﻿68 
  PEOCEEDINGS 
  OF 
  THE 
  NATIONAL. 
  MUSEUM 
  vol.67 
  

  

  where 
  the 
  reversal 
  occurred 
  incipient 
  spines 
  were 
  developed, 
  instead 
  

   of 
  a 
  swollen 
  sac 
  — 
  a 
  modification 
  which 
  became 
  dominant 
  in 
  Hydrei- 
  

   onocrinus. 
  

  

  Thus 
  the 
  position 
  of 
  the 
  anus 
  on 
  the 
  anterior 
  side 
  depends 
  upon 
  

   the 
  extent 
  to 
  which 
  the 
  tube 
  grows 
  downward 
  after 
  reversal, 
  and 
  if 
  

   this 
  is 
  continued 
  far 
  enough 
  the 
  opening 
  will 
  be 
  at 
  the 
  base 
  of 
  the 
  

   tube, 
  as 
  in 
  " 
  Scaphiocrinus 
  " 
  elegans; 
  and 
  with 
  the 
  evidence 
  afforded 
  

   by 
  the 
  above 
  mentioned 
  examples 
  it 
  is 
  clear 
  that 
  its 
  position 
  in 
  all 
  

   forms 
  where 
  located 
  below 
  the 
  distal 
  end 
  may 
  be 
  accounted 
  for. 
  

  

  The 
  genera 
  characterized 
  by 
  the 
  Inflated 
  Sac 
  fall 
  into 
  a 
  fairly 
  

   well 
  limited 
  group 
  forming 
  a 
  section 
  of 
  the 
  subfamily 
  Poterio- 
  

   crininae 
  as 
  defined 
  in 
  the 
  Zittel-Eastman 
  textbook, 
  edition 
  of 
  1913. 
  

   It 
  has 
  an 
  extreme 
  range 
  from 
  the 
  earliest 
  Lower 
  Carboniferous 
  

   through 
  the 
  principal 
  subdivisions 
  and 
  into 
  the 
  Upper 
  Carboniferous, 
  

   and 
  according 
  to 
  Wanner 
  into 
  the 
  Permian. 
  It 
  reached 
  its 
  acme 
  in 
  

   variety 
  and 
  abundance 
  in 
  the 
  Burlington 
  and 
  Keokuk, 
  and 
  in 
  extrav- 
  

   agance 
  of 
  form 
  in 
  the 
  later 
  formations. 
  Coincident 
  with 
  the 
  es- 
  

   tablishment 
  of 
  the 
  mushroom 
  form 
  in 
  the 
  Chester, 
  the 
  group 
  began 
  

   to 
  decline, 
  and 
  to 
  be 
  replaced 
  by 
  a 
  series 
  of 
  genera 
  in 
  which 
  the 
  

   strong 
  anal 
  tube 
  as 
  a 
  solid 
  structure 
  disappeared, 
  and 
  the 
  anal 
  plates 
  

   were 
  lifted 
  out 
  of 
  the 
  calyx 
  until 
  they 
  no 
  longer 
  occupied 
  a 
  place 
  

   within 
  the 
  ring 
  of 
  radials, 
  and 
  ceased 
  to 
  form 
  an 
  integral 
  part 
  of 
  the 
  

   calyx 
  wall. 
  

  

  This 
  series 
  started 
  with 
  the 
  long-lived 
  Eupachycrinus, 
  which 
  

   began, 
  as 
  I 
  now 
  know, 
  in 
  the 
  Keokuk 
  with 
  a 
  rare 
  and 
  isolated 
  spe- 
  

   cies, 
  and 
  lasted 
  through 
  the 
  Upper 
  Carboniferous 
  ; 
  and 
  which, 
  while 
  

   retaining 
  the 
  strong 
  radianal 
  and 
  anal 
  plates 
  of 
  its 
  predecessors, 
  had 
  

   only 
  a 
  remnant 
  of 
  the 
  tube. 
  The 
  change 
  progressed 
  through 
  Crom- 
  

   yocrinus, 
  Agassizocrinus, 
  Ulocrinus, 
  Erisocrinus, 
  and 
  finally 
  to 
  En- 
  

   crinus 
  in 
  the 
  Trias, 
  when 
  all 
  trace 
  of 
  anal 
  structures 
  in 
  the 
  calyx 
  

   was 
  lost. 
  

  

  The 
  section 
  with 
  a 
  tube, 
  therefore, 
  all 
  having 
  strong 
  anal 
  plates 
  

   in 
  the 
  calyx 
  to 
  support 
  it, 
  includes 
  the 
  genera 
  from 
  Poteriocrinus 
  to 
  

   Scytalocrinus 
  of 
  the 
  subfamily 
  Poteriocrininae 
  above 
  mentioned, 
  

   with 
  some 
  additions. 
  In 
  view 
  of 
  the 
  close 
  interrelationships 
  of 
  the 
  

   genera 
  comprising 
  this 
  group, 
  the 
  need 
  of 
  better 
  description 
  and 
  

   illustration 
  of 
  some 
  of 
  them, 
  and 
  the 
  presence 
  of 
  some 
  new 
  forms, 
  it 
  

   is 
  desirable 
  to 
  give, 
  along 
  with 
  the 
  new 
  matter, 
  a 
  general 
  summary 
  

   of 
  their 
  characters. 
  

  

  In 
  this 
  group, 
  specialized 
  as 
  it 
  is 
  in 
  regard 
  to 
  the 
  ventral 
  sac, 
  

   there 
  must 
  be 
  recognized, 
  as 
  already 
  intimated, 
  a 
  considerable 
  com- 
  

   plexity 
  and 
  intermingling 
  of 
  characters, 
  and 
  the 
  definition 
  of 
  genera 
  

   is 
  complicated 
  by 
  the 
  occurrence 
  of 
  exceptions 
  in 
  regard 
  to 
  char- 
  

   acters 
  which 
  in 
  some 
  other 
  groups 
  are 
  considered 
  to 
  be 
  of 
  generic 
  

   value. 
  We 
  simply 
  have 
  to 
  select 
  some 
  character 
  that 
  appears 
  to 
  be 
  

  

  