﻿art 
  9 
  UNUSUAL 
  FORMS 
  OF 
  FOSSIL 
  CRINOIDS 
  SPRINGER 
  65 
  

  

  developing 
  a 
  variety 
  of 
  peculiar 
  inflated 
  or 
  spiniferous 
  structures, 
  

   which 
  have 
  been 
  called 
  balloon-shaped 
  or 
  mushroom-shaped. 
  Some 
  

   of 
  these 
  extend 
  beyond 
  the 
  limits 
  of 
  the 
  arms, 
  surmounting 
  the 
  

   crown 
  with 
  a 
  conspicuous 
  expanded 
  appendage; 
  others 
  are 
  wholly 
  

   enclosed 
  by 
  the 
  arms. 
  All 
  are 
  enlarged 
  extensions 
  of 
  a 
  narrow 
  

   neck 
  by 
  which 
  they 
  are 
  connected 
  with 
  the 
  cup. 
  

  

  This 
  form 
  of 
  sac 
  was 
  especially 
  developed 
  in 
  the 
  Lower 
  Carbonif- 
  

   erous, 
  where 
  it 
  is 
  characteristic 
  of 
  a 
  considerable 
  group 
  of 
  genera, 
  

   beginning 
  in 
  the 
  earliest 
  members, 
  the 
  Kinderhook 
  and 
  Lower 
  Bur- 
  

   lington, 
  with 
  the 
  genus 
  Coeliocrinus, 
  a 
  variable 
  form, 
  which 
  con- 
  

   tinued 
  with 
  modifications 
  into 
  some 
  of 
  the 
  later 
  formations, 
  and 
  was 
  

   succeeded 
  toward 
  the 
  close 
  by 
  Hydreionocrinus, 
  which 
  combines 
  one 
  

   of 
  its 
  forms 
  of 
  sac 
  with 
  a 
  different 
  type 
  of 
  calyx 
  and 
  arm 
  structure, 
  

   and 
  as 
  thus 
  modified 
  passed 
  on 
  to 
  the 
  Upper 
  Carboniferous. 
  The 
  

   sac 
  with 
  the 
  infradistal 
  position 
  of 
  the 
  anus 
  is 
  subject 
  to 
  considerable 
  

   modification 
  in 
  details 
  of 
  structure, 
  and 
  in 
  one 
  form 
  or 
  another 
  

   occurs 
  as 
  a 
  conspicuous 
  feature 
  in 
  several 
  genera 
  besides 
  the 
  two 
  

   above 
  named. 
  It 
  is 
  also 
  found 
  associated 
  with 
  different 
  plans 
  of 
  

   arm 
  arrangement 
  and 
  forms 
  of 
  calyx. 
  As 
  to 
  these 
  characters 
  there 
  

   is 
  no 
  very 
  stable 
  correlation, 
  and 
  the 
  lack 
  of 
  fixity 
  in 
  this 
  respect 
  is 
  

   often 
  confusing. 
  

  

  For 
  example: 
  Pachylocrinus 
  arboreus 
  (Worthen) 
  and 
  P. 
  florealis 
  

   (Yandell 
  and 
  Shumard) 
  with 
  more 
  or 
  less 
  dichotomous 
  arms, 
  have 
  

   the 
  opening 
  toward 
  the 
  top, 
  the 
  former 
  somewhat 
  below 
  the 
  bulbous 
  

   enlargement, 
  and 
  the 
  latter 
  almost 
  at 
  the 
  very 
  top, 
  next 
  to 
  the 
  ter- 
  

   minal 
  cluster 
  of 
  a 
  few 
  spiniferous 
  plates 
  (pi. 
  16, 
  figs. 
  3-9). 
  Ahroto- 
  

   crinus 
  unicus 
  (Hall), 
  with 
  the 
  arms 
  of 
  Pachylocrinus 
  somewhat 
  

   modified 
  but 
  having 
  a 
  pentagonal 
  stem, 
  has 
  the 
  anus 
  about 
  midway 
  

   (pi. 
  17, 
  figs. 
  1-3) 
  ; 
  as 
  have 
  also 
  Scytalocrinus 
  and 
  Decadocrinus, 
  with 
  

   their 
  ten 
  unbranched 
  arms, 
  the 
  former 
  with 
  slender 
  pinnules 
  and 
  the 
  

   latter 
  with 
  strong 
  armlets 
  or 
  ramules. 
  41 
  In 
  Aulocrinus, 
  with 
  arms 
  

   and 
  pinnules 
  of 
  the 
  Decadocrinus 
  type, 
  the 
  opening 
  is 
  at 
  about 
  the 
  

   same 
  height, 
  but 
  projected 
  from 
  a 
  lateral 
  tube 
  (pi. 
  19). 
  

  

  Zeacrinus, 
  with 
  heterotomous 
  arms 
  and 
  short 
  brachials, 
  exhibits 
  

   great 
  variation 
  in 
  tube 
  structure 
  : 
  Z. 
  elegans 
  of 
  the 
  Burlington 
  has 
  a 
  

   rounded 
  sac 
  expanding 
  upwards 
  (pi. 
  21, 
  fig. 
  2a) 
  ; 
  Z. 
  commaticus 
  of 
  

   the 
  Keokuk 
  has 
  the 
  opening 
  at 
  the 
  base 
  of 
  an 
  elongated 
  sac 
  (pi. 
  22, 
  

   fig. 
  Za) 
  ; 
  while 
  the 
  typical 
  species, 
  Z. 
  wortheni 
  of 
  the 
  Chester, 
  has 
  an 
  

   altogether 
  different 
  form 
  of 
  sac, 
  being 
  a 
  pyramid 
  narrowing 
  upwards 
  

   to 
  an 
  apex, 
  but 
  having 
  the 
  opening 
  about 
  midway 
  (pi. 
  23, 
  fig. 
  2a). 
  

  

  Hydreionocrinus, 
  with 
  a 
  highly 
  distinctive 
  spiniferous 
  sac, 
  and 
  the 
  

   opening 
  in 
  at 
  least 
  one 
  species 
  about 
  midway, 
  has 
  no 
  less 
  than 
  three 
  

  

  41 
  North 
  Amer. 
  Crin. 
  Cam., 
  pi. 
  7, 
  and 
  herein, 
  pi. 
  17. 
  

   23832—26 
  5 
  

  

  