﻿art 
  9 
  UNUSUAL. 
  FORMS 
  OP 
  FOSSIL 
  CRINOIDS 
  SPRINGER 
  79 
  

  

  and 
  it 
  appears 
  that 
  in 
  the 
  very 
  latest 
  species 
  of 
  all, 
  from 
  the 
  Pennsyl- 
  

   vanian, 
  the 
  earlier 
  form 
  of 
  radianal 
  was 
  resumed. 
  Therefore 
  such 
  

   a 
  range 
  of 
  variation 
  in 
  this 
  character 
  must 
  be 
  accepted 
  for 
  the 
  

   genus. 
  

  

  So 
  in 
  regard 
  to 
  the 
  arms. 
  The 
  typical 
  species 
  have 
  the 
  arms 
  flat 
  

   and 
  closely 
  apposed, 
  touching 
  all 
  around 
  by 
  linear 
  margins 
  and 
  

   forming 
  a 
  more 
  or 
  less 
  smooth, 
  rotund 
  or 
  ellipsoidal 
  crown. 
  But 
  we 
  

   are 
  obliged 
  to 
  admit 
  as 
  exceptions 
  others 
  in 
  which 
  the 
  arms 
  are 
  

   rounded 
  and 
  not 
  in 
  close 
  contact, 
  with 
  axillaries 
  prominent 
  or 
  nodose, 
  

   as 
  for 
  instance, 
  Z. 
  asper 
  Meek 
  and 
  Worthen. 
  

  

  Again, 
  the 
  sac 
  typically 
  is 
  enclosed 
  by 
  the 
  arms; 
  but 
  in 
  some 
  

   species, 
  as 
  Z. 
  commaticus 
  and 
  Z. 
  girtyi, 
  it 
  rises 
  distinctly 
  above 
  them, 
  

   and 
  among 
  the 
  various 
  species 
  appears 
  in 
  the 
  three 
  different 
  forms 
  

   already 
  mentioned. 
  

  

  As 
  a 
  general 
  rule 
  there 
  is 
  only 
  the 
  single 
  axillary 
  primibrach 
  in 
  

   the 
  four 
  rays 
  other 
  than 
  the 
  anterior, 
  which 
  regularly 
  adds 
  one 
  or 
  

   more 
  brachials 
  between 
  that 
  and 
  the 
  radial; 
  but 
  to 
  this 
  also 
  there 
  

   are 
  a 
  few 
  exceptions. 
  

  

  Zeacrinus 
  ranges 
  through 
  the 
  entire 
  Lower 
  Carboniferous, 
  begin- 
  

   ning 
  in 
  the 
  Kinderhook, 
  culminating 
  in 
  the 
  Chester, 
  and 
  passing 
  up 
  

   into 
  the 
  Pennsylvanian. 
  It 
  has 
  hitherto 
  been 
  regarded 
  as 
  strictly 
  

   an 
  American 
  genus, 
  but 
  it 
  now 
  appears 
  prominently 
  as 
  a 
  part 
  ef 
  

   the 
  remarkable 
  crinoidal 
  fauna 
  brought 
  to 
  light 
  in 
  recent 
  years 
  by 
  

   Mr. 
  Wright 
  from 
  formations 
  in 
  Scotland 
  equivalent 
  to 
  our 
  Chester. 
  

  

  A 
  few 
  representative 
  species 
  will 
  now 
  be 
  considered 
  in 
  detail 
  : 
  

  

  ZEACRINUS 
  BURSAEFORMIS 
  White 
  

  

  Plate 
  21, 
  fig. 
  1 
  

  

  Zeacrinus 
  bursaeformis 
  White, 
  Proc. 
  Boston 
  Soc. 
  Nat. 
  Hist., 
  vol. 
  9, 
  1SG2, 
  

  

  p. 
  10. 
  

   Lower 
  Burlington 
  limestone 
  ; 
  Burlington, 
  Iowa. 
  

  

  A 
  well-defined 
  species, 
  not 
  before 
  figured; 
  represented 
  by 
  four 
  

   good 
  specimens, 
  besides 
  the 
  type 
  in 
  the 
  Museum 
  of 
  Comparative 
  

   Zoology; 
  differing 
  from 
  typical 
  species 
  by 
  having 
  the 
  calyx 
  in 
  the 
  

   form 
  of 
  an 
  inverted, 
  truncated 
  cone, 
  spreading 
  directly 
  to 
  the 
  arm 
  

   bases, 
  with 
  turbinate 
  base, 
  instead 
  of 
  concave. 
  Arms 
  flat 
  on 
  the 
  

   back, 
  rather 
  closely 
  apposed, 
  about 
  8 
  to 
  the 
  ray 
  except 
  the 
  anterior 
  

   which 
  usually 
  has 
  6. 
  To 
  the 
  foregoing 
  abstract 
  from 
  the 
  original 
  

   description 
  may 
  be 
  added 
  the 
  following 
  remark 
  by 
  the 
  author: 
  

  

  This 
  species 
  not 
  only 
  resembles 
  Zeacrinus 
  above 
  the 
  base, 
  but 
  possesses 
  those 
  

   characters 
  which 
  have 
  been 
  regarded 
  as 
  peculiar 
  to 
  that 
  genus 
  of 
  having 
  but 
  

   two 
  radials 
  (R+IBr) 
  to 
  four 
  of 
  the 
  rays, 
  and 
  a 
  greater 
  number 
  in 
  the 
  anterior 
  

   one 
  ; 
  yet 
  the 
  body 
  has 
  the 
  true 
  form 
  and 
  development 
  of 
  Poteriocrinus. 
  

  

  That 
  is, 
  it 
  is 
  one 
  of 
  the 
  striking 
  exceptions 
  to 
  the 
  type 
  which 
  upon 
  

   a 
  preponderance 
  of 
  characters 
  must 
  be 
  held 
  within 
  the 
  genus. 
  It 
  

  

  