﻿206 
  

  

  PROCEEDINGS 
  OF 
  THE 
  MALACOLOGICAL 
  SOCIETY. 
  

  

  cyipuca 
  and 
  triangular 
  in 
  anatina, 
  with 
  the 
  base 
  near 
  the 
  anterior 
  

   adductor, 
  but 
  in 
  cy(])uca 
  the 
  apex 
  lies 
  in 
  a 
  dorsal 
  plane, 
  and 
  the 
  base 
  

   is 
  a 
  little 
  farther 
  away 
  from 
  the 
  adductor 
  than 
  in 
  the 
  other 
  species. 
  

  

  The 
  anterior 
  protractor 
  pedis 
  muscle 
  is 
  larger 
  in 
  anatina 
  than 
  in 
  

   cygmca, 
  and 
  in 
  the 
  latter 
  it 
  is 
  relatively 
  farther 
  away 
  from 
  the 
  

   anterior 
  adductor. 
  Moreover, 
  in 
  ct/gncca 
  it 
  lies 
  in 
  a 
  direction 
  ventro- 
  

   posteriorly 
  to 
  the 
  anterior 
  adductor, 
  whereas 
  in 
  anatina 
  it 
  is 
  posterior 
  

   to 
  that 
  muscle. 
  

  

  Tlie 
  ])osterior 
  retractor 
  pedis 
  is 
  of 
  an 
  oblong 
  shape 
  in 
  cygmca 
  and 
  

   triangular 
  in 
  anatina. 
  In 
  anatina 
  it 
  is 
  close 
  to 
  the 
  dorsal 
  surface 
  of 
  

   the 
  posterior 
  adductor, 
  and 
  in 
  cygncca 
  is 
  a 
  little 
  farther 
  away 
  from 
  

   tlie 
  muscle 
  with 
  its 
  dorsal 
  edge 
  nearly 
  parallel 
  to 
  the 
  long 
  axis 
  of 
  the 
  

   animal. 
  

  

  In 
  describing 
  and 
  comparing 
  the 
  shape 
  of 
  the 
  exposed 
  parts 
  of 
  the 
  

   muscles 
  as 
  viewed 
  from 
  the 
  right 
  side 
  of 
  the 
  animal 
  just 
  after 
  the 
  

   removal 
  of 
  tlie 
  shell, 
  it 
  must 
  bo 
  borne 
  in 
  mind 
  that 
  there 
  is 
  often 
  

   considerable 
  variation 
  when 
  comparing 
  one 
  specimen 
  with 
  anotber 
  of 
  

   the 
  same 
  species, 
  and 
  further, 
  the 
  shape 
  on 
  the 
  riglit 
  side 
  of 
  the 
  

   animal 
  may 
  be, 
  and 
  often 
  is, 
  different 
  to 
  that 
  on 
  the 
  left 
  side. 
  Since 
  

   this 
  variation 
  even 
  extends 
  to 
  a 
  comparison 
  of 
  the 
  median 
  sections 
  it 
  

   consequently 
  does 
  not 
  follow 
  tliat 
  the 
  spreading 
  out 
  of 
  the 
  muscles 
  

   near 
  the 
  lateral 
  extremities 
  altogetlier 
  explains 
  the 
  difference 
  in 
  the 
  

   external 
  shape. 
  Great 
  as 
  the 
  variation 
  may 
  be 
  it 
  is 
  nevertheless 
  

   confined 
  within 
  certain 
  limits, 
  and 
  it 
  is 
  quite 
  possible 
  to 
  determine 
  

   the 
  species 
  by 
  an 
  examination 
  of 
  the 
  exposed 
  terminal 
  parts 
  of 
  the 
  

   muscles 
  alone. 
  

  

  JiJinbryos. 
  — 
  If 
  the 
  embryos 
  wlitm 
  ])assed 
  into 
  the 
  marsupium 
  are 
  

   examined 
  under 
  a 
  low 
  magnification 
  it 
  will 
  be 
  observed 
  that 
  tliere 
  is 
  

   a 
  difference 
  between 
  those 
  of 
  cygnrea 
  and 
  anatina 
  whicli 
  is 
  constant, 
  

   namely, 
  that 
  the 
  apical 
  region 
  or 
  growing 
  part 
  is 
  pointed 
  in 
  cygnma 
  

   (A, 
  A') 
  and 
  blunt 
  in 
  anatina 
  {B,B'). 
  Usually 
  the 
  sides 
  are 
  more 
  

   curved 
  in 
  anatina, 
  making 
  quite 
  a 
  shoulder 
  in 
  the 
  upper 
  part, 
  but 
  this 
  

   character 
  is 
  not 
  quite 
  so 
  constant. 
  The 
  base 
  or 
  hinge 
  region 
  is 
  the 
  

   same 
  in 
  botli 
  species. 
  

  

  