﻿

  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  185 
  

  

  species 
  have 
  spines 
  at 
  the 
  genal 
  angles, 
  corresponding 
  to 
  the 
  

   interocnlar 
  spines 
  of 
  Holmia, 
  and 
  young 
  Elliptocephala, 
  and 
  

   not 
  to 
  the 
  spiniform 
  projections 
  of 
  the 
  free 
  cheeks. 
  From 
  

   their 
  abbreviated 
  thorax, 
  and 
  progressive 
  loss 
  of 
  annulations 
  

   on 
  the 
  glabella 
  and 
  axis 
  of 
  the 
  pygidium, 
  they 
  must 
  also 
  be 
  

   considered 
  as 
  degraded. 
  Microdiscus, 
  the 
  earlier 
  genus, 
  has 
  

   three 
  or 
  four 
  free 
  segments, 
  and 
  in 
  some 
  species 
  (M. 
  speciosus 
  

   Ford) 
  preserves 
  the 
  normal 
  pentamerous 
  glabella 
  and 
  annulated 
  

   pygidial 
  axis, 
  while 
  the 
  later 
  genus, 
  Agnostus, 
  has 
  but 
  two 
  

   free 
  segments, 
  and 
  has 
  lost 
  the 
  annulations 
  of 
  both 
  glabella 
  and 
  

   pygidium. 
  Matthew 
  26 
  has 
  described 
  the 
  protaspis 
  stage 
  of 
  

   jKicrodiscns, 
  which 
  agrees 
  with 
  the 
  similar 
  stage 
  of 
  Ptyekopa- 
  

   ria 
  and 
  Sao. 
  

  

  Fully 
  a 
  dozen 
  generic 
  names 
  have 
  been 
  proposed 
  for 
  forms 
  

   of 
  the 
  general 
  type 
  of 
  Ag?iostus, 
  but 
  none 
  of 
  them 
  has 
  ever 
  

   come 
  into 
  current 
  use. 
  Nine 
  were 
  first 
  published 
  by 
  Corda, 
  15 
  

   but 
  as 
  Barrande 
  3 
  subsequently 
  showed 
  that 
  one 
  was 
  based 
  on 
  

   an 
  Orbicida, 
  another 
  on 
  a 
  poor 
  specimen 
  of 
  jEglina, 
  and 
  three 
  

   others 
  on 
  a 
  single 
  species, 
  this 
  grouping 
  soon 
  fell 
  into 
  disuse. 
  

   Moreover, 
  Barrande 
  was 
  inclined 
  to 
  give 
  no 
  generic 
  value 
  

   to 
  the 
  form 
  and 
  lobation 
  of 
  the 
  glabella, 
  and 
  therefore 
  all 
  the 
  

   species 
  were 
  placed 
  by 
  him 
  in 
  the 
  single 
  genus 
  Agnostics. 
  At 
  

   the 
  present 
  time, 
  more 
  weight 
  is 
  given 
  to 
  the 
  characters 
  of 
  the 
  

   glabella 
  and 
  pygidium, 
  as 
  indicating 
  generic 
  differences 
  in 
  dor- 
  

   sal 
  and 
  ventral 
  structure, 
  so 
  that 
  further 
  study 
  may 
  show 
  the 
  

   desirability 
  of 
  restoring 
  such 
  of 
  Corda's 
  names 
  as 
  were 
  founded 
  

   upon 
  natural 
  groups 
  of 
  this 
  family. 
  

  

  Family 
  II. 
  Harpedid^e 
  Barrande. 
  

  

  Cephalon 
  large, 
  margined 
  by 
  a 
  broad 
  expansion 
  or 
  limb 
  ; 
  

   glabella 
  short 
  and 
  prominent. 
  Free 
  cheeks 
  ventral, 
  continu- 
  

   ous 
  ; 
  suture 
  marginal, 
  following 
  the 
  outer 
  edge 
  of 
  the 
  limb. 
  

   Paired 
  simple 
  eye 
  spots, 
  or 
  ocelli, 
  single 
  or 
  double, 
  at 
  the 
  dis- 
  

   tal 
  ends 
  of 
  well-marked 
  eye 
  lines 
  on 
  the 
  fixed 
  cheeks, 
  extend- 
  

   ing 
  outward 
  from 
  the 
  glabella. 
  Thorax 
  of 
  from 
  twenty-five 
  

   to 
  twenty-nine 
  segments, 
  with 
  long 
  grooved 
  pleura. 
  Pygidium 
  

   (in 
  Harpes) 
  very 
  small, 
  composed 
  of 
  but 
  three 
  or 
  four 
  seg- 
  

   ments. 
  Cambrian 
  to 
  Devonian. 
  

  

  Including 
  Harpes 
  Goldfuss, 
  Harpina 
  Novak, 
  and 
  Harpides 
  ? 
  

   Bey 
  rich. 
  

  

  The 
  genus 
  Harpes 
  presents 
  considerable 
  variation 
  in 
  the 
  

   lobes 
  of 
  the 
  glabella. 
  H. 
  ungula 
  Sternberg 
  shows 
  the 
  full 
  num- 
  

   ber 
  of 
  five 
  lobes, 
  but 
  in 
  some 
  species, 
  as 
  H. 
  $ 
  Orbignyianum 
  

  

  