﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  191 
  

  

  I. 
  Paradoxince. 
  Including 
  Olenellus, 
  Ilolmia, 
  Mesonacis, 
  

   Elliptocephala, 
  Schmidtia, 
  Olenelloides, 
  Paradoxides, 
  Za- 
  

   canthoides, 
  and 
  Remopleurides. 
  Most 
  of 
  the 
  genera 
  are 
  dis- 
  

   tinguished 
  by 
  their 
  long 
  narrow 
  eyes, 
  often 
  extending 
  more 
  

   than 
  half 
  the 
  length 
  of 
  the 
  glabella, 
  bat 
  more 
  especially 
  by 
  

   the 
  rudimentary 
  character 
  of 
  the 
  pygidium. 
  In 
  Olenellus, 
  

   the 
  pygidium 
  is 
  a 
  long 
  telson-like 
  spine. 
  In 
  Ilolmia, 
  Meso- 
  

   nacis, 
  Elliptocephala, 
  and 
  Schmidtia, 
  it 
  is 
  reduced 
  to 
  a 
  small 
  

   plate 
  without 
  distinct 
  segmental 
  divisions. 
  In 
  Paradoxides, 
  

   Zacanthoides, 
  and 
  Remopleurides, 
  the 
  axis 
  may 
  show 
  from 
  one 
  

   to 
  five 
  annulations, 
  while 
  the 
  limb 
  may 
  carry 
  two 
  or 
  three 
  pairs 
  

   of 
  spines 
  or 
  may 
  be 
  entire. 
  In 
  Olenellus 
  and 
  Holmia, 
  true 
  facial 
  

   sutures 
  have 
  been 
  denied 
  by 
  some 
  authors, 
  but 
  in 
  their 
  place 
  

   false 
  sutures 
  are 
  recognized. 
  They 
  are, 
  however, 
  evidently 
  

   real 
  sutures 
  in 
  a 
  condition 
  of 
  symphysis, 
  which 
  often 
  occurs 
  in 
  

   Phacops, 
  Proetus, 
  Phillipsia, 
  etc. 
  Otherwise, 
  these 
  genera 
  

   would 
  violate 
  the 
  first 
  principle 
  of 
  trilobite 
  structure, 
  in 
  not 
  

   having 
  the 
  compound 
  eyes 
  on 
  the 
  free 
  cheek 
  pieces. 
  Olenel- 
  

   loides 
  is 
  a 
  very 
  striking 
  form, 
  but 
  its 
  pygidium 
  is 
  unknown, 
  

   and 
  the 
  head 
  structure 
  is 
  obscure. 
  The 
  elongate 
  cephalon 
  is 
  a 
  

   decidedly 
  larval 
  feature, 
  and 
  the 
  genal 
  and 
  interocular 
  (?) 
  

   spines 
  strongly 
  suggest 
  its 
  immature 
  condition, 
  and 
  point 
  to 
  

   the 
  possibility 
  of 
  its 
  being 
  the 
  young 
  of 
  Olenellus 
  or 
  a 
  related 
  

   form. 
  

  

  There 
  has 
  been 
  much 
  discussion 
  as 
  to 
  the 
  synonymy 
  and 
  

   value 
  of 
  most 
  of 
  the 
  names 
  proposed 
  as 
  genera 
  or 
  subgenera 
  

   in 
  this 
  group. 
  Paradoxides, 
  Remopleurides, 
  and 
  Zacan- 
  

   thoides 
  are 
  about 
  the 
  only 
  ones 
  that 
  have 
  escaped 
  severe 
  criti- 
  

   cism 
  in 
  recent 
  years. 
  Taking 
  the 
  type 
  of 
  each 
  of 
  the 
  others, 
  

   it 
  is 
  found 
  that 
  Elliptocephala 
  (1844) 
  was 
  based 
  on 
  the 
  species 
  

   E. 
  asaphoides 
  Emmons, 
  Olenellus 
  (1862) 
  on 
  O. 
  Thompsoni 
  

   Hall, 
  Mesonacis 
  (1885) 
  on 
  M. 
  vermontana 
  Hall 
  sp., 
  Holmia 
  

   (1890) 
  on 
  H. 
  Kjerulfi 
  Linnarsson 
  sp., 
  Schmidtia 
  (1890) 
  

   on 
  S. 
  Mickwitzi 
  Schmidt 
  sp., 
  and 
  Olenelloides 
  (1894) 
  on 
  

   0. 
  armatus 
  Peach. 
  Some 
  of 
  these 
  names 
  are 
  generally 
  

   recognized 
  as 
  subgenera 
  of 
  Olenellus 
  {Mesonacis, 
  Holmia., 
  

   Olenelloides), 
  while 
  others 
  are 
  considered 
  as 
  synonyms 
  {Ellip- 
  

   tocephala, 
  Schmidtia). 
  The 
  early 
  genera 
  were 
  described 
  from 
  

   very 
  incomplete 
  material, 
  and 
  therefore 
  lacked 
  sufficient 
  diag- 
  

   nostic 
  characters 
  to 
  clearly 
  define 
  them. 
  At 
  the 
  present 
  time, 
  

   nearly 
  or 
  quite 
  entire 
  specimens 
  representing 
  the 
  type 
  species 
  

   are 
  known, 
  and 
  it 
  is 
  possible 
  to 
  compare 
  all 
  the 
  essential 
  features 
  

   with 
  some 
  degree 
  of 
  accuracy. 
  The 
  main 
  characters 
  offering 
  

   the 
  greatest 
  variation 
  are 
  (1) 
  the 
  number 
  of 
  thoracic 
  segments 
  

   and 
  (2) 
  their 
  specialization 
  into 
  groups, 
  (3) 
  the 
  relative 
  devel- 
  

   opment 
  of 
  the 
  third 
  free 
  segment, 
  (4) 
  the 
  number 
  and 
  position 
  

   of 
  the 
  spine-bearing 
  segments, 
  (5) 
  the 
  form 
  of 
  the 
  pygidium, 
  

  

  