﻿232 
  PKOCEEDINGS 
  OF 
  THE 
  MALACOLOGICAL 
  SOCIETY. 
  

  

  of 
  (lark 
  brown 
  or 
  black. 
  Sometimes 
  the 
  forward 
  edge 
  was 
  ragged 
  

   and 
  untrimmed, 
  instead 
  of 
  being 
  sharp 
  and 
  clearh' 
  rounded. 
  

  

  In 
  the 
  cases 
  where 
  the 
  operculum 
  was 
  absent 
  altogether, 
  the 
  area 
  

   left 
  uncovered 
  was 
  not 
  smooth, 
  as 
  it 
  is 
  when 
  one 
  peels 
  off 
  an 
  

   operculum 
  from 
  a 
  perfect 
  specimen. 
  Tlie 
  surface 
  where 
  the 
  

   operculum 
  should 
  have 
  been 
  was 
  contracted, 
  wrinkled, 
  and 
  raised 
  

   into 
  knotty 
  humps, 
  as 
  if 
  the 
  animal 
  were 
  making 
  some 
  effort 
  to 
  

   compensate 
  for 
  such 
  protection 
  as 
  is 
  afforded 
  by 
  the 
  operculum, 
  by 
  

   a 
  thickening 
  of 
  the 
  integument 
  in 
  that 
  area. 
  

  

  What 
  particular 
  purpose 
  is 
  served 
  by 
  the 
  operculum 
  in 
  this 
  and 
  

   many 
  other 
  kindred 
  species 
  of 
  Mollusca 
  is 
  not 
  quite 
  clear, 
  since 
  the 
  

   animal 
  remains 
  normally 
  adherent 
  to 
  the 
  rock 
  by 
  its 
  foot, 
  on 
  some 
  

   point 
  on 
  the 
  upper 
  surface 
  of 
  whose 
  hinder 
  portion 
  the 
  operculum 
  is 
  

   carried. 
  It 
  is 
  only 
  when 
  the 
  animal 
  becomes 
  detached 
  from 
  its 
  

   position 
  that 
  the 
  operculum 
  closes 
  the 
  mouth 
  of 
  the 
  shell, 
  and 
  it 
  is 
  

   clear 
  that 
  to 
  become 
  detached 
  is 
  the 
  last 
  thing 
  which 
  is 
  desirable 
  for 
  

   species 
  which 
  normally 
  live 
  adherent 
  ; 
  in 
  many 
  cases 
  they 
  must 
  be 
  

   unable 
  to 
  regain 
  their 
  position. 
  

  

  In 
  the 
  present 
  case 
  it 
  seemed 
  probable 
  that 
  overcrowding 
  and 
  

   under-feeding 
  were 
  the 
  cause 
  of 
  the 
  defects 
  of 
  the 
  Nucella. 
  In 
  size 
  

   the 
  sliell 
  was 
  not 
  more 
  stunted 
  than 
  specimens 
  to 
  be 
  found 
  on 
  almost 
  

   any 
  exposed 
  coast. 
  But 
  there 
  must 
  have 
  been 
  thousands 
  of 
  them 
  on 
  

   the 
  perpendicular 
  cliffs 
  at 
  Lydstep, 
  clinging 
  closely 
  together, 
  and 
  

   with 
  no 
  food 
  but 
  barnacles 
  and 
  an 
  occasional 
  limpet. 
  Under 
  such 
  

   circumstances 
  it 
  is 
  not 
  surprising 
  that 
  signs 
  of 
  decadence 
  should 
  

   manifest 
  themselves. 
  

  

  It 
  never 
  seems 
  possible 
  to 
  predict 
  what 
  particular 
  form 
  of 
  decadence 
  

   will 
  result 
  from 
  any 
  given 
  diminution 
  of 
  the 
  optimum 
  of 
  environment. 
  

   The 
  onl}^ 
  case 
  at 
  all 
  parallel 
  to 
  this, 
  as 
  far 
  as 
  Nucella 
  is 
  concerned, 
  

   occurred 
  in 
  a 
  group 
  of 
  N. 
  lapillus 
  found 
  living 
  at 
  Minehead. 
  Near 
  

   the 
  harbour, 
  where 
  the 
  water 
  is 
  specially 
  muddy, 
  and 
  the 
  holding 
  

   ground 
  foul 
  with 
  decayed 
  wood 
  and 
  lumps 
  of 
  rotten 
  clay, 
  specimens 
  

   occur 
  whose 
  shells 
  are 
  riddled 
  with 
  some 
  boring 
  worm, 
  with 
  the 
  

   result 
  that 
  they 
  are 
  decollate, 
  the 
  suture 
  often 
  deeply 
  pitted, 
  and 
  

   the 
  base 
  of 
  the 
  columella 
  and 
  the 
  outer 
  lip 
  eaten 
  away 
  and 
  ragged, 
  

   so 
  that 
  the 
  whole 
  shell 
  presents 
  a 
  most 
  forlorn 
  and 
  wretched 
  

   appearance. 
  Yet, 
  in 
  every 
  case 
  examined, 
  the 
  operculum 
  was 
  

   perfectly 
  normal 
  in 
  shape, 
  size, 
  colour, 
  and 
  thickness. 
  

  

  THE 
  RADULA 
  OF 
  THE 
  GENUS 
  EUTHRIA, 
  GRAY. 
  

  

  By 
  the 
  Rev. 
  A. 
  H. 
  Cooke, 
  Sc.D., 
  F.Z.S. 
  

  

  Read 
  13th 
  April, 
  1917. 
  

  

  The 
  generic 
  name 
  Eiithria 
  appears 
  to 
  cover 
  a 
  variety 
  of 
  species 
  which 
  

   in 
  certain 
  points 
  differ 
  widely 
  from 
  one 
  another. 
  Founded 
  by 
  Gray 
  

   in 
  1850 
  (Fig. 
  Moll. 
  Anim., 
  No. 
  67), 
  its 
  type 
  is 
  the 
  ^lediterranean 
  

   cornea, 
  Jj. 
  Kobelt 
  catalogued' 
  the 
  genus 
  in 
  1878. 
  Suter 
  recognizes* 
  

  

  1 
  Jahrb. 
  Deutsch. 
  Malak. 
  Gesell., 
  vol. 
  v, 
  1878, 
  pp. 
  237-8. 
  

   - 
  Manual 
  New 
  Zealand 
  Mollusca, 
  1913, 
  pp. 
  373-81. 
  

  

  