﻿200 
  Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  

  

  ambulations 
  are 
  obsolete. 
  In 
  Enorinurus, 
  the 
  eye 
  line 
  in 
  meet- 
  

   ing 
  and 
  joining 
  the 
  anterior 
  lobe 
  of 
  the 
  glabella 
  sometimes 
  

   gives 
  the 
  appearance 
  of 
  an 
  extra 
  lobe, 
  as 
  in 
  Ogygia 
  and 
  Para- 
  

   d 
  oxides. 
  

  

  Family 
  XII. 
  Calymenid^e 
  Brongniart. 
  

  

  Cephalon 
  somewhat 
  wider 
  than 
  long. 
  Fixed 
  cheeks 
  large 
  ; 
  

   genal 
  angles 
  rounded 
  or 
  produced 
  into 
  spines. 
  Glabella 
  nar- 
  

   rowing 
  anteriorly. 
  Free 
  cheeks 
  long, 
  separate, 
  usually 
  with, 
  a 
  

   free 
  plate, 
  the 
  epistoma, 
  between 
  the 
  anterior 
  extremities. 
  

   Sutures 
  extending 
  from 
  just 
  in 
  front 
  of 
  the 
  genal 
  angles, 
  con- 
  

   verging 
  anteriorly, 
  and 
  cutting 
  the 
  margins 
  separately. 
  Eyes 
  

   small 
  ; 
  visual 
  surface 
  seldom 
  preserved. 
  Thorax 
  of 
  thirteen 
  

   segments, 
  with 
  grooved 
  pleura. 
  Pygidium 
  of 
  from 
  six 
  to 
  

   fourteen 
  segments; 
  axis 
  tapering. 
  Ordovician 
  to 
  Devonian. 
  

  

  Including 
  the 
  genera 
  and 
  subgenera 
  Calymene 
  Brongniart, 
  

   Brongniartia 
  Salter, 
  Burmeisteria 
  Salter, 
  Calymenella 
  Ber- 
  

   geron, 
  Calymenopsis 
  Munier-Chalmas 
  and 
  Bergeron, 
  Dipleura 
  

   Green, 
  llomalonotus 
  Koenig, 
  Koenigia 
  (= 
  Homalonotus) 
  

   Salter, 
  Pharostoma 
  Corda, 
  Plcesiacomia 
  Corda, 
  Ptychome- 
  

   topus 
  Schmidt, 
  and 
  Trimerus 
  Green. 
  

  

  The 
  genera 
  of 
  this 
  family 
  naturally 
  cluster 
  around 
  the 
  two 
  

   leading 
  ones, 
  Calymene 
  and 
  Homalonotus. 
  Closely 
  related 
  to 
  

   the 
  first 
  are 
  Ptychometojpus, 
  Pharostoma, 
  Calymenopsis, 
  and 
  

   Calymenella, 
  all 
  agreeing 
  in 
  having 
  the 
  glabella 
  well 
  defined 
  

   and 
  marked 
  by 
  furrows 
  or 
  indentations 
  at 
  the 
  sides, 
  correspond- 
  

   ing 
  to 
  its 
  segmental 
  nature. 
  

  

  The 
  second 
  group, 
  including 
  Brongniartia, 
  Trimerus, 
  

   Homalonotus 
  (sens, 
  str.), 
  Plmsiacomia, 
  Dipleura, 
  and 
  Bur- 
  

   meisteria, 
  agree 
  in 
  having 
  a 
  low, 
  not 
  sharply 
  defined, 
  quadrate 
  

   glabella, 
  without 
  distinct 
  furrows 
  or 
  lobes. 
  In 
  general, 
  the 
  

   axis 
  of 
  the 
  thorax 
  and 
  pygidium 
  is 
  much 
  wider 
  than 
  in 
  the 
  first 
  

   group, 
  and 
  the 
  pygidium 
  is 
  more 
  elongate 
  and 
  often 
  pointed. 
  

  

  Family 
  XIII. 
  Cheirueidje 
  Salter. 
  

   Glabella 
  well 
  defined. 
  Free 
  cheeks 
  small, 
  sometimes 
  much 
  

   reduced. 
  Sutures 
  extending 
  from 
  in 
  front 
  of 
  the 
  genal 
  angles 
  

   inward 
  to 
  the 
  eyes, 
  and 
  then 
  obliquely 
  forward, 
  cutting 
  the 
  

   anterior 
  margin 
  in 
  front 
  and 
  each 
  side 
  of 
  the 
  glabella. 
  Eyes 
  

   usually 
  small. 
  Thorax 
  composed 
  of 
  from 
  nine 
  to 
  eighteen 
  seg- 
  

   ments, 
  generally 
  eleven 
  ; 
  pleura 
  often 
  extended 
  into 
  hollow 
  

   spines. 
  Pygidium. 
  small, 
  with 
  from 
  three 
  to 
  five 
  segments; 
  

   pleural 
  elements 
  commonly 
  produced 
  into 
  spines. 
  

  

  

  