﻿308 
  Beecher 
  — 
  Structure 
  and 
  Appendages 
  of 
  Trinucleus. 
  

  

  and 
  made 
  one 
  of 
  the 
  bases 
  for 
  a 
  division 
  of 
  the 
  genus 
  into 
  

   two 
  sections 
  or 
  genera 
  — 
  Trlnucleus 
  proper 
  and 
  Tetraspis. 
  

   These 
  divisions 
  were 
  accepted 
  by 
  Salter, 
  but 
  later 
  were 
  thor- 
  

   oughly 
  discussed, 
  and 
  rejected 
  by 
  Barrande 
  (I. 
  <?., 
  p. 
  617), 
  upon 
  

   valid 
  grounds. 
  Nicholson 
  and 
  Etheridge,* 
  in 
  1879, 
  reviewed 
  

   these 
  facts 
  at 
  some 
  length, 
  and 
  gave 
  original 
  figures 
  illustrat- 
  

   ing 
  the 
  ocular 
  tubercle 
  and 
  eye-line. 
  They 
  also 
  agree 
  with 
  

   Barrande 
  in 
  recognizing 
  them 
  as 
  clearly 
  adolescent 
  char- 
  

   acters. 
  

  

  The 
  justice 
  of 
  these 
  conclusions 
  is 
  substantiated, 
  and 
  

   additional 
  results 
  are 
  reached, 
  from 
  the 
  study 
  of 
  a 
  series 
  of 
  

   Trinucleus 
  concentricus 
  Eaton, 
  found 
  associated 
  with 
  Triar- 
  

   thrus 
  Becki 
  Green, 
  in 
  the 
  Utica 
  slate, 
  near 
  Rome, 
  New 
  York. 
  

   The 
  remarkable 
  preservation 
  of 
  the 
  fossils 
  at 
  this 
  locality, 
  

   has 
  already 
  afforded 
  a 
  means 
  of 
  determining 
  all 
  the 
  principal 
  

   details 
  of 
  the 
  ventral 
  structure 
  of 
  the 
  trilobite 
  genus 
  Triar- 
  

   thrus, 
  and 
  there 
  is 
  now 
  distinct 
  evidence 
  as 
  to 
  the 
  nature 
  of 
  

   the 
  appendages 
  in 
  another 
  type 
  — 
  Trinucleus, 
  as 
  well 
  as 
  to 
  the 
  

   probable 
  significance 
  of 
  the 
  so-called 
  " 
  eye-tubercle." 
  

  

  As 
  compared 
  with 
  Triarthrus, 
  specimens 
  of 
  Trinucleus 
  

   are 
  not 
  very 
  common 
  at 
  this 
  locality, 
  and, 
  although 
  more 
  than 
  

   fifty 
  individuals 
  of 
  the 
  latter 
  have 
  been 
  obtained 
  from 
  the 
  

   collections 
  presented 
  to 
  the 
  Yale 
  Museum 
  by 
  Professor 
  Marsh, 
  

   not 
  more 
  than 
  half 
  a 
  dozen 
  of 
  these 
  are 
  adult 
  specimens, 
  and 
  

   but 
  three 
  show 
  any 
  appendages. 
  Young 
  specimens 
  of 
  all 
  

   ages 
  occur, 
  from 
  about 
  l 
  mm 
  across 
  the 
  cephalon 
  upwards, 
  and 
  

   in 
  all 
  the 
  eye-line 
  and 
  eye 
  tubercle 
  are 
  present 
  until 
  a 
  width 
  

   of 
  nearly 
  5 
  mm 
  is 
  attained, 
  when 
  in 
  the 
  present 
  species 
  these 
  

   features 
  dwindle 
  and 
  disappear, 
  leaving 
  no 
  discoverable 
  traces 
  

   in 
  the 
  adult. 
  

  

  Two 
  cephala 
  of 
  young 
  individuals, 
  without 
  the 
  free 
  cheeks, 
  

   are 
  shown 
  enlarged 
  in 
  figures 
  1 
  and 
  2 
  of 
  Plate 
  III. 
  Figure 
  2 
  

   represents 
  a 
  specimen 
  before 
  the 
  appearance 
  of 
  the 
  perforate 
  

   border, 
  and 
  figure 
  1 
  gives 
  a 
  later 
  stage, 
  having 
  two 
  rows 
  of 
  

   perforations 
  around 
  the 
  head. 
  On 
  both 
  specimens 
  the 
  eye- 
  

   line 
  is 
  clearly 
  shown, 
  extending 
  somewhat 
  obliquely 
  backward 
  

   from 
  the 
  anterior 
  lobe 
  of 
  the 
  glabella 
  to 
  the 
  central 
  area 
  of 
  the 
  

   fixed 
  cheeks, 
  enlarging 
  slightly, 
  and 
  terminating 
  in 
  a 
  rounded 
  

   node 
  or 
  tubercle 
  (a, 
  a, 
  figure 
  2). 
  

  

  In 
  seeking 
  for 
  homologous 
  features 
  in 
  other 
  trilobites, 
  the 
  

   genera 
  Harpes 
  and 
  Harpides 
  are 
  immediately 
  suggested, 
  since 
  

   they 
  have 
  similar 
  ocular 
  ridges 
  extending 
  from 
  the 
  sides 
  of 
  

   the 
  glabella, 
  and 
  ending 
  in 
  a 
  tubercle, 
  which, 
  in 
  Harpes, 
  con- 
  

   tains 
  from 
  one 
  to 
  three 
  eye-spots, 
  as 
  determined 
  by 
  Barrande. 
  

   They 
  further 
  agree 
  in 
  having 
  these 
  visual 
  organs 
  on 
  the 
  

  

  * 
  Monograph 
  of 
  the 
  Silurian 
  Fossils 
  of 
  the 
  Girvan 
  District 
  in 
  Ayrshire, 
  

   Fasc. 
  II, 
  1879. 
  

  

  