﻿204 
  rUOCKKDINfJS 
  OV 
  TIIIO 
  MAI.ACOLOGICAL 
  SOCIETY. 
  

  

  portion 
  lying 
  ventro-posteriorly. 
  The 
  ventral 
  edge 
  has 
  a 
  distinct 
  

   symmetrical 
  curve. 
  

  

  The 
  colour 
  of 
  the 
  foot 
  is 
  white 
  or 
  greyish 
  white, 
  often 
  tinged 
  with 
  

   yellow, 
  becoming 
  deeper 
  towards 
  the 
  keel, 
  where 
  at 
  times 
  orange 
  

   replaces 
  the 
  yellow. 
  The 
  mantle 
  has 
  generally 
  a 
  yellow 
  shade, 
  

   occasionally 
  passing 
  to 
  a 
  light 
  orange. 
  Some 
  pools 
  yield 
  specimens 
  

   having 
  no 
  orange 
  or 
  yellow 
  colour 
  about 
  tliem. 
  

  

  The 
  mantle 
  is 
  bordered 
  by 
  a 
  deep 
  band 
  of 
  circumpallial 
  muscles 
  

   (Fig. 
  2, 
  CAT.), 
  which 
  are 
  deeper 
  at 
  the 
  anterior 
  and 
  posterior 
  ends, 
  

   more 
  particularly 
  the 
  latter, 
  where 
  they 
  are 
  developed 
  and 
  form 
  the 
  

   siphonal 
  chambers. 
  These 
  are 
  large 
  and 
  occupy 
  the 
  wliole 
  of 
  the 
  

   posterior 
  area, 
  tlie 
  inhalent 
  {In. 
  A.) 
  being 
  uiuch 
  larger 
  than 
  the 
  

   exhalent 
  one 
  (Kv.A.). 
  The 
  former 
  is 
  bordered 
  by 
  a 
  tentacular 
  

   fringe, 
  each 
  side 
  having 
  from 
  120 
  to 
  200 
  small 
  tentacles. 
  

  

  The 
  exposed 
  terminal 
  parts 
  of 
  the 
  muscles 
  present 
  the 
  following 
  

   features 
  : 
  — 
  

  

  The 
  anterior 
  adductor 
  {A. 
  A.) 
  is 
  large 
  with 
  the 
  anterior 
  part 
  

   curved 
  and 
  the 
  posterior 
  portion 
  irregular, 
  particularly 
  the 
  ventro- 
  

   posterior 
  part. 
  

  

  The 
  posterior 
  adductor 
  {P. 
  A.) 
  is 
  also 
  large, 
  inclined 
  to 
  be 
  more 
  or 
  

   less 
  spherical 
  but 
  slightly 
  prolonged 
  ventro-posteriorly 
  and 
  flattened 
  

   antero-dorsally. 
  

  

  The 
  anterior 
  retractor 
  pedis 
  {R.P.A.) 
  has 
  a 
  somewhat 
  triangular 
  

   shape 
  with 
  the 
  base 
  close 
  to 
  the 
  posterior 
  edge 
  of 
  the 
  anterior 
  

   adductor 
  and 
  the 
  apex 
  lying 
  in 
  a 
  dorso-posterior 
  direction. 
  

  

  The 
  anterior 
  protractor 
  pedis 
  {P.P. 
  A.) 
  presents 
  an 
  irregularly 
  

   shaped 
  and 
  relatively 
  large 
  surface 
  and 
  lies 
  near 
  the 
  ventro-posterior 
  

   edjje 
  of 
  the 
  anterior 
  adductor. 
  

  

  The 
  posterior 
  retractor 
  pedis 
  {P. 
  P.P.) 
  is 
  bluntly 
  triangular, 
  with 
  

   the 
  base 
  resting 
  upon 
  the 
  dorsal 
  edge 
  of 
  the 
  posterior 
  adductor. 
  

  

  As 
  in 
  A. 
  cygncta, 
  too 
  much 
  importance 
  must 
  not 
  be 
  attached 
  to 
  the 
  

   description 
  of 
  the 
  shape 
  of 
  the 
  retractors 
  and 
  protractors 
  but 
  rather 
  

   their 
  relative 
  position 
  to 
  the 
  adductor 
  muscles. 
  

  

  The 
  two 
  species 
  contrasted. 
  

  

  Shell. 
  — 
  The 
  shell 
  is 
  relatively 
  larger 
  in 
  A. 
  ctjgncea, 
  but 
  is 
  more 
  

   oval, 
  ventricose, 
  and 
  thicker 
  in 
  A. 
  anatina 
  ; 
  the 
  periostracum 
  is 
  

   usually 
  thicker 
  and 
  of 
  a 
  darker 
  colour 
  — 
  an 
  olive 
  green 
  or 
  brown— 
  in 
  

   anatina; 
  tlie 
  umbonal 
  region 
  is 
  more 
  central 
  in 
  ajiatina; 
  the 
  ligament 
  

   is 
  longer 
  and 
  nearly 
  liidden 
  by 
  the 
  dorsal 
  growth 
  of 
  the 
  sliell 
  in 
  

   ci/f/n(sa, 
  in 
  anatina 
  it 
  is 
  exposed 
  and 
  prominent 
  ; 
  dorsal 
  ly 
  cygncea 
  is 
  

   straight, 
  whereas 
  anatina 
  is 
  curved 
  and 
  from 
  the 
  umbonal 
  region 
  

   ascending 
  for 
  some 
  distance 
  posteriorly 
  ; 
  the 
  ventral 
  part 
  of 
  the 
  

   anterior 
  edge 
  is 
  gaping 
  in 
  anatina 
  and 
  usually 
  closed 
  in 
  cygncea, 
  but 
  

   if 
  gaping 
  then 
  only 
  slightly 
  ; 
  the 
  ventral 
  margin 
  of 
  anatina 
  is 
  a 
  little 
  

   more 
  curved; 
  posteriorly 
  both 
  species 
  are 
  wedge-shaped, 
  but 
  the 
  

   extremitj- 
  is 
  more 
  pointed 
  and 
  situated 
  more 
  dorsally 
  in 
  cygncea 
  ; 
  the 
  

   muscular 
  and 
  pallial 
  scars 
  are 
  far 
  more 
  distinct 
  in 
  anatina^ 
  whilst 
  in 
  

   cygnma 
  they 
  are 
  often 
  difficult 
  to 
  trace 
  completely. 
  

  

  Dorso-ventral 
  sections 
  of 
  the 
  shell 
  show 
  cygncea 
  to 
  be 
  of 
  a 
  uniform 
  

  

  