﻿206 
  P. 
  E. 
  Raymond 
  — 
  Beecher^s 
  Classification 
  of 
  Trilobites. 
  

  

  Opisthopa?'ia. 
  

  

  31esonacida3. 
  — 
  The 
  suggestions 
  of 
  Swinnerton 
  in 
  regard 
  to 
  

   this 
  family 
  and 
  his 
  proposed 
  order 
  Protoparia 
  are 
  certainly 
  

   exceedingly 
  interesting 
  and 
  attractive, 
  and 
  offer 
  the 
  only 
  logical 
  

   explanation 
  so 
  far 
  put 
  forward 
  for 
  the 
  condition 
  seen 
  in 
  the 
  

   Mesonacidse. 
  Unfortunately, 
  to 
  accept 
  Swinnerton's 
  explana- 
  

   tion 
  involves 
  a 
  belief 
  in 
  the 
  polyphyletic 
  origin 
  of 
  the 
  trilobites, 
  

   which 
  hardly 
  seems 
  probable, 
  in 
  view 
  of 
  the 
  compactness 
  of 
  

   the 
  group. 
  

  

  There 
  can 
  be 
  no 
  doubt, 
  in 
  spite 
  of 
  statements 
  to 
  the 
  contrary, 
  

   that 
  a 
  fair 
  proportion 
  of 
  specimens 
  of 
  Mesonacids 
  show 
  distinct 
  

   traces 
  of 
  facial 
  sutures, 
  particularly 
  of 
  the 
  part 
  behind 
  the 
  eyes. 
  

   This 
  posterior 
  portion 
  of 
  the 
  sutures 
  is 
  well 
  shown 
  in 
  Walcott's* 
  

   figures 
  of 
  ElUptocephala 
  asaphoides 
  (pi. 
  24), 
  Mesonacis 
  ver- 
  

   montana 
  (pi. 
  26), 
  Callavia 
  broggeri 
  (pi. 
  27), 
  and 
  Callavia 
  

   callavei 
  (pi. 
  42, 
  — 
  this 
  shows 
  anterior 
  portion 
  of 
  sutures 
  also). 
  

   The 
  fact 
  that 
  specimens 
  often 
  show 
  the 
  facial 
  suture 
  on 
  one 
  

   half 
  of 
  the 
  cephalon 
  and 
  not 
  on 
  the 
  other, 
  and 
  that 
  the 
  anterior 
  

   portion 
  may 
  show 
  on 
  a 
  specimen 
  which 
  shows 
  no 
  trace 
  of 
  the 
  

   posterior 
  portion, 
  would 
  seem 
  to 
  indicate 
  that 
  these 
  sutures 
  are 
  

   vestigial 
  and 
  not 
  rudimentary 
  and 
  in 
  a 
  state 
  of 
  symphysis 
  and 
  

   not 
  synthesis 
  (see 
  Swiimerton, 
  p. 
  492). 
  

  

  When 
  Swinnerton 
  states 
  that 
  the 
  young 
  Mesonacids 
  do 
  not 
  

   show 
  facial 
  sutures, 
  and 
  that 
  in 
  the 
  youngest 
  known 
  specimens 
  

   the 
  eyes 
  are 
  fully 
  developed 
  and 
  on 
  the 
  dorsal 
  surface, 
  he 
  is 
  

   correct, 
  for 
  there 
  is 
  here 
  an 
  anomaly 
  among 
  trilobites. 
  While 
  

   the 
  eyes 
  come 
  in 
  at 
  the 
  margin 
  and 
  move 
  (relatively) 
  backward 
  

   through 
  the 
  growth 
  of 
  the 
  free 
  cheeks 
  during 
  development 
  

   there 
  is 
  no 
  suggestion 
  that 
  they 
  were 
  ever 
  ventral 
  or 
  came 
  

   over 
  the 
  margin. 
  The 
  facial 
  sutures 
  are 
  much 
  more 
  in 
  evi- 
  

   dence 
  in 
  the 
  adult 
  than 
  in 
  the 
  young. 
  If 
  the 
  Mesonacids 
  could 
  

   be 
  considered 
  as 
  highly 
  specialized 
  trilobites 
  occupying 
  a 
  ter- 
  

   minal 
  position 
  in 
  their 
  particular 
  line 
  of 
  development, 
  we 
  might 
  

   explain 
  this 
  as 
  a 
  case 
  of 
  " 
  earlier 
  inheritance," 
  but 
  all 
  the 
  evi- 
  

   dence 
  of 
  the 
  thorax 
  and 
  pygidium 
  seems 
  to 
  point 
  to 
  the 
  primi- 
  

   tive 
  state 
  of 
  development 
  in 
  the 
  Mesonacidae.f 
  From 
  a 
  study 
  

   of 
  the 
  young 
  stages 
  in 
  the 
  development 
  of 
  Elliptocephala 
  and 
  

   other 
  genera 
  of 
  the 
  family 
  it 
  is 
  possible 
  to 
  develop 
  a 
  theory 
  of 
  

  

  * 
  Smithsonian 
  Miscl. 
  Coll., 
  vol. 
  liii, 
  No. 
  6. 
  1910. 
  

  

  f 
  The 
  supposed 
  possession 
  by 
  Olenellus 
  of 
  a 
  telson 
  in 
  place 
  of 
  a 
  pygidiuni 
  

   has 
  been 
  used 
  as 
  an 
  argument 
  that 
  the 
  Mesonacidse 
  are 
  specialized 
  trilobites. 
  

   Personally, 
  I 
  do 
  not 
  believe 
  that 
  the 
  spine 
  seen 
  at 
  the 
  termination 
  of 
  Olenellus 
  

   is 
  a 
  pygidium, 
  or 
  takes 
  the 
  place 
  of 
  the 
  pygidium, 
  but 
  that 
  this 
  spine 
  is 
  the 
  

   normal 
  spine 
  seen 
  on 
  the 
  15th 
  segment 
  of 
  all 
  species 
  of 
  Mesonacis 
  and 
  Pae- 
  

   deumias. 
  In 
  other 
  words 
  Paedeumias 
  seems 
  to 
  me 
  to 
  be 
  a 
  complete 
  specimen 
  

   of 
  Olenellus. 
  This 
  belief 
  is 
  of 
  long 
  standing, 
  as 
  may 
  be 
  seen 
  by 
  my 
  wording 
  

   of 
  the 
  definition 
  of 
  Olenellus 
  in 
  the 
  " 
  Eastman-Zittel 
  " 
  textbook, 
  and 
  has 
  just 
  

   received 
  an 
  interesting 
  confirmation 
  in 
  the 
  discovery 
  by 
  Walcott 
  that 
  Olenel- 
  

   lus 
  gilberti 
  is 
  a 
  Mesonacis 
  (Smithsonian 
  Miscl. 
  Coll., 
  vol. 
  iv, 
  p. 
  406, 
  pi. 
  45, 
  fig. 
  3). 
  

  

  