796 
ME.  HAJS^COCK  ON  THE  OEOANIZATION  OE  THE  BEACHIOPODA. 
In  Waldheimia  australis'^  the  occlusors  have  their  inferior  or  ventral  terminations 
placed  close  together,  forming  an  elongated  oval  mass,  which  is  attached  to  the  valve 
about  one-third  the  length  of  the  shell  from  the  beak  or  perforate  extremity  on  the 
median  line.  From  this  point  they  pass  diagonally  upwards  and  forwards  towards  the 
imperforate  or  dorsal  valve,  continuing  united  for  some  distance;  they  then  separate, 
and  diverging  laterally,  become  much  attenuated  and  assume  the  form  of  tendons.  Up 
to  this  point  they  are  firmly  attached  to  the  anterior  wall  of  the  body,  but  beyond  it 
become  perfectly  free.  Each  muscle  then  almost  immediately  dmdes  into  an  anterior 
and  posterior  portion,  which  run  parallel  to  each  other,  on  either  side  of  the  alimentary 
tube  and  liver,  and  on  reaching  the  dorsal  valve  are  each  attached  to  it  by  a distinct, 
much  enlarged  extremity,  at  a httle  distance  from  the  median  line.  These  two  divisions 
of  the  occlusor  muscles  have  been  denominated  by  Professor  Owex  “ adductor  longus 
anticus,”  and  “adductor  longus  posticus”  respectively^,  and  in  this  paper  wiU  receive  the 
distinctive  appellations  of  anterior  and  posterior  occlusors.  The  former  is  considerably 
the  stouter,  and  the  enlarged  attachment  of  both  are  seen  close  together  at  the  surface 
of  the  body  after  the  shell  has  been  removed;  they  are  of  a p^Tiform  contom,  the 
pointed  end  of  the  anterior  and  larger  being  inclined  inwards  and  backwards,  that  of 
the  posterior  in  the  opposite  direction.  All  the  four  dorsal  extremities  are  of  a reddish 
flesh-colour,  resembling  muscle ; but  with  this  exception,  the  muscles  are  throughout 
dense,  glistening  and  tendon-like,  even  including  the  enlarged  ventral  extremities. 
These  muscles  are  undoubtedly  for  the  purpose  of  closing  the  valves. 
The  divaricator  muscles  arise  from  the  ventral  valve,  one  on  each  side,  a httle  in 
advance  of,  and  close  to  the  united  bases  of  the  occlusors.  These  extremities  are  large, 
fleshy,  and  of  a reddish-yellow  colour;  they  rapidly  diminish  in  size,  and  attain  the 
appearance  of  white,  glistening  tendons ; and,  havhig  the  intestine  betu-een  them,  con- 
verge as  they  pass  backwards  and  upwards  towards  the  cardinal  process,  into  which  their 
superior,  attenuated  extremities  are  inserted  in  close  contiguity. 
The  accessory  divaricators  are  a pair  of  small  muscles  Avhich  have  their  enlarged  car- 
neous  ends  attached  to  the  ventral  valve  on  each  side  of  the  median  Ime,  a httle  behind 
the  united  bases  of  the  occlusors,  and  the  termination  of  the  intestine  is  seen  imme- 
diately in  front  of  them.  At  the  surface  of  the  body  they  appear  as  two  OA  al  reddish 
discs,  and  they  become,  almost  at  once,  reduced  into  flattened  tendons,  which,  rmining 
parallel  to  each  other,  incline  upwards  and  forwards,  course  round  the  inner  extremity 
of  the  peduncular  capsule,  to  the  upper  surface  of  which  they  become  fiinily  united, 
and  as  they  advance  blend  some  of  their  fibres  with  those  of  the  divaricators,  and  go 
thus  united  with  them  to  be  inserted  into  the  extreme  point  of  the  cardinal  process. 
The  function  of  these  two  pairs  of  muscles  is  obvious  enough ; it  is  to  open  or  divari- 
cate the  valves.  The  cardinal  process  projecting  backwards  beyond  the  hmge  or  ful- 
crum formed  by  the  teeth  and  sockets,  when  these  muscles  contract  the  process  will  be 
drawn  inwards,  and  consequently  the  anterior  margins  of  the  valves  separated,  the 
action  being  that  of  a lever  of  the  first  order.  This  peculiar  mode  of  opening  the  Aulves 
' Plates  LII.  figs.  2,  10 ; LIV.  fig.  1 ; LVII.  figs.  1,  2 ; LXII.  fig.  1.  = 0^.  cit.  p.  8. 
