﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  105 
  

  

  further 
  show 
  that 
  this 
  family 
  should 
  be 
  placed 
  at 
  the 
  beginning. 
  

   The 
  nepionic 
  Dalmanites, 
  with 
  seven 
  thoracic 
  segments, 
  has 
  a 
  

   head 
  structure 
  very 
  similar 
  to 
  the 
  adult 
  Cheirurus 
  (Eccopto- 
  

   cheile), 
  fig. 
  28, 
  thus 
  making 
  the 
  Cheiruridse 
  precede 
  the 
  Pha- 
  

   copidse. 
  The 
  arrangement 
  of 
  families 
  under 
  the 
  Proparia 
  

   accordingly 
  will 
  be 
  (1) 
  the 
  Encrinuridse 
  (Placoparia 
  and 
  

   Encrinurus, 
  Plate 
  III, 
  figs. 
  24, 
  25); 
  (2) 
  the 
  Calymenidse 
  

   {Galymene 
  and 
  Dipleura, 
  figs. 
  26, 
  27) 
  ; 
  (3) 
  the 
  Cheiruridae 
  

   {Cheirurus 
  (Eccoptocheile), 
  fig. 
  28) 
  ; 
  and 
  (4) 
  the 
  Phacopidas 
  

   {Dalmanites, 
  Chas?nops, 
  Acaste, 
  Phacops, 
  figs. 
  29-33). 
  

  

  The 
  sequence 
  of 
  families 
  in 
  the 
  most 
  primitive 
  order, 
  Hypo- 
  

   paria, 
  may 
  now 
  be 
  easily 
  disposed 
  of. 
  The 
  genera 
  are 
  so 
  aber- 
  

   rant 
  and 
  offer 
  such 
  conspicuous 
  differences 
  from 
  ordinary 
  

   trilobites, 
  that 
  it 
  was 
  considered 
  better 
  to 
  delay 
  their 
  disposi- 
  

   tion 
  until 
  the 
  variations 
  in 
  structure 
  governing 
  the 
  arrange- 
  

   ment 
  of 
  families 
  in 
  the 
  higher 
  orders 
  were 
  clearly 
  shown. 
  

   The 
  degree 
  of 
  specialization 
  of 
  the 
  glabella, 
  of 
  the 
  form 
  and 
  

   character 
  of 
  the 
  fixed 
  cheeks, 
  and 
  the 
  great 
  range 
  in 
  the 
  num- 
  

   ber 
  of 
  segments 
  in 
  the 
  thorax 
  and 
  pygidium 
  are 
  strong 
  evi- 
  

   dence 
  that 
  we 
  are 
  dealing 
  with 
  the 
  terminal 
  genera 
  of 
  the 
  

   order 
  which 
  must 
  have 
  attained 
  its 
  normal 
  development 
  in 
  pre- 
  

   Cambrian 
  times. 
  Agnostus 
  and 
  Microdiscus 
  have 
  so 
  many 
  

   protaspidian 
  and 
  larval 
  characters 
  that 
  they 
  must 
  be 
  considered 
  

   more 
  primitive 
  than 
  the 
  other 
  genera, 
  although 
  in 
  some 
  

   respects 
  they 
  show 
  a 
  high 
  degree 
  of 
  specialization 
  and 
  even 
  

   degeneration, 
  as 
  will 
  be 
  noticed 
  under 
  the 
  family 
  Agnostidse. 
  

   Moreover, 
  Harpes, 
  in 
  its 
  elongate 
  cephalon, 
  persistent 
  ocelli, 
  

   and 
  many 
  thoracic 
  segments, 
  is 
  also 
  quite 
  primitive. 
  Trinu- 
  

   cleus, 
  with 
  ocelli 
  only 
  present 
  in 
  larval 
  stages, 
  a 
  transverse 
  

   cephalon, 
  and 
  genal 
  spines 
  belonging 
  to 
  the 
  free 
  cheeks, 
  is 
  con- 
  

   siderably 
  higher 
  and 
  properly 
  comes 
  last 
  in 
  the 
  order, 
  thus 
  

   making 
  the 
  arrangement 
  of 
  families 
  as 
  follows: 
  (1) 
  Agnos- 
  

   tidse 
  (Agnostus, 
  Microdiscus, 
  Plate 
  III, 
  figs. 
  9, 
  10) 
  ; 
  (2) 
  Har- 
  

   pedidae 
  (Harpes, 
  fig. 
  11); 
  and 
  (3) 
  Trinucleidse 
  (Trinucleus, 
  

   Ampyx, 
  figs. 
  12, 
  13). 
  

  

  Diagnoses 
  and 
  Discussions. 
  

   Subclass 
  Trilobita. 
  

   Marine 
  Crustacea, 
  with 
  a 
  variable 
  number 
  of 
  metameres 
  ; 
  

   body 
  covered 
  with 
  a 
  hard 
  dorsal 
  shell 
  or 
  crust, 
  longitudinally 
  

   trilobate 
  from 
  the 
  defined 
  axis 
  and 
  pleura 
  ; 
  head, 
  thorax, 
  and 
  

   abdomen 
  distinct. 
  Head 
  covered 
  with 
  a 
  cephalic 
  shield 
  com- 
  

   posed 
  of 
  a 
  primitively 
  pentamerous 
  middle 
  piece, 
  the 
  cra- 
  

   nidium, 
  and 
  two 
  side 
  pieces, 
  or 
  free 
  cheeks, 
  which 
  may 
  be 
  

   Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  Ill, 
  No. 
  14 
  —February, 
  1897 
  

  

  