﻿P. 
  E. 
  Raymond 
  — 
  Beecher's 
  Classification 
  of 
  Trilobites. 
  207 
  

  

  the 
  secondary 
  origin 
  of 
  the 
  free 
  cheeks 
  on 
  a 
  plan 
  quite 
  different 
  

   from 
  that 
  set 
  forth 
  by 
  Professor 
  Beech 
  er. 
  

  

  The 
  study 
  of 
  the 
  young 
  specimens 
  of 
  Mesonacids 
  figured 
  by 
  

   Ford,* 
  Beecher, 
  and 
  Walcott 
  indicates 
  that 
  the 
  cephalon 
  of 
  

   these 
  trilobites 
  is 
  made 
  up 
  of 
  six 
  similar 
  segments 
  each 
  consist- 
  

   ing 
  of 
  an 
  axial 
  and 
  pair 
  of 
  pleural 
  portions. 
  The 
  axial 
  por- 
  

   tions 
  are 
  ranged 
  one 
  behind 
  the 
  other 
  in 
  a 
  straight 
  line, 
  while 
  

   the 
  pleurae 
  are 
  bent 
  abruptly 
  backward 
  so 
  that 
  the 
  anterior 
  

   ones 
  closely 
  envelop 
  and 
  crowd 
  those 
  behind. 
  The 
  effect 
  of 
  

   this 
  crowding 
  has 
  been 
  to 
  eliminate 
  the 
  greater 
  part 
  of 
  the 
  

   pleurae 
  of 
  the 
  last 
  segments, 
  though 
  these 
  three 
  segments 
  com- 
  

   bined 
  produced 
  one 
  pair 
  of 
  spines 
  which 
  project 
  beyond 
  the 
  

   posterior 
  border. 
  The 
  first 
  two 
  segments 
  of 
  the 
  cephalon 
  are 
  

   more 
  or 
  less 
  coalesced, 
  and 
  their 
  pleurae 
  also 
  unite 
  to 
  produce 
  

  

  Fig. 
  3. 
  

  

  Fig. 
  3. 
  Elliptocephala 
  asaphoides 
  Emmons. 
  A 
  very 
  young 
  specimen, 
  

   representing 
  one 
  of 
  the 
  earliest 
  stages 
  known 
  in 
  the 
  development 
  of 
  a 
  

   Mesonacid. 
  After 
  Walcott. 
  Much 
  enlarged. 
  

  

  a 
  pair 
  of 
  free 
  spines. 
  In 
  the 
  smallest 
  specimens 
  known 
  these 
  

   spines 
  are 
  very 
  close 
  together 
  (Walcott, 
  loc. 
  cit. 
  pi. 
  25, 
  figs. 
  

   1, 
  2) 
  or 
  may 
  even 
  be 
  coalesced 
  (pi. 
  25, 
  fig. 
  22). 
  On 
  further 
  

   growth, 
  there 
  is 
  a 
  spreading 
  out 
  along 
  the 
  posterior 
  margin, 
  as 
  

   is 
  shown 
  by 
  the 
  fact 
  that 
  the 
  spines 
  move 
  outward. 
  The 
  

   spines 
  of 
  the 
  first 
  two 
  segments 
  separate, 
  so 
  that 
  the 
  spines 
  of 
  

   the 
  first 
  segment 
  move 
  to 
  the 
  genal 
  angles, 
  or 
  in 
  some 
  cases, 
  

   even 
  around 
  to 
  the 
  front 
  of 
  the 
  cephalon 
  (Walcott, 
  loc. 
  cit. 
  

   pi. 
  36, 
  fig. 
  14 
  ; 
  pi. 
  7, 
  figs. 
  9-11) 
  ; 
  the 
  spines 
  of 
  the 
  second 
  seg- 
  

   ment 
  move 
  outward 
  to 
  form 
  the 
  intergenal 
  spines, 
  or 
  where 
  

   the 
  3 
  pairs 
  of 
  spines 
  remain, 
  the 
  genal 
  spines 
  ; 
  and 
  the 
  inner 
  

   spines 
  of 
  the 
  young 
  are 
  either 
  suppressed 
  or, 
  in 
  exceptional 
  

   cases, 
  become 
  intergenal 
  spines. 
  If 
  then, 
  we 
  think 
  of 
  the 
  dor- 
  

   sal 
  surface 
  of 
  the 
  head 
  of 
  these 
  trilobites 
  as 
  consisting 
  of 
  6 
  

   united 
  segments, 
  the 
  first 
  act 
  in 
  forming 
  the 
  head 
  was 
  the 
  

   sharp 
  bending 
  back 
  of 
  all 
  the 
  pleurae 
  to 
  form 
  an 
  oval 
  head 
  

  

  *This 
  Journal 
  (3), 
  vol. 
  xiii, 
  p. 
  265, 
  1877 
  ; 
  vol. 
  xv, 
  p. 
  129, 
  1878 
  ; 
  vol. 
  xxii, 
  

   p. 
  250, 
  1831. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XLIII, 
  No. 
  255.— 
  March, 
  1917. 
  

   15 
  

  

  