﻿Beechev 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  193 
  

  

  Eyes 
  smooth, 
  well 
  developed, 
  sometimes 
  of 
  very 
  large 
  size, 
  

   even 
  occupying 
  the 
  entire 
  surface 
  of 
  the 
  free 
  cheeks. 
  Thorax 
  

   generally 
  composed 
  of 
  eight 
  or 
  ten 
  segments, 
  but 
  varying 
  

   from 
  five 
  to 
  ten 
  ; 
  capable 
  of 
  enrollment. 
  Pygidium 
  large, 
  

   often 
  with 
  wide 
  doublure. 
  

  

  Cambrian, 
  Ordovician, 
  and 
  Silurian. 
  

   The 
  long 
  list 
  of 
  genera 
  in 
  this 
  family 
  may 
  easily 
  be 
  divided 
  

   into 
  two 
  sections, 
  which 
  are 
  often 
  recognized 
  as 
  of 
  family 
  

   rank. 
  

  

  I. 
  Asaphlcle. 
  Including 
  the 
  genera 
  and 
  subgenera 
  Asaphus 
  

   Brongniart 
  (= 
  Cryptonymus 
  Eichwald), 
  Asaphellus 
  Callaway, 
  

   Asaphiscus 
  Meek, 
  Barrandia 
  McCoy, 
  Basilicus 
  Salter, 
  

   Battiyurellus 
  Billings, 
  Bathyuriscus 
  Meek, 
  Bathyurus 
  Bil- 
  

   lings, 
  Bolbocephalus 
  Whitfield, 
  Brachyaspis 
  Salter, 
  Bronte- 
  

   opsis 
  W. 
  Thompson, 
  Dolichomeiopus 
  Angelin, 
  Gerasaphes 
  

   Clarke, 
  Ilolasaphus 
  Matthew, 
  Homalopecten 
  Salter, 
  Isotelus 
  

   DeKay, 
  Megalaspides 
  Brogger, 
  Megalaspis 
  Angelin, 
  JViobe 
  

   Angelin, 
  Ogygia 
  Brongniart, 
  Ogygiopsis 
  Walcott, 
  Phillip- 
  

   sinella 
  Novak, 
  Platypeltis 
  Callaway, 
  Ptychopyge 
  Angelin, 
  

   and 
  titygina 
  Salter. 
  

  

  This 
  is 
  a 
  tolerably 
  homogeneous 
  group, 
  although 
  some 
  of 
  

   the 
  Cambrian 
  forms 
  have 
  a 
  sufficiently 
  archaic 
  expression 
  to 
  

   make 
  them 
  seem 
  a 
  little 
  out 
  of 
  place 
  with 
  genera 
  of 
  so 
  pro- 
  

   nounced 
  a 
  family 
  type 
  as 
  Asaphus, 
  Niobe, 
  Ptychopyge, 
  Mega- 
  

   laspis, 
  and 
  Isotelus. 
  

  

  The 
  elements 
  of 
  the 
  glabella 
  are 
  generally 
  quite 
  obscure, 
  

   and 
  even 
  its 
  limits 
  cannot 
  be 
  clearly 
  made 
  out 
  in 
  late 
  genera, 
  

   as 
  Stygina 
  and 
  Asaphus. 
  The 
  segmental 
  nature 
  of 
  the 
  gla- 
  

   bella 
  is 
  clearly 
  shown 
  in 
  Ogygia, 
  Ogygiopsis, 
  Homalopecten, 
  

   Asaphellus, 
  Bronteopsis, 
  and 
  Bathyuriscus. 
  

  

  The 
  elements 
  of 
  the 
  pygidium 
  are 
  obscurely 
  marked 
  in 
  

   Brachyaspis 
  and 
  Isotelus. 
  Phillipsinella 
  is 
  a 
  very 
  small 
  

   form, 
  and 
  probably 
  the 
  young 
  of 
  an 
  Asaphus. 
  Barrandia, 
  

   Homalopecten, 
  and 
  Stygina 
  serve 
  as 
  transition 
  genera 
  to 
  the 
  

   Illsenidse. 
  

  

  II. 
  IiAjM^idm. 
  Including 
  the 
  genera 
  and 
  subgenera 
  Illce- 
  

   nus 
  Dalman, 
  yHJglina 
  Barrande 
  (=Oyclopyge 
  Cbrda), 
  Bumas- 
  

   tus 
  Murchison, 
  Dysplanus 
  Burmeister, 
  Ectillcenus 
  Salter, 
  

   Holocephalina 
  Salter, 
  Hydrolenas 
  Salter, 
  Illcenopsis 
  Salter, 
  

   Illcenurus 
  Hall, 
  A 
  T 
  ileus 
  Dalman, 
  Octillcenus 
  Salter, 
  Panderia 
  

   Tolborth, 
  Psilocephalus 
  Salter, 
  Symphy 
  suras 
  Goldfuss, 
  and 
  

   Thaleops 
  Conrad. 
  

  

  The 
  Illsenidse 
  form 
  a 
  much 
  more 
  compact 
  group 
  than 
  the 
  

   preceding, 
  characterized 
  by 
  having 
  an 
  epistomal 
  plate 
  and 
  by 
  

  

  