﻿192 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  (6) 
  the 
  presence 
  or 
  absence 
  of 
  interocular 
  spines, 
  and 
  (7) 
  the 
  

   form 
  of 
  the 
  cephalon. 
  A 
  simple 
  variation 
  in 
  any 
  one 
  of 
  these 
  

   would 
  not 
  necessarily 
  imply 
  more 
  than 
  a 
  specific 
  difference, 
  

   but 
  the 
  genera 
  here 
  mentioned 
  exhibit 
  marked 
  changes 
  in 
  all 
  

   or 
  nearly 
  all 
  of 
  these 
  characters, 
  and 
  in 
  any 
  family 
  should 
  

   receive 
  recognition. 
  Olenellus, 
  Mesonacis, 
  and 
  Elliptocephala 
  

   are 
  more 
  closely 
  related 
  than 
  the 
  other 
  forms, 
  and 
  probably 
  

   have 
  only 
  a 
  subgeneric 
  value 
  under 
  Elliptocephala. 
  In 
  the 
  

   first 
  form 
  with 
  fourteen 
  thoracic 
  segments, 
  the 
  third 
  is 
  greatly 
  

   enlarged 
  and 
  the 
  fifteenth 
  is 
  the 
  spiniform, 
  telson-like 
  pygid- 
  

   ium. 
  In 
  Mesonacis 
  with 
  twenty-six 
  thoracic 
  segments, 
  the 
  

   third 
  is 
  somewhat 
  enlarged, 
  and 
  behind 
  the 
  narrow 
  spine- 
  

   bearing 
  fifteenth 
  segment 
  there 
  are 
  eleven 
  others 
  without 
  

   spines, 
  followed 
  by 
  the 
  small 
  plate-like 
  pygidium. 
  In 
  Ellip- 
  

   tocephala 
  with 
  eighteen 
  thoracic 
  segments, 
  the 
  cephalon 
  is 
  

   broader, 
  the 
  third 
  segment 
  is 
  not 
  enlarged 
  except 
  in 
  the 
  young, 
  

   and 
  the 
  fourteenth 
  to 
  eighteenth 
  segments 
  are 
  narrower 
  and 
  

   spine-bearing. 
  

  

  II. 
  Oryctocephaliim. 
  Including 
  Oryctocephalus, 
  Ctenopyge, 
  

   Olenoides, 
  and 
  Parabolina, 
  with 
  large 
  pygidia 
  and 
  all 
  but 
  the 
  last 
  

   one 
  or 
  two 
  pleural 
  elements 
  continued 
  into 
  spines; 
  also 
  Eu- 
  

   rycare, 
  Angelina, 
  Peltnra, 
  and 
  Prolopeltura, 
  with 
  smaller 
  and 
  

   shorter 
  pygidia 
  and 
  denticulaiionsof 
  the 
  margins 
  corresponding 
  

   to 
  the 
  pleural 
  divisions. 
  

  

  III. 
  Olenince. 
  Including 
  Olenus, 
  Agraulus, 
  Liostracus, 
  

   Acerocare, 
  Ptychoparia, 
  Solenopleura, 
  Ptychaspis, 
  Lepto- 
  

   plastus, 
  Loganellus, 
  Sphwropthalmus, 
  Parabolinella, 
  Boeckia, 
  

   Proceratopyge, 
  Cer 
  atopy 
  ge, 
  Protypus, 
  Ellipsocephalus, 
  Sao, 
  

   and 
  Triarthrus. 
  All 
  these 
  genera 
  have 
  small 
  or 
  medium-sized 
  

   pygidia, 
  with 
  from 
  two 
  to 
  eight 
  annulations 
  in 
  the 
  axis. 
  Eyes 
  

   medium 
  to 
  small, 
  at 
  the 
  ends 
  of 
  distinct 
  eye 
  lines 
  in 
  all 
  but 
  the 
  

   latest 
  genera, 
  which 
  preserve 
  this 
  character 
  only 
  during 
  the 
  

   young 
  stages. 
  Thoracic 
  segments 
  from 
  eleven 
  to 
  eighteen. 
  

  

  IY. 
  Dikelocephalinm. 
  Including 
  Dikelocephalus, 
  Asaphe- 
  

   lina, 
  and 
  Crepicephalus. 
  Eight 
  or 
  nine 
  thoracic 
  segments. 
  

   Pygidium 
  wide, 
  with 
  the 
  posterior 
  lateral 
  portion 
  often 
  pro- 
  

   duced 
  into 
  broad 
  spine-like 
  extensions. 
  Dikelocephalus 
  is 
  in 
  

   many 
  ways 
  related 
  to 
  Ogygia 
  and 
  Asaphus. 
  

  

  Family 
  VI. 
  AsAPHiDiE 
  Emmrich. 
  

  

  Cephalon 
  and 
  pygidium 
  well 
  developed 
  ; 
  glabella 
  often 
  

   obscurely 
  limited. 
  Free 
  cheeks 
  usually, 
  separate. 
  Facial 
  

   sutures 
  extending 
  forward 
  from 
  the 
  posterior 
  edge 
  of 
  the 
  

   cephalon 
  within 
  the 
  genal 
  angles, 
  and 
  cutting 
  the 
  lateral 
  or 
  

   anterior 
  margins, 
  occasionally 
  uniting 
  in 
  front 
  of 
  the 
  glabella. 
  

  

  