﻿art 
  9 
  UNUSUAL. 
  FORMS 
  OF 
  FOSSIL 
  CRINOIDS 
  SPRINGER 
  71 
  

  

  eally 
  dichotomous 
  arms, 
  it 
  will 
  include 
  some 
  intermediate 
  forms 
  in 
  

   which 
  there 
  are 
  one 
  or 
  two 
  arms 
  branching 
  unequally 
  to 
  the 
  inner 
  

   side 
  of 
  the 
  ray, 
  provided 
  these 
  are 
  rounded 
  arms, 
  not 
  abutting 
  as 
  in 
  

   Zeacrinus. 
  Observance 
  of 
  this 
  distinction 
  will 
  help 
  to 
  remedy 
  some 
  

   of 
  the 
  confusion 
  in 
  the 
  descriptions. 
  The 
  type 
  species, 
  P. 
  aequalis 
  

   (Hall) 
  of 
  the 
  Keokuk, 
  44 
  is 
  shown 
  by 
  good 
  figures 
  in 
  Geological 
  Sur- 
  

   vey 
  of 
  Illinois 
  (vol. 
  5, 
  pi. 
  15, 
  fig. 
  6), 
  in 
  Zittel-Eastman 
  (1913, 
  p. 
  222, 
  

   fig. 
  323), 
  and 
  herein 
  (pi. 
  16, 
  fig. 
  1). 
  It 
  is 
  a 
  very 
  abundant 
  species, 
  

   and 
  specimens 
  are 
  to 
  be 
  seen 
  in 
  almost 
  all 
  collections. 
  Sac 
  and 
  

   opening 
  are 
  similar 
  to 
  those 
  of 
  P. 
  arboreus, 
  but 
  rarely 
  seen. 
  

   Other 
  good 
  examples 
  of 
  this 
  type 
  are 
  : 
  

  

  Pachylocrinus 
  (Potcriocrinus) 
  concinnus 
  Meek 
  and 
  Wokthen, 
  Geol. 
  Surv. 
  

  

  Illinois, 
  vol. 
  5, 
  1873, 
  pi. 
  14, 
  fig. 
  3. 
  

   Pachylocr. 
  (Poteriocr.) 
  jesupi 
  Whitfield, 
  Bull. 
  1, 
  Aruer. 
  Mus. 
  Nat. 
  Hist, 
  

  

  1881, 
  pis. 
  1, 
  2. 
  Syn. 
  of 
  P. 
  swallovi, 
  Meek 
  and 
  Worthen, 
  Geol. 
  Surv. 
  

  

  111., 
  vol. 
  2, 
  pi. 
  16, 
  figs. 
  4«, 
  b. 
  

   Pachylocr. 
  (Poteriocr.) 
  coxa 
  mis 
  Wokthen, 
  Geol. 
  Surv. 
  Illinois, 
  vol. 
  7, 
  

  

  1883, 
  pi. 
  27, 
  fig. 
  1. 
  12 
  arms 
  to 
  the 
  ray. 
  

   Pachylocr. 
  {poteriocr.) 
  spartarius 
  Miller 
  and 
  Gijrley, 
  Journ. 
  Cin. 
  Soc. 
  

  

  Nat. 
  Hist., 
  1S90, 
  pi. 
  7, 
  figs. 
  1, 
  2, 
  3. 
  S 
  arms 
  to 
  the 
  ray. 
  

  

  Notwithstanding 
  the 
  abundance 
  of 
  species 
  belong 
  to 
  this 
  genus, 
  

   specimens 
  showing 
  the 
  position 
  of 
  the 
  anal 
  opening 
  are 
  rare. 
  To 
  

   illustrate 
  it 
  I 
  have 
  used 
  two 
  species 
  from 
  the 
  later 
  formations, 
  in 
  

   one 
  of 
  which 
  the 
  opening 
  is 
  the 
  nearest 
  to 
  the 
  distal 
  end 
  of 
  any 
  I 
  

   have 
  seen. 
  

  

  PACHYLOCRINUS 
  AQUALIS 
  (Hall) 
  

  

  Plate 
  16, 
  fig. 
  10 
  

  

  Scaphiocrinus 
  aqualis 
  Hall, 
  Supp. 
  Geol. 
  Iowa, 
  1860, 
  p. 
  83. 
  

   Lower 
  Burlington 
  limestone 
  ; 
  Burlington, 
  Iowa. 
  

  

  I 
  am 
  figuring 
  a 
  characteristic 
  specimen 
  of 
  this 
  species, 
  which 
  is 
  

   one 
  of 
  the 
  most 
  conspicuous 
  of 
  its 
  formation, 
  but 
  has 
  not 
  before 
  been 
  

   illustrated. 
  It 
  attains 
  a 
  large 
  size, 
  some 
  specimens 
  being 
  consider- 
  

   ably 
  larger 
  than 
  the 
  one 
  here 
  figured, 
  which 
  I 
  have 
  selected 
  for 
  its 
  

   excellent 
  showing 
  of 
  the 
  sac. 
  I 
  am 
  glad 
  of 
  the 
  opportunity 
  to 
  fur- 
  

   nish 
  an 
  authentic 
  figure 
  of 
  the 
  species, 
  to 
  help 
  clarify 
  the 
  confusion 
  

   heretofore 
  existing 
  between 
  the 
  name 
  and 
  that 
  of 
  P. 
  aequalis 
  of 
  the 
  

   Keokuk. 
  

  

  PACHYLOCRINUS 
  ARBOREUS 
  (Worthen) 
  

  

  Plate 
  16, 
  figs. 
  3-7 
  

  

  Zeacrinus 
  arboreus 
  Worthen, 
  Geol. 
  Surv. 
  111., 
  vol. 
  5, 
  p. 
  534, 
  pi. 
  20, 
  fig. 
  5. 
  — 
  

   Springer, 
  Amer. 
  Geol., 
  vol. 
  26, 
  1900, 
  pi. 
  16, 
  figs. 
  1S-23. 
  

  

  Chester 
  group, 
  Ohara 
  formation 
  (formerly 
  called 
  St. 
  Louis) 
  ; 
  Huntsville, 
  

   Alabama. 
  

  

  This 
  is 
  a 
  prolific 
  species, 
  found 
  in 
  good 
  preservation, 
  and 
  repre- 
  

   sented 
  in 
  the 
  collection 
  by 
  upwards 
  of 
  one 
  hundred 
  specimens. 
  The 
  

  

  "Boston 
  Journ. 
  Nat. 
  Hist., 
  1861, 
  p. 
  316. 
  

  

  