﻿350 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  fourth 
  spines. 
  My 
  figure 
  shows 
  that 
  considerable 
  variation 
  occurs 
  even 
  

   in 
  a 
  single 
  specimen. 
  There 
  are 
  two 
  pairs 
  of 
  incisions, 
  the 
  first 
  between 
  

   the 
  first 
  and 
  second 
  lobes 
  and 
  the 
  second 
  outside 
  the 
  latter 
  lobe. 
  The 
  

   first 
  incisions 
  are 
  usually 
  very 
  deep, 
  while 
  both 
  pairs 
  are 
  narrow. 
  

  

  The 
  ends 
  of 
  the 
  lobes 
  are 
  heavily 
  chitinized. 
  The 
  chitinous 
  processes 
  

   extending 
  along 
  the 
  mesal 
  margins 
  of 
  the 
  median 
  lobes 
  I 
  have 
  not 
  found 
  

   large 
  and 
  definite 
  so 
  frequently 
  as 
  in 
  A. 
  forbesi. 
  Those 
  on 
  the 
  inner 
  

   margin 
  of 
  the 
  first 
  incision 
  are 
  of 
  the 
  general 
  type 
  of 
  the 
  latter 
  species 
  

   and 
  are 
  strongly 
  curved 
  toward 
  the 
  lobe, 
  but 
  are 
  more 
  usually 
  broader 
  at 
  

   the 
  base 
  of 
  the 
  process, 
  that 
  is, 
  toward 
  the 
  tip 
  and 
  outer 
  edge 
  of 
  the 
  lobe. 
  

   This 
  is 
  not, 
  however, 
  a 
  very 
  positive 
  difi"erence. 
  The 
  opposite 
  processes 
  

   nearly 
  equal 
  these 
  in 
  size, 
  which 
  is 
  not 
  the 
  case 
  in 
  A. 
  forbesi. 
  Some- 
  

   times 
  I 
  have 
  not 
  distinguished 
  the 
  process 
  of 
  the 
  outer 
  edge 
  of 
  the 
  second 
  

   incision, 
  but 
  it 
  is 
  often 
  present 
  of 
  about 
  the 
  same 
  size 
  as 
  that 
  of 
  the 
  

   inner 
  edge. 
  

  

  Ventral 
  glands 
  are 
  wanting 
  even 
  in 
  the 
  adult. 
  The 
  chitinous 
  thick- 
  

   enings 
  of 
  this 
  region 
  are 
  present 
  however 
  as 
  definite 
  and 
  narrow 
  dark 
  

   areas 
  in 
  well 
  cleared 
  specimens. 
  They 
  are 
  twice 
  bent 
  and 
  of 
  the 
  shape 
  

   shown 
  in 
  the 
  figure. 
  These 
  will 
  serve 
  to 
  differentiate 
  the 
  adult 
  A. 
  p 
  e 
  r- 
  

   n 
  i 
  c 
  i 
  o 
  s 
  u 
  s 
  from 
  the 
  immature 
  forms 
  of 
  any 
  of 
  these 
  species, 
  even 
  though 
  

   eggs 
  and 
  young 
  are 
  not 
  present 
  to 
  certify 
  to 
  the 
  maturity 
  of 
  the 
  form. 
  

  

  The 
  anus 
  is 
  of 
  medium 
  size, 
  and 
  is 
  about 
  as 
  far 
  from 
  the 
  ends 
  of 
  the 
  

   chitinous 
  processes 
  as 
  these 
  are 
  from 
  the 
  tips 
  of 
  the 
  lobes. 
  

  

  The 
  transverse 
  chitinous 
  processes 
  are 
  frequently 
  broad 
  and 
  some- 
  

   what 
  irregular. 
  They 
  are 
  not 
  so 
  usually 
  prominent 
  and 
  definite 
  as 
  in 
  

   A. 
  ostreaeformis. 
  The 
  dorsal 
  pores 
  are 
  comparatively 
  tew, 
  small 
  

   and 
  inconspicuous. 
  They 
  are 
  usually 
  present 
  in 
  traces 
  of 
  three 
  short 
  

   lines; 
  the 
  first 
  runs 
  from 
  the 
  second 
  incision; 
  the 
  second 
  just 
  laterad 
  of 
  

   this 
  and 
  the 
  third, 
  consisting 
  of 
  only 
  a 
  few 
  scattering 
  pores, 
  is 
  still 
  farther 
  

   laterad. 
  Quite 
  frequently 
  a 
  single 
  pore 
  is 
  to 
  be 
  seen 
  anterior 
  to 
  the 
  

   lateral 
  transverse 
  thickening. 
  

  

  The 
  second 
  stage 
  (pi. 
  12, 
  fig. 
  2) 
  is 
  much 
  the 
  same 
  in 
  general 
  arrange- 
  

   ment 
  and 
  in 
  the 
  outline 
  of 
  parts 
  except 
  that 
  the 
  ventral 
  chitinous 
  thicken- 
  

   ings 
  are 
  parenthesis-shaped 
  and 
  lack 
  definiteness, 
  the 
  dorsal 
  pores 
  are 
  

   fewer 
  and 
  more 
  scattering, 
  and 
  the 
  plates 
  are 
  not 
  always 
  so 
  easily 
  detected. 
  

   It 
  may 
  be 
  most 
  easily 
  confounded 
  in 
  this 
  stage 
  with 
  A. 
  forbesi, 
  but 
  

   it 
  usually 
  displays 
  plates 
  enough 
  in 
  carefully 
  prepared 
  specimens 
  to 
  sepa- 
  

   rate 
  it 
  from 
  that 
  form, 
  and 
  will 
  show 
  also, 
  on 
  greater 
  familiarity, 
  the 
  same 
  

   difference 
  in 
  relative 
  size 
  of 
  the 
  chitinous 
  processes 
  and 
  in 
  relative 
  posi- 
  

   tion 
  of 
  the 
  lobes 
  which 
  is 
  to 
  be 
  marked 
  in 
  the 
  adult 
  forms. 
  

  

  