﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  195 
  

  

  Arethusina 
  Barrtfnde, 
  Brachymetopus 
  McCoy, 
  Cehnus 
  An- 
  

   gelin, 
  Cordania 
  Clarke, 
  Crotalurus 
  Volborth, 
  Cyphaspis 
  Bur- 
  

   meister, 
  Dechenella 
  Kayser, 
  Griffithides 
  Portlock, 
  Phaeton- 
  

   ella 
  Novak, 
  Phillipsia 
  Portlock, 
  Prionopeltis 
  Corda, 
  

   Pseudophillipsia 
  Gemmellaro, 
  Schmidtella 
  Tschernyschew, 
  

   Tropidocoryphe 
  Novak, 
  and 
  Xiphogomium 
  Corda. 
  

  

  The 
  genera 
  of 
  this 
  family 
  readily 
  fall 
  into 
  a 
  series 
  express- 
  

   ing 
  more 
  or 
  less 
  closely 
  the 
  development 
  and 
  specialization 
  of 
  

   various 
  characters. 
  Arethusina 
  is 
  the 
  only 
  genus 
  retaining 
  

   the 
  archaic 
  eye 
  lines, 
  and 
  both 
  on 
  this 
  account 
  and 
  for 
  the 
  

   comparatively 
  forward 
  position 
  of 
  the 
  eyes 
  (itself 
  a 
  nepionic 
  

   character), 
  together 
  with 
  the 
  large 
  number 
  of 
  thoracic 
  seg- 
  

   ments, 
  it 
  stands 
  near 
  the 
  base 
  of 
  the 
  series. 
  

  

  The 
  eyes 
  gradually 
  approach 
  the 
  axis, 
  and 
  move 
  backward 
  

   through 
  the 
  genera 
  Tropidocoryphe, 
  Cyphaspis, 
  Proetus, 
  

   Prionopeltis, 
  Phillipsia, 
  and 
  Griffithides. 
  Concurrent 
  with 
  

   this 
  variation, 
  there 
  is 
  a 
  reduction 
  of 
  the 
  fixed 
  cheeks 
  and 
  

   extension 
  of 
  the 
  glabella. 
  In 
  Arethusina, 
  Tropidocoryphe, 
  

   Cordania, 
  and 
  Cyphaspis, 
  the 
  fixed 
  cheeks 
  are 
  about 
  the 
  size 
  

   of 
  the 
  free 
  cheeks, 
  and 
  occupy 
  a 
  large 
  portion 
  of 
  the 
  cranid- 
  

   ium. 
  They 
  are 
  more 
  reduced 
  in 
  Proetus 
  and 
  Prionopeltis, 
  

   and 
  in 
  Phillipsia 
  and 
  Griffithides, 
  they 
  form 
  only 
  a 
  narrow 
  

   border 
  to 
  the 
  glabella. 
  The 
  lobation 
  of 
  the 
  glabella 
  varies 
  

   greatly, 
  and 
  few 
  species 
  retain 
  evidences 
  of 
  its 
  original 
  seg- 
  

   mental 
  nature. 
  Some 
  Proetus 
  and 
  Dechenella 
  show 
  this 
  

   feature, 
  but 
  in 
  many 
  Phillipsia 
  and 
  Griffithides 
  the 
  elements 
  

   cannot 
  be 
  made 
  out. 
  In 
  Proetus, 
  there 
  is 
  often 
  a 
  small 
  acces- 
  

   sory 
  lobe 
  developed 
  at 
  the 
  ends 
  of 
  the 
  neck 
  ring, 
  which 
  is 
  only 
  

   of 
  interest 
  as 
  being 
  homologous 
  with 
  similar 
  lobes 
  in 
  many 
  of 
  

   the 
  Lichadidee 
  and 
  Acidaspidge, 
  where 
  they 
  often 
  become 
  very 
  

   conspicuous. 
  In 
  all 
  the 
  Proetidse, 
  the 
  oblique 
  lobes 
  of 
  the 
  

   fourth 
  annulus 
  of 
  the 
  glabella 
  are 
  also 
  important 
  in 
  this 
  con- 
  

   nection, 
  as 
  here 
  again 
  is 
  marked 
  the 
  inception 
  of 
  side 
  axial 
  

   lobes, 
  which 
  become 
  prominent 
  features 
  in 
  higher 
  genera, 
  

   indicating 
  greater 
  specialization 
  of 
  the 
  organs 
  and 
  appendages 
  

   of 
  the 
  head. 
  

  

  Family 
  VIII. 
  Bronteidje 
  Barrande. 
  

  

  Dorsal 
  shield 
  broadly 
  elliptical. 
  Cephalon 
  less 
  than 
  one- 
  

   third 
  the 
  entire 
  length 
  ; 
  glabella 
  rapidly 
  expanding 
  in 
  front, 
  

   with 
  faint 
  indications 
  of 
  lobes. 
  Free 
  cheeks 
  larger 
  than 
  the 
  

   fixed 
  cheeks. 
  Facial 
  sutures 
  extending 
  from 
  the 
  posterior 
  mar- 
  

   gin 
  just 
  behind 
  the 
  eyes 
  abruptly 
  inward 
  around 
  the 
  palpebral 
  

  

  