﻿162 
  ME. 
  F. 
  A. 
  BATHER 
  ON 
  TRIGONOCRINUS 
  PROM 
  

  

  radius 
  with 
  its 
  adjoining 
  interradius, 
  and 
  the 
  decrease 
  in 
  size 
  of 
  two 
  

   radii 
  and 
  the 
  interradins 
  between 
  them. 
  

  

  Instances 
  of 
  deviation 
  from 
  regular 
  penta-symmetry 
  are 
  not 
  un- 
  

   common 
  among 
  Echinodermata. 
  The 
  cause 
  of 
  such 
  deviation 
  may 
  be 
  

   one 
  of 
  many. 
  Thus, 
  the 
  increased 
  importance 
  of 
  the 
  anal 
  interradius 
  

   in 
  most 
  Crinoidea 
  from 
  Palaeozoic 
  strata 
  tends 
  to 
  hexa-symmetry 
  ; 
  

   the 
  structure 
  here 
  is 
  affected 
  by 
  function 
  : 
  Herbert 
  Spencer* 
  has 
  

   compared 
  the 
  typical 
  upright 
  condition 
  of 
  the 
  iso-pentamerous 
  

   crinoid 
  to 
  that 
  of 
  an 
  upright 
  flower 
  ; 
  surrounded 
  on 
  all 
  sides 
  by 
  an 
  

   equal 
  medium, 
  it 
  retains 
  its 
  symmetry, 
  but 
  should 
  the 
  position 
  on 
  

   the 
  stalk 
  be 
  altered, 
  the 
  radii 
  differ 
  in 
  their 
  growth, 
  e. 
  g. 
  Eug. 
  nutans, 
  

   Eudesicrinus, 
  Holopus 
  i 
  in 
  the 
  later 
  and 
  more 
  differentiated 
  among 
  

   the 
  Echinoidea 
  a 
  marked 
  bilateral 
  s}^mmetry 
  is 
  superinduced 
  on 
  the 
  

   ground-plan 
  ; 
  the 
  advantage 
  of 
  this 
  to 
  a 
  crawling 
  animal 
  is 
  obvious. 
  

  

  The 
  methods 
  through 
  which 
  organic 
  forms 
  undergo 
  change 
  have 
  

   been 
  variously 
  classified, 
  and 
  the 
  late 
  Dr. 
  Asa 
  Gray 
  distinguished 
  

   as 
  many 
  as 
  9f. 
  The 
  deviations 
  from 
  the 
  central 
  type 
  of 
  Crinoidea 
  

   may 
  be 
  referred 
  to 
  one 
  of 
  three 
  categories, 
  which 
  may, 
  for 
  brevity, 
  

   be 
  termed: 
  — 
  (i) 
  Sport, 
  (ii) 
  Hypertrophy 
  or 
  Atrophy, 
  (iii) 
  Fusion 
  

   or 
  Eission. 
  In 
  the 
  consideration 
  of 
  homologies 
  it 
  is 
  important 
  that 
  

   these 
  methods 
  should 
  be 
  discriminated. 
  

  

  (i) 
  Under 
  the 
  term 
  " 
  Sport 
  " 
  I 
  include 
  the 
  sudden 
  addition 
  or 
  

   subtraction 
  of 
  a 
  part, 
  the 
  latter 
  being 
  the 
  more 
  common. 
  The 
  4- 
  

   rayed 
  specimens 
  of 
  Eugeniacrinus 
  alluded 
  to 
  above, 
  or 
  the 
  6-rayed 
  

   Eug. 
  caryophyllatus, 
  also 
  figured 
  by 
  RosinusJ, 
  are 
  parallelled 
  by 
  

   similar 
  variations 
  in 
  all 
  polymeric 
  forms. 
  The 
  shortness 
  of 
  the 
  list 
  

   in 
  the 
  appendix, 
  though 
  I 
  fear 
  it 
  is 
  not 
  an 
  exhaustive 
  one, 
  shows 
  

   the 
  relative 
  rarity 
  of 
  such 
  cases 
  in 
  the 
  later 
  Crinoidea. 
  Some 
  of 
  

   the 
  examples 
  prove 
  that 
  it 
  is 
  possible 
  for 
  a 
  crinoid 
  to 
  grow 
  to 
  full 
  

   stature 
  and 
  to 
  enjoy 
  life 
  while 
  devoid 
  of 
  one 
  of 
  its 
  pentameres 
  ; 
  but 
  

   there 
  is 
  nothing 
  to 
  show 
  that 
  its 
  peculiarity 
  is 
  inherited 
  by 
  its 
  off- 
  

   spring. 
  There 
  is, 
  however, 
  always 
  the 
  chance 
  that, 
  perhaps 
  under 
  

   changed 
  conditions, 
  a 
  mere 
  sport 
  may 
  be 
  in 
  earnest 
  of 
  use 
  to 
  the 
  

   animal; 
  it 
  would 
  thus 
  be 
  preserved 
  and, 
  possibly, 
  perpetuated. 
  

   Many 
  sports, 
  of 
  which 
  man 
  at 
  all 
  events 
  cannot 
  see 
  the 
  advantage, 
  

   have 
  been 
  truly 
  transmitted, 
  and 
  the 
  descendants 
  thus 
  modified 
  

   have 
  in 
  many 
  cases 
  ousted 
  the 
  original 
  form§. 
  It 
  is, 
  as 
  we 
  have 
  

   seen, 
  a 
  general 
  belief 
  that 
  Eugeniacrinus 
  shows 
  a 
  tendency 
  to 
  tetra- 
  

   merism. 
  Perhaps 
  it 
  does 
  show 
  a 
  greater 
  tendency 
  than 
  some 
  genera, 
  

   e.g. 
  Pentacrinus. 
  The 
  odds 
  in 
  favour 
  of 
  such 
  a 
  sport 
  becoming 
  

   permanent 
  are 
  therefore 
  greater, 
  and 
  'Tetracrinus, 
  in 
  accordance 
  

   with 
  its 
  perfect 
  symmetry, 
  is 
  best 
  explained 
  in 
  this 
  manner 
  ; 
  it 
  is 
  

   further 
  to 
  be 
  noticed 
  that 
  the 
  genus 
  only 
  contains 
  one 
  species, 
  

   which 
  had 
  a 
  very 
  short 
  geological 
  life. 
  

  

  * 
  ' 
  Principles 
  of 
  Biology,' 
  issue 
  of 
  1880, 
  vol. 
  ii. 
  p. 
  178. 
  

  

  t 
  P. 
  E. 
  Geddes, 
  art. 
  "Morphology," 
  Enoyclop. 
  Brit. 
  ed. 
  ix. 
  vol. 
  xvi. 
  p. 
  844 
  

   (Edinburgh, 
  1883). 
  

  

  \ 
  Loo. 
  cit. 
  tab. 
  iii. 
  Classis 
  B, 
  No. 
  1. 
  

  

  § 
  For 
  several 
  instances, 
  with 
  references, 
  see 
  St. 
  George 
  Mivart, 
  ' 
  On 
  the 
  

   Genesis 
  of 
  Species,' 
  2nd 
  ed. 
  chap. 
  iv. 
  pp. 
  112-117. 
  London, 
  1871. 
  

  

  