﻿231 
  

  

  ON 
  THE 
  ANATOMY 
  OF 
  THE 
  BEITISH 
  SPECIES 
  OF 
  THE 
  GENUS 
  

  

  PSAMMOBIA. 
  

  

  By 
  H. 
  H. 
  Bloomer, 
  F.L.S. 
  

  

  Read 
  11th 
  November, 
  1910. 
  

  

  PLATES 
  IX 
  AND 
  X. 
  

  

  PsAMMOBiA 
  vESPEKTiNA 
  (Chemnitz). 
  

  

  The 
  animal, 
  in 
  a 
  quiescent 
  state, 
  lies 
  with 
  the 
  valves 
  of 
  the 
  shell 
  

   a 
  little 
  agape, 
  the 
  adductor 
  muscles 
  partly 
  relaxed, 
  and 
  the 
  pallial 
  

   lobes, 
  excepting 
  near 
  the 
  beak 
  and 
  ligament, 
  protruding 
  some 
  distance 
  

   beyond 
  the 
  edge 
  of 
  the 
  shell, 
  especially 
  those 
  portions 
  bordering 
  the 
  

   pedal 
  aperture 
  (Fig. 
  1). 
  Externally 
  nearly 
  the 
  whole 
  of 
  the 
  pro- 
  

   jecting 
  parts 
  is 
  covered 
  with 
  the 
  usual 
  dusky-brown 
  periostracum, 
  

   which 
  passes 
  from 
  the 
  shell 
  to 
  near 
  the 
  mantle-fringe. 
  In 
  a 
  normal 
  

   condition 
  the 
  free 
  edge 
  of 
  the 
  lobes 
  curves 
  inwardly, 
  so 
  that 
  the 
  

   fringe 
  is 
  not 
  often 
  exposed 
  to 
  view. 
  The 
  exposure, 
  however, 
  may 
  

   take 
  place 
  immediately 
  preceding 
  and 
  during 
  the 
  process 
  of 
  moving, 
  

   when 
  the 
  animal 
  approaches 
  complete 
  expansion, 
  and 
  the 
  pallial 
  

   margin 
  is 
  uncurved. 
  

  

  The 
  foot 
  (Fig. 
  2, 
  F.) 
  is 
  of 
  a 
  milk-white 
  colour, 
  very 
  muscular, 
  yet 
  

   very 
  sensitive. 
  It 
  can 
  be 
  protruded 
  a 
  great 
  distance, 
  and 
  shows 
  

   considerable 
  power 
  of 
  mobility, 
  projecting 
  from 
  any 
  part 
  of 
  and 
  

   moving 
  all 
  along 
  the 
  pedal 
  aperture, 
  bending 
  from 
  side 
  to 
  side, 
  

   swelling 
  or 
  contracting, 
  lengthening 
  or 
  shortening, 
  and 
  capable, 
  

   either 
  rapidly 
  or 
  slowly, 
  of 
  continually 
  changing 
  its 
  shape 
  and 
  

   position. 
  From 
  my 
  own 
  observation 
  the 
  mode 
  of 
  locomotion 
  is 
  

   either 
  by 
  leaping 
  or 
  creeping, 
  and 
  in 
  this 
  function 
  the 
  foot 
  plays 
  

   the 
  principal 
  part. 
  In 
  leaping 
  the 
  foot 
  is 
  pushed 
  out 
  anteriorly 
  

   with 
  a 
  slow, 
  undulatory 
  movement 
  ; 
  whilst 
  lengthening, 
  the 
  distal 
  

   end 
  bends, 
  and 
  passes 
  underneath 
  the 
  other 
  portion, 
  then, 
  with 
  great 
  

   rapidity 
  and 
  muscular 
  force, 
  it 
  is 
  straightened, 
  and 
  the 
  animal 
  

   thrown 
  forwards. 
  On 
  the 
  other 
  hand, 
  the 
  foot 
  may 
  be 
  thrust 
  out 
  

   sideways, 
  passed 
  underneath 
  the 
  shell, 
  and 
  with 
  the 
  same 
  action 
  

   the 
  animal 
  is 
  turned 
  over 
  or 
  sent 
  some 
  distance 
  laterally. 
  

  

  In 
  creeping, 
  the 
  distal 
  end 
  of 
  the 
  extended 
  foot 
  seizes 
  the 
  ground, 
  

   contracts, 
  and 
  draws 
  the 
  body 
  along, 
  It 
  is 
  possible 
  that 
  this 
  species 
  

   also 
  swims 
  in 
  the 
  same 
  manner 
  as 
  the 
  smaller 
  ones, 
  but 
  I 
  have 
  

   never 
  seen 
  it 
  do 
  so. 
  

  

  External 
  Characters. 
  — 
  An 
  average-sized 
  animal 
  measures 
  about 
  

   52 
  mm. 
  antero-posteriorly 
  by 
  28 
  mm. 
  dorso-ventrally. 
  Dorsally 
  it 
  

   is 
  curved 
  with 
  a 
  depression 
  under 
  the 
  hinge 
  region. 
  The 
  other 
  

   sides 
  are 
  likewise 
  curved, 
  except 
  the 
  postero- 
  ventral 
  part, 
  which 
  is 
  

   angular, 
  and 
  projects 
  more 
  posteriorly 
  than 
  any 
  other 
  portion. 
  

  

  The 
  pallium, 
  or 
  mantle, 
  is 
  a 
  thin 
  membraneous 
  covering 
  bordered 
  

   by 
  a 
  muscular 
  band, 
  decreasing 
  in 
  depth 
  as 
  it 
  passes 
  rearwardly. 
  

   The 
  pedal 
  aperture 
  reaches 
  from 
  the 
  anterior 
  adductor 
  muscle 
  to 
  

   the 
  musculus 
  cruciformis, 
  consequently, 
  between 
  these 
  two 
  points 
  

  

  