﻿BI.OOMER 
  & 
  OVKKTON 
  : 
  ON 
  AXODONTA 
  AND 
  rSEUDANODONTA. 
  205 
  

  

  tliickness, 
  whilst 
  in 
  anatina 
  it 
  greatly 
  increases 
  towards 
  the 
  ventral 
  

   edge. 
  This 
  condition 
  is 
  also 
  noticeable 
  in 
  young 
  specimens 
  of 
  

   anatina. 
  

  

  Animal. 
  — 
  In 
  cygncta 
  there 
  is 
  a 
  comparatively 
  straight 
  and 
  hori- 
  

   zontal 
  (to 
  the 
  long 
  axis 
  of 
  the 
  animal) 
  dorsal 
  edge, 
  whereas 
  in 
  anatina 
  

   from 
  the 
  umboual 
  region 
  it 
  is 
  slightly 
  curved 
  and 
  ascending 
  for 
  some 
  

   distance 
  posteriorly. 
  Anteriorly 
  cygnma 
  is 
  slightly 
  more 
  curved 
  than 
  

   anatina. 
  Ventrally 
  the 
  edge 
  is 
  more 
  curved 
  in 
  anatina 
  than 
  in 
  cggncea,- 
  

   whilst 
  posteriorly 
  both 
  species 
  are 
  triangular, 
  being 
  sharper 
  in 
  cygnma 
  

   than 
  anatina. 
  

  

  The 
  siphonal 
  apertures 
  are 
  comparatively 
  larger 
  in 
  anatina 
  than 
  in 
  

   ci/gnoia, 
  but 
  in 
  both 
  the 
  inhalent 
  is 
  larger 
  than 
  the 
  exhalent 
  aperture. 
  

   Further 
  in 
  cygncca 
  they 
  are 
  situated 
  near 
  the 
  centre 
  of 
  the 
  posterior 
  

   border, 
  and, 
  lying 
  in 
  an 
  inclined 
  plane, 
  have 
  a 
  slightly 
  dorsal 
  aspect. 
  

   In 
  anatina 
  they 
  are 
  more 
  vertical, 
  occupy 
  a 
  greater 
  surface, 
  and 
  have 
  

   a 
  more 
  ventral 
  position. 
  

  

  The 
  inhalent 
  aperture 
  is 
  bordered 
  by 
  a 
  tentacular 
  fringe. 
  The 
  

   number 
  of 
  tentacular 
  processes 
  is 
  less 
  for 
  cygncca 
  than 
  anatina, 
  being 
  

   always 
  under 
  100 
  on 
  each 
  side 
  for 
  cygncea 
  and 
  more 
  than 
  100 
  for 
  

   anatina, 
  approximately 
  50 
  to 
  70 
  for 
  cygncea 
  and 
  from 
  120 
  to 
  200 
  

   for 
  anatina. 
  Though 
  the 
  processes 
  are 
  fewer 
  in 
  number 
  in 
  cygncea 
  

   they 
  are 
  larger 
  in 
  size. 
  

  

  The 
  colour 
  of 
  the 
  mantle 
  and 
  foot 
  of 
  cygncea 
  is 
  usually 
  of 
  an 
  orange 
  

   shade, 
  whilst 
  that 
  of 
  anatina 
  is, 
  when 
  coloured, 
  of 
  a 
  yellow 
  shade. 
  

   The 
  intensity 
  of 
  the 
  colouring 
  is 
  invariably 
  greater 
  in 
  cygncea 
  than 
  

   anatina. 
  

  

  The 
  question 
  of 
  coloration 
  deserves 
  closer 
  attention 
  from 
  the 
  point 
  

   of 
  view 
  of 
  periodicity 
  as 
  well 
  as 
  distribution. 
  

  

  In 
  cygnma 
  the 
  labial 
  palps, 
  that 
  is 
  those 
  portions 
  below 
  the 
  line 
  of 
  

   attachment 
  to 
  the 
  body 
  of 
  the 
  animal, 
  are 
  not 
  proportionately 
  so 
  large 
  

   in 
  area 
  as 
  in 
  anatina, 
  and 
  in 
  anatina 
  the 
  distal 
  part 
  is 
  pointed 
  and 
  

   more 
  produced 
  rearwardly 
  than 
  in 
  cygncea. 
  

  

  The 
  gills 
  in 
  cygncea 
  ascend 
  gradually 
  in 
  a 
  dorso 
  anterior 
  direction, 
  

   whilst 
  in 
  anatina 
  they 
  ascend 
  much 
  more 
  rapidly 
  and 
  the 
  dorso- 
  

   anterior 
  portion 
  lies 
  more 
  dorsally 
  than 
  in 
  cygncea. 
  

  

  The 
  circumpallial 
  muscles 
  of 
  anatina 
  form 
  a 
  considerably 
  deeper 
  

   band 
  than 
  that 
  possessed 
  by 
  cygncea. 
  

  

  The 
  exposed 
  terminal 
  parts 
  of 
  the 
  muscles 
  of 
  the 
  right 
  side, 
  viz. 
  : 
  — 
  

  

  The 
  anterior 
  adductor 
  muscle 
  is 
  curved 
  on 
  the 
  anterior 
  edge 
  in 
  both 
  

   species. 
  J)ovsdi\\y 
  cygncea 
  has 
  a 
  sharper 
  curve 
  than 
  anatina. 
  Ventrally 
  

   cygncea 
  has 
  a 
  rounder 
  edge 
  than 
  anatina. 
  Posteriorly 
  cygncea 
  has 
  

   a 
  more 
  uneven 
  edge 
  than 
  anatina. 
  Further 
  the 
  muscle 
  is 
  relatively 
  

   larger 
  in 
  anatina 
  than 
  in 
  cygncea, 
  but 
  in 
  both 
  species 
  it 
  has 
  a 
  tendency 
  

   to 
  vary 
  in 
  shape. 
  

  

  The 
  posterior 
  adductor 
  muscle 
  in 
  cygncea 
  is 
  broadly 
  crescent-shaped, 
  

   whilst 
  in 
  anatina 
  it 
  is 
  somewhat 
  spherical. 
  In 
  anatina 
  it 
  is 
  com- 
  

   paratively 
  very 
  lai'ge, 
  and 
  is 
  situated 
  slightly 
  more 
  dorsally 
  than 
  in 
  

   cygncea. 
  This 
  muscle, 
  like 
  the 
  anterior 
  adductor, 
  has 
  a 
  disposition 
  

   to 
  vary 
  in 
  shape, 
  though 
  in 
  a 
  lesser 
  degree. 
  

  

  Tlie 
  anterior 
  retractor 
  pedis 
  muscle 
  is 
  of 
  an 
  irregular 
  shape 
  in 
  

  

  