﻿Beecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilobites. 
  197 
  

  

  Most 
  of 
  the 
  forms 
  of 
  this 
  family 
  are 
  above 
  the 
  average 
  size 
  

   of 
  trilobites, 
  and 
  several 
  species 
  are 
  among 
  the 
  largest 
  of 
  the 
  

   class. 
  They 
  are 
  all 
  thin-shelled, 
  and 
  were 
  loosely 
  articulated, 
  

   so 
  that 
  entire 
  specimens 
  are 
  extremely 
  rare. 
  

  

  A 
  great 
  diversity 
  is 
  shown 
  in 
  the 
  form 
  and 
  lobation 
  of 
  the 
  

   glabella. 
  In 
  Lichas 
  (sens, 
  str.), 
  Platymetopus, 
  and 
  Ze-iolichas, 
  

   the 
  anterior 
  lobe 
  dominates 
  and 
  is 
  continuous 
  with 
  the 
  axis. 
  

   In 
  Hoplolichas 
  and 
  Homolichas, 
  the 
  lateral 
  lobes 
  are 
  strongly 
  

   defined, 
  and 
  each 
  is 
  nearly 
  equal 
  in 
  size 
  to 
  the 
  central 
  lobe. 
  

   Dicranogmus, 
  Onc/wlichas, 
  Co?iolic/ias, 
  Jletopias, 
  Arctinuras, 
  

   and 
  Arges 
  show 
  the 
  lateral 
  lobes 
  divided 
  transversely 
  into 
  two 
  

   or 
  three 
  smaller 
  ones. 
  Lastly 
  in 
  Ceratolichas, 
  and 
  more 
  

   especially 
  in 
  Terataspis, 
  the 
  central 
  lobe 
  becomes 
  a 
  prominent 
  

   ovoid 
  or 
  globular 
  extension. 
  These 
  variations 
  evidently 
  indi- 
  

   cate 
  differences 
  in 
  the 
  relative 
  development 
  of 
  the 
  appendages 
  

   and 
  organs 
  of 
  the 
  head, 
  and 
  therefore 
  are 
  of 
  considerable 
  mor- 
  

   phological 
  importance. 
  

  

  The 
  pygidium 
  is 
  composed 
  of 
  few 
  distinct 
  segments. 
  The 
  

   annulated 
  portion 
  of 
  the 
  axis 
  is 
  generally 
  short, 
  and 
  the 
  denta- 
  

   tions 
  on 
  the 
  border 
  of 
  the 
  limb, 
  corresponding 
  to 
  the 
  pleural 
  

   grooves, 
  range 
  from 
  two 
  to 
  four 
  on 
  each 
  side. 
  Leiolichas 
  is 
  

   the 
  only 
  form 
  which 
  has 
  an 
  entire 
  pygidial 
  margin. 
  

  

  Family 
  X. 
  Acidaspim: 
  Barrande. 
  

  

  Dorsal 
  shield 
  spinose. 
  Cephalon 
  transversely 
  semi-elliptical, 
  

   quadrate, 
  or 
  trapezoidal 
  : 
  genal 
  angles 
  spiniform. 
  Glabella 
  

   with 
  one 
  large 
  median 
  axial 
  lobe 
  and 
  two 
  or 
  three 
  lateral 
  

   lobes. 
  Free 
  cheeks 
  large, 
  separate. 
  Sutures 
  extending 
  from 
  

   within 
  the 
  genal 
  angles 
  abruptly 
  inward 
  to 
  the 
  eyes, 
  and 
  then 
  

   forward, 
  cutting 
  the 
  anterior 
  margin 
  each 
  side 
  of 
  the 
  glabella. 
  

   Eyes 
  small, 
  often 
  prominent. 
  Thorax 
  of 
  eight 
  to 
  ten 
  segments, 
  

   with 
  ridged 
  pleura 
  extended 
  into 
  hollow 
  spines. 
  Pygidium 
  

   usually 
  small, 
  with 
  spinous 
  margin. 
  Ordovician 
  to 
  Devonian. 
  

  

  Including 
  the 
  genera 
  and 
  subgenera 
  Acidaspis 
  Murchison, 
  

   Ancyropyge 
  Clarke, 
  Ceratocephala 
  Warder, 
  Dicixmurus 
  Con- 
  

   rad, 
  Odontopleura 
  Emmrich, 
  and 
  Selenopeltis 
  Corda. 
  

  

  In 
  this 
  family 
  and 
  the 
  Lichadidse 
  is 
  shown 
  the 
  highest 
  

   expression 
  of 
  differentiation 
  and 
  specialization 
  of 
  the 
  Opistho- 
  

   paria. 
  The 
  primitive 
  pentamerous 
  lobation 
  of 
  the 
  axis 
  of 
  the 
  

   cranidium 
  is 
  entirely 
  obscured, 
  and 
  is 
  only 
  clearly 
  seen 
  in 
  the 
  

   protaspis 
  and 
  early 
  nepionic 
  stages. 
  These 
  two 
  families 
  are 
  

   very 
  closely 
  related, 
  the 
  chief 
  differences 
  being 
  noticed 
  in 
  the 
  

   size 
  and 
  character 
  of 
  the 
  pygidium, 
  and 
  the 
  ribbed 
  or 
  grooved 
  

  

  Am. 
  Jouk. 
  Sci. 
  — 
  Fourth 
  Series, 
  Yol. 
  Ill, 
  No. 
  15. 
  — 
  M^rch, 
  1897. 
  

   14 
  

  

  