﻿aut9 
  UNUSUAL 
  FORMS 
  OF 
  FOSSIL 
  CRINOIDS 
  SPRINGER 
  41 
  

  

  forth 
  is 
  constant 
  throughout. 
  But 
  this 
  does 
  not 
  apply 
  to 
  the 
  type 
  

   species, 
  A. 
  shumardi, 
  which 
  is 
  from 
  a 
  higher 
  horizon, 
  the 
  Glen 
  Dean 
  

   formation 
  of 
  the 
  upper 
  Chester; 
  of 
  this 
  five 
  specimens 
  are 
  known, 
  

   and 
  in 
  every 
  one 
  of 
  them 
  the 
  arms 
  are 
  erect, 
  and 
  there 
  is 
  no 
  sign 
  of 
  

   indentations 
  upon 
  the 
  calyx, 
  while 
  the 
  species 
  is 
  well 
  characterized 
  

   otherwise 
  by 
  the 
  less 
  height 
  and 
  greater 
  width 
  of 
  the 
  radial 
  plates. 
  

  

  Therefore 
  it 
  must 
  be 
  conceded 
  that 
  while 
  this 
  specialization 
  has 
  

   nothing 
  to 
  do 
  with 
  the 
  genus 
  as 
  a 
  controlling 
  character, 
  since 
  in 
  five 
  

   of 
  the 
  genera 
  in 
  which 
  it 
  occurs 
  both 
  types 
  of 
  arms 
  are 
  found, 
  it 
  

   does 
  hold 
  good 
  for 
  the 
  species. 
  

  

  While 
  reference 
  should 
  be 
  had 
  to 
  the 
  ample 
  illustrations 
  given 
  by 
  

   Wachsmuth 
  and 
  Springer 
  on 
  plate 
  80 
  of 
  the 
  Camerata 
  monograph, 
  

   some 
  of 
  which 
  I 
  reproduce, 
  I 
  am 
  for 
  convenience 
  giving 
  some 
  addi- 
  

   tional 
  figures, 
  especially 
  a 
  new 
  one 
  of 
  the 
  rarer 
  species, 
  A. 
  shumardi. 
  

  

  ADDITIONAL 
  ELEMENTS 
  IN 
  THE 
  CALYX 
  

  

  In 
  connection 
  with 
  the 
  genera 
  last 
  above 
  discussed, 
  another 
  singu- 
  

   lar 
  modification 
  is 
  to 
  be 
  considered. 
  

  

  As 
  before 
  stated, 
  the 
  dorsal 
  cup 
  of 
  a 
  crinoid 
  consists 
  primarily 
  of 
  

   a 
  circlet 
  of 
  radials 
  supporting 
  the 
  arms, 
  and 
  one 
  or 
  two 
  rings 
  of 
  

   basals 
  below 
  them, 
  plus 
  interradial 
  structures 
  if 
  present. 
  While 
  the 
  

   latter 
  may 
  or 
  may 
  not 
  be 
  present, 
  and 
  when 
  present 
  are 
  regarded 
  as 
  

   secondary 
  elements 
  which 
  exhibit 
  a 
  wide 
  range 
  of 
  variation 
  in 
  form 
  

   and 
  number, 
  any 
  departure 
  from 
  the 
  normal 
  two, 
  or 
  three, 
  rings 
  of 
  

   primary 
  plates 
  has 
  been 
  considered 
  as 
  extremely 
  exceptional. 
  

   Therefore 
  the 
  occurrence 
  of 
  numerous 
  additional 
  sets 
  or 
  series 
  of 
  

   plates 
  between 
  the 
  radial 
  and 
  basal 
  circlets 
  in 
  Acrocrinus, 
  the 
  last 
  

   survivor 
  of 
  the 
  Camerata, 
  has 
  been 
  regarded 
  as 
  a 
  structure 
  sui 
  gen- 
  

   eris, 
  appearing 
  suddenly 
  at 
  the 
  end 
  of 
  the 
  series, 
  as 
  a 
  reversion 
  to 
  

   their 
  cystid 
  ancestry. 
  

  

  While 
  such 
  a 
  multiplication 
  of 
  plates 
  is 
  not 
  uncommon 
  among 
  

   the 
  irregular, 
  many-plated 
  cystid 
  types, 
  the 
  definite 
  insertion 
  of 
  an 
  

   extra 
  ring 
  of 
  primary 
  plates 
  in 
  a 
  form 
  of 
  otherwise 
  regular 
  construc- 
  

   tion 
  is 
  to 
  be 
  noted 
  in 
  the 
  Ordovician 
  genus 
  Macrocystella, 
  thus 
  pro- 
  

   ducing 
  a 
  dorsal 
  cup 
  of 
  4 
  rings 
  of 
  plates. 
  This 
  modification 
  was 
  not 
  

   followed 
  up 
  in 
  either 
  of 
  the 
  orders 
  of 
  the 
  crinoids. 
  But 
  it 
  is 
  now 
  

   of 
  interest 
  to 
  note 
  that 
  the 
  extensive 
  development 
  of 
  such 
  addi- 
  

   tional 
  plates 
  in 
  Acrocrinus 
  is 
  not 
  the 
  sudden 
  occurrence 
  that 
  we 
  

   have 
  hitherto 
  supposed, 
  but 
  is 
  the 
  culmination 
  of 
  one 
  of 
  the 
  several 
  

   remarkable 
  modifications 
  that 
  took 
  place 
  in 
  the 
  generic 
  types 
  repre- 
  

   sented 
  by 
  Dichocrinus 
  and 
  its 
  derivitives. 
  As 
  evidence 
  of 
  this 
  fact 
  

   I 
  am 
  able 
  to 
  offer 
  well-marked 
  specimens 
  of 
  two 
  species 
  from 
  widely 
  

   different 
  horizons. 
  

   23832—26 
  1 
  

  

  