﻿FOURTH 
  REPORT 
  OF 
  THE 
  DIRECTOR 
  I907 
  163 
  

  

  and 
  illustrated. 
  1 
  This- 
  attachment 
  is 
  so 
  effected 
  that 
  the 
  mouth 
  of 
  

   the 
  shell 
  is 
  seated 
  directly 
  over 
  the 
  anal 
  aperture 
  of 
  the 
  crinoid 
  so 
  

   that 
  the 
  former 
  may 
  catch 
  the 
  digestal 
  waste 
  of 
  the 
  latter. 
  Upon 
  

   this 
  waste 
  the 
  Platyceras 
  palpably 
  sustains 
  itself. 
  So 
  many 
  

   instances 
  of 
  this 
  conjunction 
  have 
  passed 
  under 
  examination 
  that 
  

   no 
  question 
  can 
  arise 
  as 
  to 
  the 
  fact 
  that 
  such 
  attachment 
  is 
  solely 
  

   for 
  feeding 
  purposes. 
  Suggestions 
  which 
  have 
  been 
  occasionally 
  

   made 
  that 
  the 
  attachment 
  is 
  rather 
  accidental 
  than 
  otherwise, 
  as 
  

   attachment 
  to 
  some 
  substantial 
  object 
  is 
  the 
  habit 
  of 
  the 
  gastropod, 
  

   are 
  not 
  borne 
  out 
  by 
  the 
  evidence 
  afforded 
  by 
  multitudes 
  of 
  these 
  

   cases. 
  It 
  is 
  quite 
  certain, 
  however, 
  that 
  in 
  the 
  Devonic 
  and 
  Carbonic 
  

   faunas 
  where 
  this 
  habit 
  became 
  most 
  prevalent, 
  there 
  was 
  always 
  a 
  

   predominant 
  percentage 
  of 
  the 
  gastropods 
  that 
  did 
  not 
  lend 
  them- 
  

   selves 
  to 
  it 
  ; 
  nor 
  have 
  we 
  reason 
  yet 
  to 
  conclude 
  that 
  the 
  habit 
  once 
  

   inaugurated 
  necessarily 
  continued 
  during 
  the 
  remaining 
  life 
  of 
  the 
  

   individual. 
  It 
  did 
  continue 
  for 
  a 
  considerable 
  period 
  of 
  the 
  shell's 
  

   existence 
  as 
  the 
  very 
  instructive 
  figure 
  6 
  on 
  plate 
  6 
  indicates, 
  the 
  

   concentric 
  scars 
  being 
  the 
  successive 
  impressions 
  of 
  the 
  lip 
  of 
  

   the 
  shell 
  as 
  its 
  growth 
  enlarged, 
  while 
  its 
  position 
  relative 
  to 
  the 
  

   after 
  opening 
  of 
  the 
  crinoid 
  is 
  unaltered. 
  

  

  The 
  history 
  of 
  this 
  form 
  of 
  dependence 
  is 
  extraordinary 
  and 
  

   illuminating. 
  Throughout 
  the 
  Siluric 
  the 
  crinoids 
  and 
  cystids 
  

   abounded 
  but 
  mollusks 
  of 
  the 
  limpetlike 
  construction 
  of 
  Platy- 
  

   ceras 
  were 
  few. 
  Moreover 
  the 
  crinoids 
  were 
  for 
  the 
  most 
  part 
  

   built 
  with 
  slender 
  domes 
  well 
  hedged 
  about 
  by 
  delicate 
  arms, 
  and 
  

   on 
  these 
  domes 
  the 
  mollusk 
  might 
  have 
  found 
  difficulty 
  in 
  securing 
  

   a 
  footing. 
  

  

  The 
  earliest 
  intimation 
  of 
  the 
  tendency 
  on 
  the 
  part 
  of 
  a 
  mollusk 
  

   to 
  seek 
  its 
  food 
  from 
  the 
  rejectamenta 
  of 
  the 
  crinoid 
  is 
  afforded 
  by 
  

   an 
  example 
  of 
  a 
  Lower 
  Siluric 
  Glyptocrinus 
  which 
  holds 
  within 
  its 
  

   arms 
  and 
  in 
  a 
  feeding 
  posture 
  a 
  shell 
  of 
  the 
  holostomatous 
  gastro- 
  

   pod 
  Cyclonema 
  bilix. 
  One 
  might 
  regard 
  the 
  occurrence 
  

   accidental 
  if 
  it 
  had 
  not 
  been 
  observed 
  more 
  than 
  once. 
  

  

  In 
  the 
  Upper 
  Siluric, 
  Platyceras 
  had 
  become 
  somewhat 
  more 
  

   abundant 
  but 
  its 
  numerical 
  development 
  did 
  not 
  reach 
  that 
  of 
  the 
  

   allied 
  mollusk 
  Diaphorostoma 
  and 
  in 
  plate 
  6, 
  figure 
  i 
  , 
  we 
  have 
  an 
  illus- 
  

   tration 
  of 
  a 
  small 
  shell 
  of 
  this 
  latter 
  genus 
  attached 
  over 
  the 
  after 
  

   of 
  the 
  cystid 
  Caryocrinus 
  ornatus 
  (Rochester 
  shale) 
  . 
  

   Thus 
  far 
  in 
  time 
  no 
  examples 
  have 
  come 
  to 
  our 
  observation 
  of 
  

  

  *See 
  particularly 
  C. 
  R. 
  Keyes. 
  Synopsis 
  of 
  American 
  Carbonic 
  Calyptraeidae. 
  Acad. 
  Nat. 
  

   Sci. 
  Phila. 
  Proc. 
  1890. 
  p. 
  150. 
  The 
  author 
  here 
  records 
  a 
  long 
  list 
  of 
  these 
  parasitic 
  asso- 
  

   ciations 
  and 
  especially 
  indicates 
  the 
  effect 
  of 
  this 
  condition 
  in 
  modifying 
  the 
  aperture 
  of 
  

   the 
  gastropod. 
  

  

  