﻿166 
  ME. 
  F. 
  A. 
  BATHER 
  ON 
  TKIGONOCMNUS 
  FROM 
  

  

  Pentamerous 
  symmetry 
  still 
  dominates 
  the 
  Crinoidea. 
  Rum- 
  

   maging 
  the 
  records 
  of 
  the 
  past, 
  we 
  do 
  find 
  4-rayed 
  genera 
  — 
  e. 
  g. 
  

   Tiaracrinus 
  quadrifrons, 
  Schultze, 
  Tetramerocrinites 
  formosus, 
  

   Austin 
  *, 
  and 
  Tetracrinus 
  ; 
  but 
  in 
  the 
  long 
  run 
  every 
  attempt 
  at 
  

   this 
  and 
  other 
  compositions 
  has 
  failed. 
  Nature, 
  " 
  so 
  careful 
  of 
  the 
  

   type," 
  has 
  probably 
  kept 
  it 
  true 
  by 
  a 
  restraining 
  selective 
  action 
  

   dependent 
  on 
  obvious 
  mechanical 
  principles. 
  In 
  the 
  pentamerous 
  

   calyx 
  every 
  line 
  of 
  weakness 
  is 
  met 
  halfway 
  by 
  a 
  solid 
  plate: 
  but 
  

   whenever 
  a 
  hexamerous 
  or 
  tetramerous 
  condition 
  is 
  attained, 
  the 
  

   lines 
  of 
  weakness 
  pass 
  right 
  across 
  the 
  calyx; 
  such 
  forms 
  are 
  there- 
  

   fore 
  liable 
  to 
  destruction. 
  Simple 
  Fusion 
  does 
  not 
  alter 
  original 
  

   form 
  ; 
  so 
  far 
  as 
  this 
  goes, 
  the 
  tripartite 
  base 
  of 
  many 
  Palaeozoic 
  

   crinoids 
  is 
  still 
  of 
  pentamerous 
  symmetry. 
  Atrophy 
  of 
  the 
  two 
  

   larger 
  basals 
  and 
  Hypertrophy 
  of 
  the 
  smaller 
  one 
  are 
  required 
  to 
  

   induce 
  iso-trimerism 
  f 
  . 
  This 
  once 
  attained, 
  the 
  same 
  opposition 
  to 
  

   lines 
  of 
  weakness 
  is 
  found, 
  and 
  structural 
  equilibrium 
  is 
  maintained. 
  

   The 
  other 
  classes 
  of 
  the 
  Echinodermata, 
  while 
  similarly 
  retaining 
  

   penta-symmetry 
  as 
  the 
  normal 
  plan, 
  do 
  often 
  produce 
  4-rayed 
  and 
  

   6-rayed 
  forms. 
  A 
  few 
  examples 
  are 
  quoted 
  in 
  the 
  Appendix. 
  Some 
  

   writers 
  have 
  regarded 
  these 
  as 
  due 
  to 
  reversion 
  J. 
  However 
  that 
  may 
  

   be, 
  we 
  may 
  well 
  suppose 
  that 
  the 
  conditions, 
  mechanical 
  or 
  otherwise, 
  

   which 
  are 
  now 
  merely 
  restraining, 
  were 
  once 
  evolutionary 
  in 
  their 
  

   action. 
  The 
  constant 
  occurrence 
  in 
  the 
  " 
  Ur-Gruppe 
  " 
  — 
  Cystoidea 
  — 
  of 
  

   tetramerous 
  or 
  hexamerous 
  systems 
  and, 
  a 
  priori, 
  the 
  simpler 
  nature 
  

   of 
  such 
  division, 
  renders 
  it 
  quite 
  possible 
  that 
  the 
  Echinodermata 
  were 
  

   at 
  first 
  less 
  definite 
  in 
  their 
  plan 
  of 
  structure 
  ; 
  and 
  that, 
  of 
  the 
  many 
  

   types 
  which 
  at 
  first 
  took 
  the 
  field 
  in 
  the 
  struggle 
  for 
  existence, 
  the 
  

   pentamerous 
  type 
  was 
  victorious 
  ; 
  finally, 
  that 
  this 
  type 
  has 
  become 
  

   fixed, 
  and 
  to 
  it 
  all 
  Echinodermata 
  subjected, 
  through 
  methods 
  above 
  

   described, 
  controlled 
  by 
  natural 
  selection 
  §. 
  

  

  Such 
  a 
  view 
  would 
  lend 
  fresh 
  interest 
  to 
  the 
  stem-structure 
  pre- 
  

   valent 
  in 
  the 
  Larviformia, 
  W. 
  and 
  S. 
  Here, 
  as 
  a 
  general 
  rule, 
  four, 
  

   and 
  not 
  five, 
  canals 
  surround 
  the 
  central 
  canal 
  of 
  the 
  stem. 
  True, 
  this 
  

   is 
  not 
  invariably 
  connected 
  with 
  four 
  radii 
  in 
  the 
  calyx 
  ; 
  at 
  the 
  same 
  

  

  * 
  Perhaps 
  only 
  a 
  sport. 
  T. 
  and 
  T. 
  Austin, 
  " 
  Description 
  of 
  several 
  new 
  

   Grenera 
  and 
  Species 
  of 
  Crinoidea? 
  Ann. 
  & 
  Mag. 
  Nat. 
  Hist. 
  ser. 
  1, 
  xi. 
  pp. 
  195- 
  

   207 
  (London, 
  1843); 
  see 
  p. 
  203. 
  

  

  t 
  See 
  E. 
  Beyricb, 
  " 
  On 
  the 
  Base 
  (Pelvis) 
  of 
  the 
  Crinoidea 
  Brackiata" 
  Ann. 
  

   & 
  Mag. 
  Nat. 
  Hist. 
  ser. 
  4, 
  vii. 
  pp. 
  393-411, 
  1871. 
  Translated 
  by 
  W 
  : 
  S. 
  Dallas 
  

   from 
  Monatsber. 
  d. 
  k. 
  preuss. 
  Akad. 
  d. 
  Wiss. 
  zu 
  Berlin, 
  part 
  for 
  Jan. 
  1871, 
  

   pp. 
  33-55. 
  

  

  j 
  Wilhelm 
  Haacke," 
  Zur 
  Morphologie 
  der 
  Seeigel-schale," 
  Zoologischer 
  Anzei- 
  

   ger, 
  herausg. 
  J. 
  V. 
  Car 
  us, 
  viii. 
  Jahrg. 
  pp. 
  490-493 
  (Leipzig, 
  1885) 
  ; 
  and 
  " 
  Ueber 
  

   die 
  urspriinglichen 
  Grundzahlen 
  der 
  Medusen 
  und 
  Echinodermen," 
  ibid. 
  pp. 
  505- 
  

   507(1885). 
  

  

  § 
  Some 
  will 
  no 
  doubt 
  regard 
  the 
  Holothurians 
  with 
  their 
  soft 
  bodies 
  as 
  fur- 
  

   nishing 
  a 
  fatal 
  objection 
  to 
  the 
  above 
  hypothesis. 
  This, 
  however, 
  can 
  only 
  be 
  in 
  

   so 
  far 
  as 
  the 
  explanation 
  relies 
  on 
  mechanical 
  principles. 
  But 
  the 
  spicules 
  and 
  

   degenerate 
  plates 
  of 
  the 
  Holothurians 
  may 
  be 
  evidence 
  that 
  they 
  are 
  descended 
  

   from 
  ancestors 
  in 
  which 
  the 
  typical 
  skeleton 
  was 
  better 
  developed 
  ; 
  they 
  may 
  stand 
  

   in 
  the 
  same 
  relation 
  to 
  the 
  rest 
  of 
  the 
  Echinoderms 
  as 
  the 
  now 
  naked 
  Octopoda 
  

   do 
  to 
  the 
  shell-bearing 
  Cephalopoda. 
  — 
  Kote 
  added 
  by 
  Author, 
  Jan. 
  15, 
  1889. 
  

  

  