﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  199 
  

  

  ginal 
  free 
  cheeks, 
  presenting 
  in 
  general 
  the 
  appearance 
  of 
  Atops, 
  

   Conocoryphe, 
  Ctenocephalus, 
  etc., 
  of 
  the 
  Conocoryphidse. 
  The 
  

   somewhat 
  higher 
  genera 
  Cybele 
  and 
  Encrinurus 
  have 
  inter- 
  

   mediate 
  or 
  transitional 
  characters, 
  leading 
  to 
  the 
  other 
  families. 
  

   The 
  Cheiruridse 
  show 
  a 
  greater 
  amount 
  of 
  differentiation 
  and 
  

   progressive 
  and 
  regressive 
  evolution 
  than 
  any 
  other 
  in 
  this 
  

   order. 
  Crotalocephalus 
  and 
  Sphmrexochus 
  seem 
  to 
  express 
  the 
  

   highest 
  development, 
  and 
  Deiphon 
  and 
  Onycopyge 
  show 
  the 
  

   effects 
  of 
  over 
  specialization, 
  resulting 
  in 
  degeneration. 
  The 
  

   Calymenidse, 
  in 
  their 
  small 
  eyes 
  and 
  narrow 
  free 
  cheeks, 
  have 
  

   decided 
  affinities 
  with 
  the 
  lower 
  genera. 
  The 
  same 
  may 
  be 
  

   said 
  of 
  Trimerocephalus 
  of 
  the 
  Phacopidse, 
  though 
  the 
  other 
  

   genera 
  of 
  this 
  family 
  possess 
  large 
  eyes, 
  situated 
  well 
  back 
  and 
  

   close 
  to 
  the 
  glabella. 
  For 
  these 
  and 
  other 
  reasons, 
  the 
  family 
  

   is 
  placed 
  at 
  the 
  end 
  of 
  the 
  order, 
  as 
  expressing 
  its 
  highest 
  

   development. 
  

  

  Family 
  XI. 
  Enckinueid^: 
  Linnarsson. 
  

  

  Cephalon 
  narrow, 
  transverse. 
  Fixed 
  cheeks 
  very 
  large. 
  

   Free 
  cheeks 
  long, 
  narrow, 
  separate, 
  sometimes 
  with 
  a 
  free 
  

   plate 
  between 
  the 
  anterior 
  extremities. 
  Sutures 
  extending 
  from 
  

   in 
  front 
  of 
  the 
  genal 
  angles 
  obliquely 
  forward, 
  and 
  cutting 
  the 
  

   anterior 
  margin 
  in 
  front 
  of 
  the 
  glabella. 
  Eyes 
  very 
  small 
  or 
  

   absent. 
  Thorax 
  of 
  from 
  nine 
  to 
  twelve 
  segments, 
  with 
  ridged 
  

   pleura. 
  Pygidium 
  generally 
  composed 
  of 
  many 
  segments; 
  

   limb 
  with 
  strong 
  ribs 
  usually 
  less 
  in 
  number 
  than 
  the 
  annula- 
  

   tions 
  of 
  the 
  axis. 
  Ordovician 
  and 
  Silurian. 
  

  

  Including 
  the 
  genera 
  Encrinurus 
  Emmrich 
  (Cromus 
  Bar- 
  

   rande), 
  Areia 
  Barrande, 
  Cybele 
  Loven, 
  Dindymene 
  Corda, 
  

   Placoparia 
  Corda, 
  and 
  Prosopiscus 
  Salter. 
  

  

  The 
  Conocoryphidse 
  were 
  shown 
  to 
  be 
  the 
  radical 
  of 
  the 
  

   order 
  Opisthoparia, 
  and 
  for 
  similar 
  reasons, 
  the 
  Encrinuridae 
  

   may 
  now 
  be 
  taken 
  as 
  the 
  primitive 
  family 
  of 
  the 
  Proparia. 
  

   The 
  cephala 
  of 
  Areia 
  and 
  Placoparia 
  have 
  many 
  resemblances 
  

   to 
  Conocoryphe, 
  but 
  the 
  fixed 
  cheeks 
  bear 
  the 
  genal 
  angles 
  and 
  

   spines, 
  while 
  in 
  the 
  latter 
  genus 
  they 
  are 
  on 
  the 
  free 
  cheeks. 
  

   In 
  both 
  families, 
  the 
  free 
  cheeks 
  are 
  narrow 
  and 
  marginal, 
  and 
  

   the 
  eyes 
  are 
  absent 
  or 
  rudimentary. 
  Both 
  these 
  characters 
  are 
  

   decidedly 
  larval. 
  Other 
  primitive 
  and 
  larval 
  features 
  belong- 
  

   ing 
  to 
  the 
  Encrinuridse 
  are 
  the 
  eye 
  lines 
  in 
  Cybele 
  and 
  Encri- 
  

   nurus, 
  the 
  undefined 
  and 
  expanded 
  termination 
  of 
  the 
  glabella 
  

   in 
  Dindymene 
  and 
  Encrinurus, 
  and 
  the 
  pentamerous 
  head 
  

   axis 
  in 
  all 
  but 
  Dindymene, 
  in 
  which 
  the 
  four 
  anterior 
  lobes 
  or 
  

  

  