798 
ME.  HANCOCK  ON  THE  OEGANIZATION  OF  THE  BEACHIOPODA. 
ventral.  The  former  of  these  are  attached  to  the  ventral  surface  of  the  peduncle, 
forming  a dense  mass  by  the  intermingling  of  their  fibres ; they  pass,  one  from  each 
side  of  the  peduncle  near  its  inner  extremity  and  just  above  the  origin  of  the  peduncular 
muscle,  through  the  capsule  forwards  and  upwards,  diverging  a little  and  having  the 
divaricators  between  them,  to  be  inserted  into  the  hinge-plate,  one  on  each  side,  a little 
in  advance,  of  the  cardial  process.  At  the  point  of  insertion  each  seems  as  if  formed  of 
two  portions.  These  muscles,  throughout  their  whole  length,  are  yellowish,  fibrous  and 
fleshy,  and  not  much  enlarged  at  theu  insertions. 
The  ventral  adjustors  pass  from  the  inner  extremity  of  the  peduncle,  and  penetrate 
the  capsule  on  each  side,  a little  above  the  dorsal  pair ; they  originate  in  a firm  mass 
cemented  to  the  dorsal  surface  of  the  peduncle,  very  similar  to  that  which  connects  the 
other  pau  to  this  organ : the  two  masses  give  a bulbous  form  to  the  inner  extremity  of 
the  peduncle.  At  first  the  muscles  are  delicate  and  tendinous;  but  coursing  down- 
wards and  forwards  they  rapidly  enlarge  and  become  attached  to  the  ventral  valve,  one 
on  each  side  of,  and  a little  behind,  the  expanded  base  of  the  divaricators.  They  form 
the  external  pair  of  muscular  discs  seen,  when  the  shell  is  removed,  clustered  together 
towards  the  umbonal  region  of  the  ventral  surface.  These  discs  are  of  a reddish  colour, 
and  in  size  and  shape  resemble  those  of  the  divaricators,  being  of  an  iiTegular  oval 
form. 
The  two  pairs  of  adjustor  muscles  are  evidently  for  the  purpose  of  mo'cing  the  shell 
upon  the  peduncle,  and  to  some  extent  are  antagonistic  to  each  other.  "SMien  the  dorsal 
pair  are  contracted  and  the  ventral  relaxed,  the  shell  will  be  depressed,  and  they  elevate 
it  on  this  action  of  the  two  muscles  being  reversed.  But  their  chief  function  would 
appear  to  be,  to  rotate  the  shell  from  side  to  side ; this  will  be  effected  by  the  right  and 
left  muscles  of  each  pair  alternately  contracting  and  relaxing.  The  shell  will  be  fii'mly 
fixed  and  the  peduncle  withdrawn  to  the  fullest  extent,  when  both  these  pairs  and  the 
peduncular  muscle  are  in  a state  of  contraction. 
The  arrangement  of  the  muscles  is  essentially  the  same  in  all  the  articulated  Brachio- 
pods  that  have  come  under  my  observation.  There  are,  however,  one  or  two  rather 
interesting  modifications  of  detail,  even  in  the  Terebratulidce^  which  require  particular 
notice.  The  first  of  these  occurs  in  a species  so  like  W.  australis,  that  1 took  it,  at  first, 
to  belong  to  that  form,  but  am  now  inclmed  to  think  that  it  must  be  distinct  from  it*. 
In  this^  the  divaricators  and  their  accessories  are  united  so  as  to  form  only  a single  pah, 
the  attachment  of  the  muscle  to  the  ventral  valve  extendmg  all  the  way  from  the  pedim- 
cular  muscle  to  considerably  in  advance  of  the  occlusors.  The  connectmg  portion,  how- 
ever, is  much  attenuated,  forming  as  it  were  a narrow  neck  of  commimication  between 
^ It  appears  that  more  than  one  species  is  included  under  the  name  of  Australis.  The  specimen  above 
alluded  to  was  very  large  and  fine,  with  the  anterior  margins  of  the  valves  much  and  suddenly  bent  inn  ards, 
and  the  ribs  few  and  very  large ; it  resembled  figs.  25  and  28,  plate  69,  of  the  ‘ Thesaurus  Couchyliorum  ’ 
of  SOWDEBT. 
^ Plates  LII.  fig.  3 ; LIV.  fig.  1 ; LXII.  fig.  4. 
