
  descriptions, 
  but 
  it 
  can 
  be 
  seen 
  in 
  all 
  well-preserved 
  speci- 
  

   mens. 
  In 
  Trinucleus' 
  29 
  and 
  Harpes, 
  it 
  follows 
  the 
  edge 
  of 
  the 
  

   cephalon, 
  and 
  separates 
  the 
  dorsal 
  from 
  the 
  ventral 
  plate 
  of 
  the 
  

   pitted 
  limb. 
  Since 
  eye 
  spots 
  occur 
  on 
  the 
  fixed 
  cheeks 
  in 
  the 
  

   young 
  Trinucleus 
  and 
  adult 
  Harpes, 
  it 
  is 
  probable 
  that 
  this 
  

   character 
  is 
  a 
  primitive 
  one 
  in 
  this 
  order, 
  and 
  has 
  been 
  lost 
  in 
  

   Agnostus, 
  Microdiscus, 
  Ampyx, 
  and 
  Dionide. 
  

  

  The 
  ontogeny 
  of 
  Sao, 
  Ptychoparia, 
  Triarthrus, 
  Dalmanites, 
  

   etc., 
  shows 
  that 
  the 
  true 
  eyes 
  and 
  free 
  cheeks 
  are 
  first 
  devel- 
  

   oped 
  v.entrally, 
  appearing 
  later 
  at 
  the 
  margin, 
  and 
  then 
  on 
  the 
  

   dorsal 
  side 
  of 
  the 
  cephalon. 
  Therefore, 
  the 
  Agnostidse, 
  Trinu- 
  

   cleidse, 
  and 
  Harpedidse 
  have 
  a 
  very 
  primitive 
  head 
  structure, 
  

   characteristic 
  of 
  the 
  early 
  larval 
  forms 
  of 
  higher 
  families. 
  

   Other 
  secondary 
  features 
  show 
  that 
  this 
  order, 
  though 
  the 
  

   most 
  primitive 
  in 
  many 
  respects, 
  is 
  more 
  specialized 
  that 
  either 
  

   of 
  the 
  others, 
  except 
  in 
  their 
  highest 
  genera. 
  The 
  characters 
  

   referred 
  to 
  are 
  the 
  glabella 
  and 
  pygidium. 
  Very 
  few 
  species 
  

   show 
  the 
  primitive 
  segmentation 
  of 
  the 
  glabella, 
  it 
  being 
  

   usually 
  smooth 
  and 
  inflated, 
  and 
  resembling 
  in 
  its 
  specializa- 
  

   tion 
  such 
  higher 
  genera 
  as 
  Proetus, 
  Asapkus, 
  and 
  Lichas. 
  

   The 
  pygidium 
  often 
  fails 
  to 
  indicate 
  its 
  true 
  number 
  of 
  seg- 
  

   ments. 
  Some 
  Agnostus 
  and 
  Microdiscus 
  show 
  no 
  segments 
  

   either 
  on 
  the 
  axis 
  or 
  limb 
  of 
  the 
  pygidium. 
  Trinucleus 
  and 
  

   others 
  may 
  have 
  a 
  many 
  annulated 
  axis 
  and 
  fewer 
  grooves 
  

   on 
  the 
  pleural 
  portions. 
  The 
  number 
  of 
  appendages 
  corre- 
  

   sponds 
  to 
  the 
  axial 
  divisions, 
  as 
  determined 
  by 
  the 
  writer.* 
  

   The 
  multiplication 
  of 
  segments 
  in 
  the 
  pygidium 
  and 
  their 
  con- 
  

   sequent 
  crowding 
  makes 
  them 
  quite 
  rudimentary. 
  

  

  Family 
  I. 
  Agnostid^ 
  Dalman. 
  

  

  Small 
  forms, 
  having 
  the 
  cephalon 
  and 
  pygidium 
  elongate, 
  

   nearly 
  equal, 
  and 
  similar 
  in 
  form 
  and 
  markings. 
  Free 
  cheeks 
  

   ventral, 
  continuous 
  ; 
  suture 
  marginal 
  or 
  ventral. 
  Eyes 
  want- 
  

   ing. 
  Thorax 
  composed 
  of 
  from 
  two 
  to 
  four 
  segments, 
  with 
  

   grooved 
  pleura. 
  Cambrian 
  and 
  Ordovician. 
  

  

  Including 
  the 
  genera 
  Agnostus 
  Brongniart, 
  and 
  Microdiscus 
  

   Emmons. 
  

  

  The 
  genera 
  in 
  this 
  family 
  are 
  primitive 
  in 
  their 
  form 
  and 
  

   structure, 
  as 
  shown 
  by 
  their 
  ventral 
  free 
  cheeks, 
  marginal 
  or 
  

   ventral 
  suture, 
  elongate 
  cephalon, 
  and 
  large 
  pygidium. 
  Some 
  

  

  