﻿202 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  Family 
  XIV. 
  Phacopidje 
  Salter. 
  

  

  Glabella 
  tumid, 
  widest 
  in 
  front. 
  Free 
  cheeks 
  continuous^ 
  

   united 
  anteriorly. 
  Suture 
  extending 
  from 
  in 
  front 
  of 
  the 
  

   genal 
  angles 
  inward 
  to 
  the 
  eyes, 
  and 
  then 
  forward 
  around 
  the 
  

   glabella. 
  Eyes 
  generally 
  large, 
  and 
  always 
  with 
  distinct 
  facets, 
  

   schizochroal. 
  Thorax 
  with 
  eleven 
  segments, 
  with 
  grooved 
  

   pleura. 
  Pygidium 
  usually 
  large 
  and 
  of 
  many 
  segments 
  ; 
  limb 
  

   ribbed 
  ; 
  margin 
  entire 
  or 
  dentate. 
  Ordovician 
  to 
  Devonian. 
  

  

  Including 
  the 
  genera 
  and 
  subgenera 
  jP/iacops 
  Emmrich? 
  

   Acaste 
  Gold 
  fuss, 
  Chasmops 
  McCoy, 
  Coronura 
  Hall, 
  Cory 
  

   cephalus 
  Hall 
  and 
  Clarke, 
  Cryphceus 
  Green, 
  Dalmanites 
  

   Emmrich 
  (Hausmannia 
  Hall 
  and 
  Clarke), 
  Homalops 
  Remele 
  

   and 
  Dames, 
  Monorachos 
  Schmidt, 
  Odontocephalus 
  Conrad,. 
  

   Pterygometopus 
  Schmidt, 
  Symphoria 
  Clarke, 
  and 
  Trimero- 
  

   cephalus 
  McCoy. 
  

  

  The 
  last 
  family 
  of 
  trilobites 
  comprises 
  forms 
  which 
  are 
  com- 
  

   monly 
  believed 
  to 
  be 
  the 
  most 
  highly 
  organized 
  of 
  the 
  class, 
  

   and 
  certain 
  it 
  is 
  that 
  a 
  high 
  degree 
  of 
  organization 
  is 
  mani- 
  

   fested. 
  Some 
  of 
  the 
  characters 
  may 
  be 
  considered 
  as 
  progres- 
  

   sive, 
  while 
  others 
  are 
  larval 
  or 
  possessed 
  chiefly 
  by 
  the 
  most 
  

   primitive 
  families, 
  and 
  are 
  therefore 
  to 
  be 
  looked 
  upon 
  as 
  

   regressive. 
  Schizochroal 
  eyes 
  occur 
  in 
  no 
  other 
  family, 
  and 
  

   this 
  feature 
  is 
  apparently 
  indeterminate. 
  The 
  complete 
  union 
  

   of 
  the 
  free 
  cheeks, 
  carrying 
  the 
  doublure 
  of 
  the 
  sides 
  and 
  front 
  

   of 
  the 
  cephalon, 
  can 
  be 
  best 
  homologized 
  with 
  similar 
  struc- 
  

   tures 
  in 
  some 
  of 
  the 
  lowest 
  genera, 
  and 
  is 
  a 
  retention 
  of 
  the 
  

   complete 
  occular 
  segment. 
  The 
  glabella, 
  though 
  considerably 
  

   enlarged 
  anteriorly, 
  does 
  not 
  attain 
  the 
  degree 
  of 
  specialization 
  

   shown 
  in 
  Lichas 
  and 
  Acidaspis. 
  Only 
  (Jhasmops 
  and 
  related 
  

   forms 
  {Monorachos, 
  Homalops, 
  Si/mphoria, 
  and 
  Coronura) 
  

   have 
  separate 
  or 
  accessory 
  lobes. 
  The 
  margin 
  of 
  the 
  cephalon 
  

   shows 
  even 
  greater 
  diversity 
  than 
  in 
  any 
  other 
  family. 
  It 
  may 
  

   be 
  plain 
  (Phacops, 
  Cryphceus), 
  notched 
  (Cory 
  cephalus), 
  den- 
  

   ticulated 
  (Odontocephalus), 
  or 
  extended 
  in 
  front 
  as 
  a 
  spinose, 
  

   spatulate, 
  or 
  dentate 
  process 
  (Dalmanites 
  nasutus 
  Conrad, 
  JD. 
  

   tridens 
  Hall, 
  etc.). 
  The 
  pygidium 
  has 
  a 
  range 
  almost 
  as 
  great, 
  

   though 
  in 
  this 
  respect 
  it 
  is 
  equalled 
  in 
  the 
  Lichadidse, 
  Acidas- 
  

   pidse, 
  and 
  some 
  of 
  the 
  Olenidse. 
  In 
  America, 
  the 
  section 
  typi- 
  

   lied 
  by 
  Dalmanites 
  culminated 
  during 
  the 
  lower 
  Devonian. 
  

   Not 
  only 
  are 
  the 
  largest 
  forms 
  found 
  here 
  (Coronura 
  diurus 
  

   Green, 
  C. 
  myrmecophorus 
  Green, 
  J), 
  tridens 
  Hall, 
  etc.), 
  but 
  

   also 
  the 
  most 
  ornate 
  and 
  specialized, 
  as 
  Corycephalus, 
  Odonto- 
  

   cephalus, 
  and 
  Coronura. 
  

  

  Yale 
  Museum, 
  New 
  Haven, 
  Conn., 
  December 
  28th, 
  1896. 
  

  

  

  