﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  183 
  

  

  next 
  investigator 
  may 
  show 
  belong 
  to 
  another 
  or 
  to 
  several 
  

   genera. 
  Our 
  ideas 
  of 
  a 
  genus 
  are 
  naturally 
  based 
  mainly 
  upon 
  

   the 
  species 
  with 
  which 
  we 
  are 
  most 
  familiar. 
  Until 
  forced 
  to 
  

   make 
  authoritative 
  comparative 
  statements, 
  it 
  does 
  not 
  occur 
  to 
  

   one 
  that 
  the 
  type 
  of 
  the 
  genus 
  under 
  consideration 
  may 
  be 
  

   quite 
  different. 
  An 
  American 
  student's 
  conception 
  of 
  Homa- 
  

   lonotus 
  will 
  probably 
  be 
  formed 
  largely 
  upon 
  the 
  species 
  com- 
  

   monly 
  known 
  as 
  H. 
  delphinocephahts 
  Green, 
  from 
  the 
  Niagara, 
  

   and 
  II. 
  DeKayi 
  Green, 
  from 
  the 
  Hamilton. 
  The 
  first 
  time 
  

   the 
  type 
  of 
  the 
  genus, 
  II. 
  Knighli 
  Murchison, 
  is 
  seen 
  he 
  will 
  

   be 
  puzzled 
  to 
  place 
  it. 
  Similar 
  examples 
  could 
  be 
  multiplied 
  

   indefinitely, 
  and 
  only 
  show 
  that 
  the 
  type 
  must 
  be 
  taken 
  as 
  the 
  

   ultimate 
  unit 
  of 
  comparison. 
  

  

  Diagnoses 
  and 
  Discussions 
  of 
  Orders 
  and 
  Families. 
  

  

  Order 
  A. 
  HYPOPARIA, 
  n. 
  ord. 
  

  

  (Wo 
  under, 
  and 
  Trapad 
  cheek 
  piece.) 
  

  

  Free 
  cheeks 
  forming 
  a 
  continuous 
  marginal 
  ventral 
  plate 
  of 
  

   the 
  cephalon, 
  and 
  in 
  some 
  forms 
  also 
  extending 
  over 
  the 
  dorsal 
  

   side 
  at 
  the 
  genal 
  angles. 
  Suture 
  ventral, 
  marginal, 
  or 
  submar- 
  

   ginal. 
  Compound 
  paired 
  eyes 
  absent 
  ; 
  simple 
  eyes 
  may 
  occur 
  

   on 
  each 
  fixed 
  cheek, 
  singly 
  or 
  in 
  pairs. 
  

  

  Including 
  the 
  families 
  Agnostidse, 
  Harpedidse, 
  and 
  Trinu- 
  

   cleidse. 
  

  

  This 
  order 
  includes 
  the 
  groups 
  C 
  and 
  D, 
  or 
  the 
  Ampycini 
  

   and 
  Agnostini 
  of 
  Salter, 
  and 
  also 
  the 
  family 
  Harpedidse 
  of 
  

   that 
  author, 
  which 
  he 
  included 
  in 
  the 
  Asaphini. 
  The 
  special 
  

   recognition 
  of 
  characters, 
  however, 
  between 
  Salter's 
  groups 
  and 
  

   the 
  order 
  here 
  proposed 
  is 
  different. 
  

  

  The 
  presence 
  of 
  a 
  part 
  homologous 
  with 
  the 
  free 
  cheeks 
  of 
  

   other 
  trilobites 
  has 
  generally 
  been 
  more 
  or 
  less 
  overlooked 
  

   in 
  the 
  families 
  of 
  this 
  order. 
  In 
  Trinucleus, 
  Dionide, 
  and 
  

   Harpes, 
  the 
  sutures 
  have 
  been 
  correctly 
  determined 
  by 
  Bar- 
  

   rand 
  e. 
  3 
  Likewise, 
  Angelin 
  2 
  gave 
  the 
  right 
  structure 
  in 
  Anipyx, 
  

   but 
  in 
  Agnostus, 
  this 
  feature 
  has 
  escaped 
  notice. 
  The 
  exami- 
  

   nation 
  of 
  extensive 
  series 
  of 
  Agnostus, 
  in 
  the 
  National 
  Museum 
  

   and 
  in 
  the 
  Museum 
  of 
  Comparative 
  Zoology,* 
  has 
  proved 
  that 
  

   under 
  favorable 
  conditions 
  of 
  preservation 
  this 
  genus 
  shows 
  a 
  

   distinct 
  plate, 
  separated 
  from 
  the 
  cranidium 
  by 
  a 
  suture, 
  and 
  

   it 
  can 
  be 
  compared 
  only 
  with 
  the 
  free 
  cheeks 
  in 
  other 
  trilobites, 
  

  

  * 
  In 
  the 
  former, 
  through 
  the 
  courtesy 
  of 
  C. 
  D. 
  Walcott 
  and 
  C. 
  Schuchert, 
  and 
  

   in 
  the 
  latter, 
  of 
  A. 
  Agassiz 
  and 
  It. 
  T. 
  Jackson. 
  

  

  