﻿304 
  T. 
  H. 
  CARPENTER 
  ON 
  THE 
  RELATIONS 
  OF 
  

  

  radial 
  is 
  supported 
  upon 
  two 
  armless 
  plates, 
  the 
  left 
  one 
  of 
  which 
  is 
  

   the 
  true 
  azygos 
  plate. 
  This 
  may 
  be, 
  and 
  probably 
  is, 
  one 
  of 
  the 
  

   variations 
  of 
  growth 
  to 
  which 
  this 
  early 
  and 
  simple 
  type 
  is 
  subject. 
  

   A 
  somewhat 
  similar 
  case 
  occurs 
  in 
  Allar/ecrinus*, 
  some 
  small 
  indi- 
  

   viduals 
  having 
  but 
  three 
  distinct 
  arm-facets, 
  with 
  an 
  imperfect 
  

   fourth 
  one, 
  but 
  none 
  at 
  all 
  on 
  the 
  fifth 
  radial. 
  In 
  each 
  case, 
  how- 
  

   ever, 
  the 
  varieties 
  are 
  generically 
  identical. 
  On 
  the 
  other 
  hand, 
  

   Baerocrinus, 
  if 
  rightly 
  described 
  by 
  Grewingk 
  (fig. 
  I. 
  e 
  ; 
  PI. 
  XI. 
  

   fig. 
  1), 
  represents 
  to 
  my 
  mind 
  an 
  altogether 
  different 
  generic 
  type, 
  

   although 
  it 
  is 
  regarded 
  by 
  Zittel 
  and 
  Waohsmuth, 
  as 
  well 
  as 
  by 
  

   Schmidt, 
  as 
  synonymous 
  with 
  Hybocrinus. 
  For 
  the 
  calyx 
  consists 
  

   of 
  but 
  ten 
  plates, 
  which 
  are 
  arranged 
  in 
  two 
  alternating 
  rows 
  

   without 
  any 
  indication 
  of 
  anal 
  plates. 
  This 
  does 
  not 
  agree 
  at 
  all 
  

   with 
  Billings's 
  analysis 
  of 
  the 
  Hybocrinus-calyx. 
  

  

  Further, 
  the 
  outline 
  of 
  the 
  Baerocrinus-caljx 
  is 
  triangular 
  (fig. 
  

   II. 
  b), 
  and 
  not 
  pentagonal, 
  as 
  is 
  that 
  of 
  77. 
  dipentas 
  (fig. 
  II. 
  a), 
  

   while 
  only 
  three 
  of 
  the 
  upper 
  series 
  of 
  plates 
  bear 
  arms, 
  and 
  there 
  

   is 
  no 
  anal 
  plate 
  liko 
  that 
  of 
  Hybocrinus. 
  In 
  face 
  of 
  the 
  great 
  

   difference 
  between 
  the 
  characters 
  of 
  the 
  two 
  calyces, 
  which, 
  how- 
  

   ever, 
  is 
  described 
  by 
  Schmidt 
  as 
  a 
  ''general 
  correspondence," 
  I 
  prefer 
  

   to 
  regard 
  Baerocrinus 
  and 
  Hybocrinus 
  as 
  distinct 
  types. 
  The 
  simi- 
  

   larity 
  of 
  the 
  arms 
  on 
  which 
  Schmidt 
  insists 
  seems 
  to 
  me 
  to 
  be 
  a 
  

   comparatively 
  unimportant 
  character 
  ; 
  and 
  his 
  suggestion 
  as 
  to 
  the 
  

   loss 
  of 
  a 
  fourth 
  arm-bearing 
  plate 
  from 
  the 
  Erras 
  fossil, 
  which 
  

   would 
  render 
  it 
  more 
  similar 
  to 
  Hybocrinus, 
  is, 
  I 
  think, 
  hardly 
  

   warranted 
  by 
  the 
  condition 
  of 
  the 
  specimen. 
  If 
  one 
  be 
  supposed 
  

   lost, 
  why 
  not 
  two, 
  one 
  from 
  the 
  upper 
  face 
  of 
  each 
  armless 
  plate 
  ? 
  

   and 
  even 
  if 
  one 
  were 
  missing, 
  it 
  is 
  not 
  easy 
  to 
  see 
  where 
  it 
  could 
  be 
  

   inserted 
  so 
  as 
  to 
  make 
  the 
  calyx 
  look 
  like 
  that 
  of 
  a 
  Hybocrinus. 
  

  

  If, 
  then, 
  as 
  seems 
  to 
  me 
  probable, 
  Baerocrinus 
  is 
  generically 
  dis- 
  

   tinct 
  from 
  Hybocrinus, 
  it 
  occupies 
  a 
  somewhat 
  unique 
  position 
  

   among 
  the 
  Crinoidea. 
  It 
  is, 
  perhaps, 
  best 
  regarded 
  as 
  a 
  permanent 
  

   larval 
  form 
  which 
  has 
  only 
  developed 
  three 
  of 
  its 
  five 
  arms. 
  I 
  can 
  

   naturally 
  offer 
  no 
  opinion 
  respecting 
  " 
  Volborth's 
  organ," 
  which 
  is 
  

   attributed 
  by 
  Schmidt, 
  the 
  latest 
  writer 
  on 
  the 
  subject, 
  to 
  purely 
  

   mechanical 
  causes 
  ; 
  but 
  it 
  has 
  struck 
  me 
  as 
  possible 
  that 
  it 
  may 
  repre- 
  

   sent 
  the 
  anal 
  opening, 
  which 
  does 
  occupy 
  a 
  somewhat 
  similar 
  posi- 
  

   tion 
  between 
  the 
  radials 
  and 
  basals 
  at 
  one 
  period 
  of 
  Crinoid 
  deve- 
  

   lopment. 
  Should 
  this 
  ever 
  turn 
  out 
  to 
  be 
  the 
  case, 
  Yolborth 
  will 
  

   not 
  have 
  been 
  so 
  very 
  far 
  wrong 
  after 
  all 
  ; 
  but 
  the 
  true 
  nature 
  of 
  

   Baerocrinus 
  must 
  remain 
  somewhat 
  uncertain 
  until 
  other 
  examples 
  

   of 
  it 
  are 
  met 
  with. 
  

  

  Of 
  the 
  three 
  species 
  of 
  Hybocrinus 
  described 
  by 
  Billings, 
  two 
  

   occur 
  in 
  the 
  Trenton 
  Limestone 
  at 
  Ottawa, 
  and 
  one 
  in 
  the 
  Chazy 
  

   Limestone. 
  But 
  the 
  differences 
  of 
  the 
  latter 
  from 
  the 
  H. 
  tumidus 
  of 
  

   the 
  Trenton 
  group 
  are 
  so 
  slight 
  that 
  Billingsf 
  had 
  "much 
  doubt 
  as 
  

   to 
  the 
  propriety 
  of 
  separating 
  it 
  therefrom." 
  It 
  is, 
  however, 
  re- 
  

   tained 
  as 
  a 
  distinct 
  species 
  in 
  Messrs. 
  Wachsmuth 
  and 
  Springer's 
  

  

  * 
  Ann. 
  & 
  Mag. 
  Nat. 
  Hist. 
  ser. 
  5. 
  vol. 
  vii. 
  pp. 
  282-289, 
  pis. 
  xy. 
  and 
  xvi. 
  

   t 
  Canadian 
  Organic 
  Eemains, 
  Decade 
  iv. 
  p. 
  24. 
  

  

  