﻿8 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.. 
  67 
  

  

  Eight 
  or 
  ten 
  different 
  plans 
  of 
  structure 
  and 
  arrangement 
  of 
  the 
  

   cirri 
  and 
  columnals, 
  some 
  of 
  them 
  widely 
  different, 
  are 
  seen 
  in 
  the 
  

   material 
  thus 
  far 
  discovered, 
  and 
  in 
  those 
  cases 
  where 
  we 
  are 
  able 
  

   to 
  test 
  them 
  in 
  considerable 
  numbers 
  of 
  specimens 
  it 
  is 
  found 
  that 
  

   they 
  hold 
  good 
  with 
  remarkable 
  constancy. 
  

  

  The 
  Silurian 
  species 
  of 
  Europe 
  and 
  American 
  fall 
  into 
  two 
  groups 
  

   which 
  are 
  somewhat 
  parallel 
  ; 
  one 
  in 
  which 
  the 
  cirri, 
  with 
  many 
  vari- 
  

   ations, 
  are 
  borne 
  upon 
  the 
  margins 
  formed 
  by 
  the 
  two 
  ends 
  of 
  the 
  

   crescentic 
  or 
  elliptic 
  columnals, 
  which 
  is 
  the 
  most 
  frequent 
  condi- 
  

   tion, 
  and 
  the 
  other 
  in 
  which 
  they 
  spring 
  more 
  or 
  less 
  from 
  the 
  back 
  

   of 
  the 
  stem. 
  

  

  As 
  usually 
  found, 
  we 
  have 
  for 
  comparison 
  only 
  the 
  coiled 
  proxi- 
  

   mal 
  or 
  middle 
  portion 
  of 
  the 
  stem, 
  the 
  longer 
  uncoiled 
  or 
  broadly 
  

   curved 
  portion 
  being 
  only 
  exceptionally 
  recovered. 
  

  

  If 
  we 
  trace 
  the 
  course 
  of 
  the 
  stem 
  from 
  the 
  point 
  where 
  the 
  slender 
  

   circular 
  portion 
  proximal 
  to 
  the 
  calyx 
  enlarges 
  and 
  changes 
  to 
  a 
  

   bilateral 
  form 
  with 
  elliptic 
  section, 
  and 
  the 
  reverse 
  curve 
  begins 
  

   (regions 
  1 
  and 
  2 
  of 
  Bather's 
  description), 
  we 
  find 
  that 
  the 
  involute 
  

   curve 
  usually 
  proceeds 
  for 
  about 
  one 
  to 
  one 
  and 
  a 
  half 
  coils, 
  and 
  

   then 
  the 
  stem 
  does 
  one 
  of 
  two 
  things; 
  either 
  1, 
  tapers 
  off 
  rapidly 
  to 
  

   a 
  narrow 
  pointed 
  end 
  which 
  clings 
  rather 
  closely 
  to 
  the 
  preceding 
  

   coil; 
  or 
  2, 
  deviates 
  from 
  this 
  course 
  and 
  goes 
  off 
  in 
  an 
  increasingly 
  

   wide 
  curve 
  (which 
  sometimes 
  become? 
  almost 
  straight) 
  for 
  a 
  con- 
  

   siderable 
  distance, 
  without 
  any 
  marked 
  diminution 
  in 
  width, 
  prob- 
  

   ably 
  to 
  a 
  terminal 
  of 
  attachment. 
  The 
  first 
  we 
  call 
  a 
  " 
  close 
  coil," 
  

   which 
  in 
  some 
  species 
  is 
  all 
  there 
  is 
  ; 
  and 
  the 
  second 
  a 
  ' 
  : 
  loose 
  coil 
  " 
  : 
  

   and 
  when 
  in 
  the 
  discussion 
  of 
  species 
  measurement 
  is 
  given 
  of 
  the 
  

   diameter 
  of 
  coil, 
  it 
  means 
  the 
  same 
  thing 
  in 
  both 
  types, 
  namely, 
  the 
  

   primary 
  coil 
  before 
  the 
  deviation 
  into 
  a 
  broader 
  curve. 
  We 
  can 
  not 
  

   fix 
  a 
  very 
  accurate 
  limit 
  for 
  this 
  distinction, 
  but 
  it 
  serves 
  a 
  conven- 
  

   ient 
  purpose 
  in 
  description. 
  The 
  " 
  close 
  coil 
  " 
  represents 
  the 
  unat- 
  

   tached 
  form; 
  the 
  "loose 
  coil" 
  the 
  form 
  which 
  may 
  have 
  been 
  tem- 
  

   porarily 
  or 
  permanently 
  attached 
  by 
  the 
  stem 
  to 
  the 
  sea 
  bottom 
  or 
  to 
  

   other 
  objects. 
  

  

  Genotype. 
  — 
  Myelodactylus 
  convolutus 
  Hall. 
  

  

  Distribution. 
  — 
  Silurian 
  to 
  Lower 
  Devonian; 
  North 
  America, 
  Eng- 
  

   land, 
  Sweden. 
  

  

  MYELODACTYLUS 
  CONVOLUTUS 
  Hall 
  

  

  Plate 
  1, 
  figs. 
  1-8 
  

  

  Myelodactylu* 
  convolutus 
  Hall, 
  Pal. 
  New 
  York, 
  vol. 
  2, 
  1S52, 
  p. 
  192, 
  pi. 
  45, 
  

  

  figs. 
  5a, 
  6, 
  Ga-h. 
  

   Hcrpctocrinus 
  convolutus, 
  Bathke, 
  Crinoidea 
  of 
  Gotland, 
  1893, 
  p. 
  48, 
  pi. 
  2, 
  

  

  figs. 
  50-53. 
  

  

  Coil 
  close 
  in 
  proximal 
  region; 
  open 
  and 
  broadly 
  convolute 
  distal- 
  

   wards. 
  Columnals 
  very 
  short, 
  quadrangular 
  and 
  uniform. 
  Cirri 
  

  

  