﻿26 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.67 
  

  

  The 
  arrangement 
  of 
  columnals 
  varies 
  from 
  uniformly 
  short 
  

   throughout 
  to 
  alternation 
  with 
  longer 
  ones 
  on 
  different 
  plans. 
  The 
  

   arms 
  vary 
  from 
  10, 
  the 
  usual 
  number, 
  to 
  20, 
  30, 
  and 
  40; 
  from 
  uni- 
  

   serial 
  to 
  biserial; 
  and 
  from 
  dichotomous 
  to 
  a 
  heterotomous 
  arrange- 
  

   ment. 
  They 
  also 
  in 
  some 
  species 
  take 
  on 
  a 
  recumbent 
  habitus 
  — 
  a 
  

   modification 
  which 
  occurs 
  independently 
  among 
  the 
  Ehodocrinidae, 
  

   Melocrinidae, 
  Batocrinidae 
  and 
  Platycrinidae. 
  A 
  reduction 
  of 
  

   primibrachs 
  to 
  a 
  single 
  small 
  axillary 
  plate 
  forms 
  the 
  genus 
  Talaro- 
  

   crinus, 
  with 
  the 
  calyx 
  elements 
  otherwise 
  similar 
  to 
  Dichocrinus, 
  but 
  

   with 
  incipient 
  changes 
  in 
  the 
  tegmen 
  by 
  the 
  enlargement 
  of 
  the 
  axil- 
  

   lary 
  ambulacral, 
  or 
  radial 
  dome 
  plate, 
  which 
  when 
  developed 
  into 
  

   huge 
  wing-like 
  processes 
  produces 
  the 
  remarkable 
  specialization 
  of 
  

   Pterotocrinus. 
  

  

  Finally 
  the 
  calyx 
  itself 
  begins 
  to 
  add 
  a 
  new 
  element, 
  in 
  the 
  form 
  

   of 
  additional 
  rings 
  of 
  plates 
  between 
  basals 
  and 
  radials, 
  leading 
  to 
  

   the 
  extraordinary 
  multiplication 
  of 
  such 
  plates 
  which 
  we 
  find 
  in 
  

   Acrocrinus, 
  the 
  latest 
  survivor 
  of 
  the 
  Camerata. 
  

  

  In 
  the 
  present 
  form, 
  owing 
  to 
  the 
  more 
  solid 
  construction 
  of 
  the 
  

   calyx 
  and 
  the 
  simple 
  character 
  of 
  the 
  arms, 
  there 
  is 
  no 
  such 
  irregular- 
  

   ity 
  or 
  deformity 
  as 
  has 
  been 
  observed 
  in 
  Myelodactylus, 
  only 
  a 
  slight 
  

   distortion 
  at 
  the 
  base 
  due 
  to 
  pressure; 
  and 
  for 
  the 
  same 
  reason 
  the 
  

   crown 
  is 
  more 
  prominent 
  and 
  better 
  exposed. 
  Therefore 
  it 
  is 
  found 
  

   in 
  perfect 
  condition 
  in 
  several 
  of 
  the 
  species, 
  so 
  that 
  it 
  may 
  be 
  fully 
  

   inspected. 
  Throughout 
  the 
  long 
  geological 
  range 
  in 
  which 
  the 
  genus 
  

   persisted 
  — 
  from 
  the 
  Burlington 
  to 
  the 
  later 
  Chester 
  — 
  we 
  find 
  that 
  

   the 
  calyx 
  has 
  undergone 
  little 
  material 
  change, 
  and 
  from 
  that 
  alone 
  

   it 
  will 
  be 
  difficult 
  to 
  differentiate 
  some 
  of 
  the 
  species. 
  In 
  all 
  where 
  

   the 
  arms 
  are 
  known 
  they 
  are 
  ten 
  in 
  number, 
  and 
  usualy 
  uniserial, 
  

   the 
  brachials 
  often 
  passing 
  into 
  cuneate 
  form, 
  and 
  perhaps 
  interlock- 
  

   ing 
  toward 
  the 
  extremities. 
  

  

  But 
  as 
  in 
  the 
  Silurian 
  type, 
  the 
  specialized 
  stem 
  offers 
  good 
  specific 
  

   characters 
  resulting 
  from 
  modifications 
  upon 
  a 
  new 
  plan. 
  In 
  all 
  

   species, 
  as 
  before, 
  the 
  columnals 
  in 
  the 
  proximal, 
  rounded 
  and 
  non- 
  

   cirriferous 
  region 
  of 
  the 
  stem 
  are 
  very 
  short 
  and 
  uniform; 
  but 
  be- 
  

   yond 
  this, 
  where 
  the 
  stem 
  becomes 
  elliptic, 
  the 
  columnals 
  are 
  of 
  dif- 
  

   ferent 
  lengths, 
  parallel, 
  and 
  have 
  marginal 
  cirri 
  at 
  each 
  end 
  forming 
  

   two 
  rows 
  along 
  the 
  inner 
  or 
  concave 
  side 
  of 
  the 
  stem. 
  These 
  usually 
  

   spring 
  from 
  the 
  suture 
  between 
  paired 
  or 
  doubled 
  nodals, 
  with 
  one, 
  

   two 
  or 
  three 
  internodals 
  interposed, 
  each 
  about 
  the 
  size 
  of 
  the 
  com- 
  

   bined 
  nodal 
  pair. 
  No 
  wedge-formed 
  columnals 
  have 
  been 
  observed. 
  

   By 
  far 
  the 
  most 
  frequent 
  plan 
  is 
  that 
  of 
  duplicate 
  or 
  triplicate 
  cirri, 
  

   of 
  which 
  the 
  outer 
  ones 
  are 
  borne 
  upon 
  facets 
  at 
  the 
  junction 
  of 
  the 
  

   nodal 
  segments, 
  forming 
  clusters 
  which 
  diminish 
  in 
  size 
  inwards. 
  

   The 
  cirrus-facet 
  seems 
  to 
  lie 
  directly 
  above 
  the 
  suture. 
  

  

  