﻿46 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol. 
  «7 
  

  

  The 
  essential 
  calyx 
  elements 
  of 
  Pterotocrinus 
  are 
  the 
  same 
  as 
  

   those 
  of 
  Dichocrinus, 
  namely, 
  two 
  basals, 
  a 
  ring 
  of 
  radials, 
  and 
  an 
  

   anal 
  plate 
  in 
  line 
  with 
  them. 
  The 
  line 
  of 
  succession 
  between 
  them 
  

   through 
  the 
  genus 
  Talarocrinus, 
  as 
  was 
  stated 
  by 
  Wachsmuth 
  and 
  

   Springer 
  when 
  proposing 
  it, 
  is 
  plain 
  and 
  evident. 
  It 
  is 
  intermediate 
  

   in 
  structure, 
  and 
  partly 
  so 
  in 
  time. 
  While 
  Dichocrinus 
  carries 
  its 
  

   strong 
  and 
  simple 
  calyx 
  through 
  from 
  the 
  Kinderhook 
  to 
  the 
  end 
  of 
  

   the 
  Lower 
  Carboniferous, 
  Talarocrinus 
  as 
  one 
  of 
  its 
  off-shoots 
  begins 
  

   with 
  an 
  isolated 
  species 
  in 
  the 
  Warsaw, 
  and 
  develops 
  mainly 
  in 
  the 
  

   lower 
  Chester; 
  and 
  Pterotocrinus, 
  although 
  first 
  occurring 
  in 
  one 
  of 
  

   the 
  lower 
  formations 
  of 
  the 
  Chester, 
  is 
  characteristically 
  a 
  genus 
  of 
  

   the 
  upper 
  Chester. 
  Talarocrinus 
  was 
  short 
  lived, 
  sharply 
  limited 
  

   in 
  time, 
  and 
  is 
  found 
  by 
  the 
  geologists 
  to 
  be 
  an 
  excellent 
  horizon 
  

   marker. 
  (See 
  pis. 
  13, 
  14.) 
  

  

  Talarocrinus 
  has 
  the 
  same 
  basals, 
  radials, 
  and 
  anal 
  plate 
  as 
  

   Dichocrinus, 
  but 
  drops 
  one 
  primibrach, 
  retaining 
  a 
  small 
  triangular 
  

   axillary. 
  In 
  Pterotocrinus 
  the 
  minute 
  primibrach, 
  sometimes 
  in- 
  

   visible, 
  is 
  followed 
  by 
  single 
  secundibrachs, 
  also 
  axillary, 
  which 
  

   often 
  together 
  with 
  the 
  outer 
  tertibrachs 
  rest 
  almost 
  horizontally 
  

   within 
  the 
  radial 
  facet 
  and 
  are 
  suturally 
  connected 
  with 
  it, 
  so 
  that 
  

   the 
  ray 
  to 
  that 
  extent 
  is 
  not 
  free, 
  but 
  is 
  incorporated 
  in 
  the 
  dorsal 
  

   cup. 
  The 
  Talarocrinus 
  tegmen 
  enlarges 
  with 
  a 
  variety 
  of 
  more 
  or 
  

   less 
  tumid 
  plates, 
  among 
  which 
  the 
  axillary 
  ambulacral 
  especially 
  

   begins 
  to 
  develop 
  into 
  prominence, 
  from 
  low 
  convex 
  to 
  sharp, 
  no- 
  

   dose 
  or 
  spiniferous 
  (pi. 
  13, 
  figs. 
  1-15; 
  pi. 
  14, 
  fig. 
  6). 
  Not 
  all 
  of 
  

   these 
  plates 
  are 
  so 
  modified 
  ; 
  among 
  several 
  hundred 
  well 
  preserved 
  

   specimens 
  in 
  which 
  the 
  general 
  tendency 
  can 
  be 
  observed, 
  it 
  appears 
  

   that 
  about 
  half 
  of 
  them 
  are 
  distinctly 
  enlarged. 
  Some 
  of 
  these 
  are 
  

   shown 
  by 
  figures 
  on 
  plate 
  78 
  of 
  the 
  Camerata 
  mongraph, 
  and 
  the 
  

   genus 
  has 
  also 
  been 
  extensively 
  illustrated 
  by 
  Ulrich 
  23 
  and 
  by 
  

   Weller, 
  24 
  from 
  whose 
  figures 
  the 
  many 
  variations 
  can 
  be 
  studied 
  in 
  

   detail. 
  

  

  The 
  modified 
  ambulacral 
  in 
  Pterotocrinus 
  is 
  seated 
  in 
  a 
  distinct 
  

   facet, 
  from 
  which 
  it 
  sometimes 
  falls 
  out 
  (pi. 
  13, 
  fig. 
  20), 
  and 
  the 
  

   same 
  thing 
  occurs 
  in 
  Talarocrinus 
  (pi. 
  13, 
  fig. 
  13). 
  Along 
  with 
  

   these 
  tendencies, 
  it 
  is 
  now 
  further 
  instructive 
  to 
  see 
  that 
  occasion- 
  

   ally 
  in 
  Talarocrinus 
  the 
  anal 
  plate 
  is 
  reduced 
  to 
  the 
  acuminate 
  piece 
  

   of 
  Pterotocrinus 
  (pi. 
  13, 
  fig. 
  5). 
  

  

  23 
  Mississippian 
  formations 
  of 
  western 
  Kentucky, 
  1917, 
  pis. 
  S, 
  9, 
  10. 
  

  

  24 
  Illinois 
  State 
  Geol. 
  Surv. 
  Bulletin 
  41, 
  1920, 
  pi. 
  G. 
  

  

  