﻿42 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol. 
  07 
  

  

  ACROCRINUS 
  PRAECURSOR, 
  new 
  species 
  

  

  Plate 
  12, 
  fig. 
  1 
  

  

  I 
  have 
  given 
  this 
  name 
  to 
  an 
  isolated 
  specimen 
  from 
  the 
  Burlington 
  

   limestone 
  consisting 
  of 
  the 
  calyx 
  and 
  part 
  of 
  the 
  cuneate 
  arms, 
  some- 
  

   what 
  displaced, 
  at 
  first 
  supposed 
  to 
  be 
  merely 
  an 
  abnormal 
  Dicho- 
  

   crinus. 
  The 
  bisected 
  base 
  and 
  the 
  radials 
  with 
  the 
  interposed 
  anal 
  

   plate 
  are 
  distinct, 
  and 
  in 
  good 
  condition. 
  Between 
  these 
  two 
  primi- 
  

   tive 
  rings 
  of 
  plates 
  is 
  interpolated 
  a 
  wide 
  band 
  of 
  supplementary 
  

   pieces 
  occupying 
  more 
  than 
  half 
  the 
  total 
  height 
  of 
  the 
  dorsal 
  cup. 
  

   Those 
  next 
  to 
  the 
  radials 
  are 
  more 
  than 
  half 
  their 
  size, 
  and 
  form 
  a 
  

   ring 
  alternating 
  with 
  them 
  ; 
  from 
  there 
  down 
  to 
  the 
  basals 
  the 
  plates 
  

   diminish 
  rapidly 
  in 
  size, 
  and 
  the 
  alternation 
  becomes 
  irregular, 
  but 
  

   represents 
  the 
  equivalent 
  of 
  at 
  least 
  three 
  additional 
  rings 
  of 
  plates. 
  

   The 
  smaller 
  size 
  of 
  the 
  lower 
  plates 
  would 
  indicate 
  that 
  they 
  were 
  

   the 
  latest 
  formed. 
  The 
  anal 
  plate, 
  which 
  is 
  fully 
  as 
  large 
  as 
  the 
  

   radials, 
  is 
  succeeded 
  downward 
  by 
  a 
  diminishing 
  vertical 
  series 
  of 
  

   three 
  plates 
  in 
  line 
  with 
  the 
  interbasal 
  suture. 
  

  

  As 
  compared 
  with 
  species 
  of 
  Dichocrinus 
  in 
  the 
  same 
  formation 
  

   the 
  radials 
  are 
  very 
  much 
  shorter, 
  the 
  space 
  which 
  they 
  ordinarily 
  

   occupy 
  being 
  in 
  part 
  taken 
  by 
  the 
  anomalous 
  additional 
  plates. 
  As 
  

   the 
  relative 
  height 
  of 
  the 
  radials 
  becomes 
  important, 
  the 
  following 
  

   measurements 
  of 
  the 
  type 
  specimen 
  may 
  be 
  noted: 
  total 
  height 
  of 
  

   calyx 
  9 
  mm. 
  ; 
  of 
  radials 
  2.5 
  mm. 
  ; 
  of 
  basals 
  1.5 
  mm. 
  ; 
  of 
  band 
  of 
  sup- 
  

   plementary 
  plates 
  5 
  mm. 
  Thus 
  the 
  radials 
  occupy 
  27 
  per 
  cent 
  of 
  the 
  

   height 
  of 
  the 
  cup 
  ; 
  whereas 
  in 
  four 
  of 
  the 
  principal 
  species 
  of 
  Dicho- 
  

   cHnus 
  of 
  the 
  Upper 
  Burlington 
  limestone 
  the 
  radials 
  constitute 
  

   from 
  60 
  to 
  66 
  per 
  cent 
  of 
  the 
  height 
  of 
  the 
  cup; 
  and 
  in 
  no 
  other 
  

   species 
  from 
  any 
  formation 
  do 
  they 
  occupy 
  less 
  than 
  50 
  per 
  cent. 
  

   In 
  other 
  words, 
  these 
  are 
  not 
  the 
  radials 
  of 
  Dichocrinus. 
  The 
  arms, 
  

   ten 
  in 
  number, 
  and 
  relatively 
  slender, 
  are 
  somewhat 
  displaced 
  in 
  

   the 
  fossil, 
  and 
  those 
  which 
  appear 
  directly 
  above 
  the 
  anal 
  series 
  do 
  

   not 
  belong 
  there. 
  

  

  From 
  the 
  fact 
  that 
  no 
  such 
  a 
  specimen 
  has 
  ever 
  been 
  seen 
  before 
  in 
  

   all 
  the 
  numerous 
  collections 
  made 
  at 
  Burlington 
  during 
  more 
  than 
  

   half 
  a 
  century, 
  it 
  might 
  be 
  suggested 
  that 
  this 
  is 
  a 
  mere 
  sporadic 
  

   occurrence. 
  And 
  perhaps 
  it 
  is. 
  Nevertheless 
  it 
  is 
  a 
  definite 
  struc- 
  

   ture, 
  foreshadowing 
  one 
  of 
  the 
  most 
  remarkable 
  derivatives 
  of 
  

   Dichocrinus, 
  and 
  containing 
  all 
  its 
  essential 
  characters; 
  therefore 
  a 
  

   place 
  for 
  it 
  must 
  be 
  found. 
  The 
  question 
  is 
  whether 
  to 
  call 
  it 
  a 
  de- 
  

   layed 
  Dichocrinus 
  or 
  a 
  premature 
  Acrocrinus? 
  This 
  question 
  has 
  

   in 
  fact 
  been 
  answered 
  in 
  advance; 
  for 
  just 
  such 
  a 
  contingency 
  as 
  is 
  

   here 
  presented 
  was 
  provided 
  for 
  by 
  Wachsmuth 
  and 
  Springer 
  in 
  

   the 
  Camerata 
  Monograph, 
  when 
  in 
  discussing 
  these 
  genera 
  we 
  said 
  

   on 
  page 
  804: 
  

  

  