﻿186 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  Barrande, 
  the 
  structure 
  is 
  like 
  Oyphaspis, 
  with 
  separate 
  

   basal 
  lobes. 
  Arraphus 
  Angelin 
  was 
  apparently 
  based 
  upon 
  a 
  

   specimen 
  of 
  Harpes 
  denuded 
  of 
  the 
  pitted 
  border. 
  Harpides 
  

   Beyrich 
  is 
  imperfectly 
  known 
  but 
  seems 
  to 
  belong 
  here. 
  The 
  

   ocular 
  ridges 
  and 
  tubercles 
  on 
  the 
  fixed 
  cheeks, 
  the 
  broad 
  limb, 
  

   the 
  glabella, 
  and 
  the 
  narrow 
  weak 
  thoracic 
  segments 
  are 
  all 
  in 
  

   accord 
  with 
  Harpes, 
  though 
  in 
  other 
  features 
  it 
  has 
  affinities 
  

   with 
  the 
  Conocoryphidse. 
  

  

  In 
  many 
  respects, 
  Harpes 
  is 
  one 
  of 
  the 
  most 
  interesting 
  

   genera 
  of 
  trilobites 
  since 
  it 
  is 
  so 
  unlike 
  other 
  forms. 
  The 
  

   broad 
  hippocrepian 
  pitted 
  limb 
  of 
  the 
  cephalon 
  has 
  its 
  counter- 
  

   part 
  in 
  Trinucleus 
  and 
  Dionide, 
  although 
  not 
  so 
  well 
  devel- 
  

   oped 
  in 
  these 
  genera. 
  The 
  head 
  is 
  also 
  comparatively 
  longer 
  

   and 
  larger, 
  both 
  features 
  being 
  decidedly 
  larval. 
  It 
  is 
  the 
  

   only 
  family 
  known 
  in 
  which 
  functional 
  visual 
  spots, 
  or 
  ocelli, 
  

   are 
  situated 
  on 
  the 
  fixed 
  cheeks. 
  The 
  young 
  Trimicleus 
  has 
  

   similar 
  eye 
  spots, 
  or 
  ocelli. 
  The 
  great 
  number 
  of 
  free 
  seg- 
  

   ments 
  in 
  the 
  Harpedidae 
  is 
  another 
  primitive 
  character, 
  

   although 
  the 
  cephalon 
  (in 
  Harpes) 
  still 
  remains 
  larger 
  than 
  

   the 
  thorax 
  and 
  pygidium. 
  

  

  Family 
  III. 
  Trinucleld^ 
  Barrande. 
  

  

  Cephalon 
  larger 
  than 
  the 
  thorax 
  or 
  pygidium 
  ; 
  genal 
  angles 
  

   produced 
  into 
  spines. 
  Free 
  cheeks 
  continuous, 
  almost 
  wholly 
  

   ventral, 
  carrying 
  the 
  genal 
  spines 
  ; 
  suture 
  marginal 
  or 
  submar- 
  

   ginal. 
  Paired 
  simple 
  eyes 
  or 
  ocelli 
  generally 
  absent 
  in 
  adult 
  

   forms 
  ; 
  compound 
  eyes 
  wanting. 
  Segments 
  of 
  thorax 
  five 
  or 
  

   six 
  in 
  number, 
  with 
  grooved 
  pleura. 
  Pygidium 
  triangular; 
  

   margin 
  entire 
  ; 
  axis 
  with 
  a 
  number 
  of 
  annulations 
  ; 
  limb 
  

   grooved. 
  Ordovician 
  and 
  Silurian. 
  

  

  Including 
  the 
  genera 
  and 
  subgenera 
  Trinucleus 
  Lhwyd, 
  

   Ampyx 
  Dalman, 
  Dionide 
  Barrande, 
  Endymionia 
  ? 
  Billings, 
  

   Lonchodomus 
  Angelin, 
  Raphioplwrus 
  Angelin, 
  and 
  Salteria 
  ? 
  

   W. 
  Thompson. 
  

  

  The 
  leading 
  genera 
  of 
  this 
  family 
  form 
  a 
  tolerably 
  homoge- 
  

   neous 
  group, 
  although 
  each 
  has 
  sometimes 
  been 
  recognized 
  as 
  

   characterizing 
  a 
  separate 
  family. 
  Trinucleus 
  and 
  Dionide 
  

   have 
  a 
  broad 
  pitted 
  border, 
  but 
  this 
  hardly 
  seems 
  of 
  sufficient 
  

   importance 
  to 
  remove 
  them 
  far 
  from 
  Ampyx, 
  since 
  the 
  three 
  

   genera 
  agree 
  in 
  nearly 
  all 
  important 
  structural 
  details, 
  as 
  the 
  

   extent 
  and 
  character 
  of 
  the 
  free 
  cheeks, 
  the 
  glabella, 
  the 
  num- 
  

   ber 
  of 
  free 
  segments, 
  and 
  the 
  character 
  of 
  the 
  pygidium. 
  Lon- 
  

   chodomus 
  and 
  Raphiophorus 
  of 
  Angelin 
  are 
  commonly 
  ad- 
  

   mitted 
  as 
  subgenera 
  of 
  Ampyx. 
  

  

  

  