﻿310 
  Beecher 
  — 
  Structure 
  and 
  Appendages 
  of 
  Trinucleus. 
  

  

  side. 
  In 
  this 
  specimen 
  the 
  pyrite 
  which 
  replaced 
  the 
  chitinous 
  

   remains 
  of 
  the 
  animal 
  has 
  decomposed, 
  and 
  the 
  dorsal 
  crust 
  

   weathered 
  away, 
  exposing 
  below 
  the 
  stems 
  of 
  the 
  exopodites, 
  

   with 
  their 
  fringes 
  extending 
  over 
  the 
  entire 
  pleural 
  areas 
  on 
  

   both 
  sides. 
  A 
  pygidium, 
  with 
  three 
  attached 
  thoracic 
  segments, 
  

   from 
  another 
  entire 
  specimen 
  (figures 
  5 
  and 
  6), 
  preserves 
  the 
  

   details 
  of 
  the 
  appendages 
  in 
  the 
  most 
  perfect 
  and 
  satisfactory 
  

   manner. 
  As 
  both 
  halves 
  showed 
  essentially 
  the 
  same 
  extent 
  

   and 
  disposition 
  of 
  the 
  fringes 
  on 
  the 
  dorsal 
  side, 
  the 
  specimen 
  

   was 
  cut 
  in 
  two 
  along 
  the 
  center 
  of 
  the 
  axis, 
  and 
  the 
  left 
  side 
  

   was 
  then 
  imbedded 
  in 
  paraffine. 
  By 
  careful 
  preparation 
  the 
  

   appendages 
  were 
  exposed 
  from 
  the 
  ventral 
  side. 
  

  

  The 
  cephala 
  of 
  the 
  three 
  specimens 
  described 
  are 
  considerably 
  

   compressed, 
  and 
  from 
  them 
  a 
  very 
  imperfect 
  knowledge 
  of 
  the 
  

   mouth 
  parts 
  could 
  be 
  obtained, 
  so 
  that 
  this 
  information 
  must 
  be 
  

   left 
  to 
  future 
  discovery. 
  

  

  JEndopodites. 
  — 
  The 
  three 
  posterior 
  thoracic 
  endopodites 
  are 
  

   very 
  similar, 
  and 
  in 
  a 
  general 
  way 
  closely 
  resemble 
  those 
  of 
  

   Triarthrus 
  from 
  the 
  same 
  region 
  of 
  the 
  thorax. 
  They 
  are, 
  

   however, 
  comparatively 
  shorter 
  and 
  stouter, 
  and 
  could 
  not 
  be 
  

   extended 
  beyond 
  the 
  ends 
  of 
  the 
  pleura. 
  The 
  two 
  distal 
  

   joints 
  are 
  cylindrical, 
  with 
  well-marked 
  articular 
  surfaces 
  and 
  

   ridges. 
  The 
  joints 
  preceding 
  these 
  proximally 
  become 
  much 
  

   wider, 
  flattened, 
  and 
  produced 
  into 
  transverse 
  extensions 
  which 
  

   carry 
  large 
  tufts 
  of 
  setae 
  at 
  the 
  end, 
  as 
  also 
  does 
  the 
  end 
  of 
  the 
  

   last 
  joint 
  of 
  the 
  limb 
  (dactylopodite). 
  

  

  The 
  endopodites 
  on 
  the 
  pygidium 
  offer 
  no 
  conspicuous 
  dif- 
  

   ferences 
  from 
  those 
  just 
  described, 
  except 
  that 
  a 
  gradual 
  

   change 
  in 
  form 
  is 
  manifest 
  as 
  the 
  terminal 
  limbs 
  are 
  reached. 
  

   The 
  separate 
  endites 
  become 
  more 
  and 
  more 
  transversely 
  cylin- 
  

   drical, 
  until 
  the 
  whole 
  limb 
  appears 
  to 
  be 
  made 
  up 
  of 
  cylindri- 
  

   cal 
  segments 
  transverse 
  to 
  its 
  length. 
  A 
  similar 
  condition 
  was 
  

   observed 
  in 
  the 
  young 
  of 
  Triarthrus* 
  

  

  Exopodites. 
  — 
  These 
  seem 
  to 
  be 
  composed 
  of 
  slender 
  joints, 
  

   the 
  distal 
  exites 
  being 
  long 
  and 
  slightly 
  curved 
  outwards. 
  

   They 
  carry 
  very 
  long, 
  close 
  set, 
  overlapping, 
  lamellose 
  fringes, 
  

   which 
  evidently 
  had 
  a 
  branchial 
  function. 
  Some 
  of 
  the 
  lamel- 
  

   lae 
  are 
  spiniferous. 
  The 
  exopodites 
  become 
  shorter 
  on 
  the 
  

   pygidium, 
  and 
  apparently 
  are 
  represented 
  near 
  the 
  end 
  of 
  the 
  

   series 
  of 
  limbs 
  by 
  the 
  oval 
  plates 
  indicated 
  at 
  <?, 
  figure 
  6. 
  If 
  

   this 
  interpretation 
  is 
  correct, 
  the 
  posterior 
  exopodites 
  are 
  simple 
  

   flabella 
  attached 
  to 
  the 
  limbs, 
  as 
  in 
  Apus. 
  

  

  Both 
  Professors 
  A. 
  E. 
  Yerrill 
  and 
  S. 
  I. 
  Smith 
  agree 
  that 
  

   the 
  characters 
  of 
  the 
  appendages 
  in 
  Trinucleus 
  indicate 
  an 
  

   animal 
  of 
  burrowing 
  habit, 
  which 
  probably 
  lived 
  in 
  the 
  soft 
  

  

  * 
  This 
  Journal, 
  vol. 
  xlvii, 
  PI. 
  YII, 
  fig. 
  3, 
  April, 
  1894. 
  

  

  