﻿78 
  PKOCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.67 
  

  

  main 
  for 
  the 
  typical 
  species, 
  and 
  my 
  chief 
  object 
  in 
  introducing 
  

   it 
  now 
  is 
  to 
  illustrate 
  more 
  fully 
  the 
  form 
  and 
  structure 
  of 
  the 
  

   ventral 
  sac, 
  which 
  is 
  especially 
  notable 
  for 
  its 
  variability. 
  Later 
  ac- 
  

   quired 
  specimens 
  confirm 
  the 
  description 
  of 
  the 
  sac 
  given 
  on 
  page 
  

   127 
  of 
  the 
  work 
  above 
  cited 
  as 
  to 
  some 
  of 
  the 
  species, 
  but 
  exhibit 
  

   great 
  modification 
  of 
  it 
  in 
  others. 
  They 
  also 
  enable 
  me 
  to 
  show, 
  

   what 
  was 
  not 
  before 
  known, 
  the 
  position 
  of 
  the 
  anal 
  opening, 
  which 
  

   notwithstanding 
  the 
  acuminate 
  form 
  of 
  the 
  sac 
  as 
  described, 
  hinting 
  

   somewhat 
  at 
  the 
  possibility 
  of 
  a 
  distal 
  opening, 
  proves 
  to 
  be 
  strictly 
  

   in 
  accordance 
  with 
  the 
  type 
  characteristic 
  of 
  the 
  other 
  genera, 
  

   namely, 
  at 
  the 
  anterior 
  side, 
  about 
  midway. 
  

  

  The 
  differences 
  in 
  form 
  of 
  the 
  sac 
  bear 
  some 
  relation 
  to 
  the 
  dif- 
  

   ferent 
  geological 
  positions 
  of 
  the 
  species 
  in 
  which 
  they 
  occur, 
  and 
  

   the 
  great 
  stratigraphic 
  range 
  of 
  the 
  genus, 
  from 
  the 
  Kinderhook 
  to 
  

   the 
  Coal 
  Measures, 
  offers 
  an 
  excellent 
  field 
  for 
  modification 
  of 
  such 
  a 
  

   special 
  structure. 
  

  

  The 
  dominant 
  characters 
  of 
  the 
  genus 
  are 
  the 
  rather 
  flat, 
  closely 
  

   abutting, 
  infolding 
  arms, 
  meeting 
  by 
  linear 
  margins, 
  branching 
  re- 
  

   peatedly 
  but 
  only 
  from 
  the 
  outer 
  arms 
  of 
  the 
  ray 
  toward 
  the 
  inner 
  

   side 
  of 
  the 
  dichotom 
  ; 
  and 
  the 
  uniserial 
  brachials, 
  usually 
  short, 
  wide, 
  

   and 
  quadrangular, 
  but 
  with 
  exceptions 
  in 
  the 
  last 
  respect 
  in 
  the 
  

   lower 
  part. 
  This 
  excludes 
  nearly 
  all 
  species 
  with 
  rounded, 
  divergent 
  

   arms, 
  and 
  with 
  wedge-shaped 
  brachials, 
  the 
  inclusion 
  of 
  which 
  by 
  

   authors 
  has 
  been 
  the 
  cause 
  of 
  considerable 
  confusion; 
  but 
  neverthe- 
  

   less 
  allowance 
  must 
  be 
  made 
  for 
  intermediate 
  forms, 
  some 
  of 
  which 
  

   have 
  to 
  be 
  included. 
  It 
  was 
  formerly 
  supposed 
  that 
  the 
  genus 
  re- 
  

   quired 
  also 
  a 
  depressed 
  cup, 
  basin-shaped, 
  with 
  more 
  or 
  less 
  con- 
  

   cave 
  base; 
  but 
  we 
  are 
  compelled 
  to 
  admit 
  an 
  exception 
  in 
  a 
  well 
  

   marked, 
  otherwise 
  characteristic 
  species 
  with 
  an 
  elongate, 
  turbinate 
  

   calyx, 
  as 
  was 
  recognized 
  in 
  the 
  diagnosis 
  made 
  by 
  Wachsmuth 
  and 
  

   Springer. 
  In 
  fact, 
  it 
  seems 
  after 
  a 
  final 
  review 
  of 
  the 
  material 
  now 
  

   available 
  that 
  the 
  only 
  stable 
  character 
  is 
  the 
  heterotomous 
  arm- 
  

   branching, 
  with 
  mostly 
  uniserial, 
  short 
  and 
  distally 
  quadrangular 
  

   brachials. 
  Within 
  this 
  may 
  occur: 
  depressed 
  or 
  turbinate 
  calyx; 
  

   wide 
  or 
  narrow 
  anal 
  area; 
  long 
  or 
  short 
  RA; 
  flat 
  or 
  rounded 
  arms; 
  

   club 
  or 
  balloon-shaped 
  or 
  pyramidal 
  sac 
  ; 
  round 
  or 
  pentagonal 
  stem. 
  

  

  Variations 
  in 
  the 
  structure 
  of 
  the 
  anal 
  side 
  have 
  caused 
  some 
  con- 
  

   fusion 
  in 
  the 
  attempt 
  to 
  formulate 
  a 
  generic 
  diagnosis 
  which 
  would 
  

   reconcile 
  the 
  differences 
  between 
  the 
  earlier 
  and 
  later 
  species. 
  In 
  

   the 
  typical 
  Chester 
  species, 
  Z. 
  wortheni, 
  the 
  radianal 
  is 
  a 
  long 
  and 
  

   narrow 
  plate, 
  often 
  passing 
  down 
  between 
  two 
  basals 
  almost 
  to 
  a 
  con- 
  

   tact 
  with 
  the 
  infrabasals 
  — 
  a 
  form 
  and 
  position 
  so 
  unusual 
  that 
  it 
  

   has 
  attracted 
  special 
  attention. 
  But 
  all 
  species 
  from 
  the 
  earlier 
  

   formations 
  are 
  now 
  known 
  to 
  have 
  a 
  short 
  and 
  broad 
  radianal; 
  

  

  