﻿abt9 
  UNUSUAL. 
  FORMS 
  OF 
  FOSSIL 
  CRINOIDS 
  STRINGER 
  69 
  

  

  dominant, 
  and 
  hold 
  to 
  it 
  in 
  spite 
  of 
  exceptions 
  and 
  uncertain 
  cor- 
  

   relations. 
  

  

  In 
  the 
  mode 
  of 
  arm 
  branching, 
  for 
  instance, 
  between 
  a 
  ray 
  with 
  

   two 
  or 
  more 
  subordinate 
  branches 
  always 
  borne 
  on 
  the 
  same 
  side 
  of 
  

   a 
  main 
  arm, 
  as 
  in 
  Hydreionocrinus 
  depressus 
  — 
  heterotomous 
  — 
  and 
  

   one 
  in 
  which 
  the 
  arms 
  divide 
  by 
  successive 
  nearly 
  equal 
  bifurcations, 
  

   as 
  in 
  Pachylocrinus 
  aequalis 
  — 
  dichotomous 
  — 
  there 
  is 
  a 
  clear 
  distinc- 
  

   tion. 
  The 
  latter, 
  with 
  two 
  or 
  more 
  full 
  bifurcations, 
  would 
  give 
  8 
  

   or 
  more 
  arms 
  to 
  the 
  ray. 
  But 
  if 
  only 
  1 
  arm 
  of 
  the 
  second 
  bifurca- 
  

   tion 
  branches, 
  that 
  would 
  give 
  but 
  6 
  arms 
  to 
  the 
  ray, 
  making 
  an 
  in- 
  

   termediate 
  condition 
  which 
  may 
  occur 
  under 
  either 
  category, 
  and 
  

   has 
  to 
  be 
  dealt 
  with 
  according 
  to 
  circumstances, 
  as 
  for 
  example 
  in 
  

   the 
  heterotomous 
  Zeacrinus 
  elegans, 
  or 
  the 
  dichotomous 
  Pacliylo- 
  

   crinus 
  arboreus. 
  So 
  also 
  the 
  stem 
  varies 
  from 
  circular 
  to 
  penta- 
  

   gonal, 
  occasionally 
  both 
  in 
  one 
  species, 
  and 
  sometimes 
  both 
  in 
  forms 
  

   not 
  otherwise 
  separable 
  generically. 
  In 
  the 
  case 
  of 
  Abrotocrinus 
  I 
  

   have 
  utilized 
  Miller 
  and 
  Gurley's 
  genus, 
  separated 
  only 
  by 
  this 
  

   character, 
  because 
  it 
  offers 
  a 
  convenient 
  means 
  of 
  subdividing 
  the 
  

   unwieldy 
  genus 
  Pachylocrinus. 
  It 
  is 
  not 
  a 
  very 
  reliable 
  character 
  

   in 
  this 
  group. 
  Pachylocrinus 
  aequalis, 
  with 
  usually 
  a 
  round 
  stem, 
  

   has 
  it 
  sometimes 
  pentagonal 
  next 
  to 
  the 
  calyx, 
  and 
  Abrotocrinus 
  

   unicus 
  has 
  the 
  stem 
  both 
  ways, 
  but 
  mostly 
  pentagonal. 
  Again, 
  in 
  

   Zeacrinus, 
  the 
  character 
  most 
  relied 
  on 
  is 
  the 
  very 
  short 
  brachials, 
  

   which 
  we 
  call 
  quadrangular, 
  notwithstanding 
  the 
  fact 
  that 
  in 
  various 
  

   specimens 
  some 
  of 
  the 
  lower 
  brachials 
  are 
  clearly 
  cuneiform. 
  Such 
  

   exceptions, 
  being 
  part 
  of 
  the 
  infinite 
  variety 
  in 
  nature, 
  must 
  not 
  dis- 
  

   turb 
  us. 
  

  

  The 
  following 
  table 
  shows 
  the 
  relations 
  of 
  the 
  genera 
  as 
  they 
  

   appear 
  in 
  the 
  light 
  of 
  present 
  information 
  : 
  

  

  ANALYSIS 
  OF 
  THE 
  GENERA 
  

  

  Subfamily 
  Poteriocrininae. 
  

  

  Section 
  A. 
  Poteriocrininae 
  with 
  elongated 
  ventral 
  sac, 
  in 
  which 
  

   the 
  anal 
  opening 
  is 
  below 
  the 
  distal 
  end, 
  and 
  at 
  the 
  anterior 
  side. 
  

   Anal 
  plates, 
  including 
  radianal, 
  strongly 
  developed 
  within 
  the 
  cup. 
  

  

  IBB 
  5: 
  2-3 
  anals 
  in 
  the 
  calyx. 
  

  

  Radial 
  facet 
  round, 
  not 
  filling 
  distal 
  face 
  of 
  R. 
  

  

  Sac 
  tapering 
  distally 
  to 
  an 
  apex 
  beyond 
  the 
  arms 
  Poteriocrinus. 
  

  

  Radial 
  facet 
  straight, 
  filling 
  distal 
  face 
  of 
  R. 
  

   Sac 
  enlarging 
  distally 
  to 
  a 
  more 
  or 
  less 
  rounded, 
  

  

  nodose 
  or 
  spiniferous 
  terminal. 
  

   Arms 
  branching 
  on 
  or 
  beyond 
  IIBr. 
  

   Branching 
  usually 
  more 
  than 
  once. 
  

   Dichotomous, 
  uniserial, 
  brachials 
  cuneiform. 
  

   Sac 
  strong, 
  club-shaped, 
  occasionally 
  spinose. 
  

  

  