MR.  R.  OWEN  ON  THE  ANATOMY  OF  THE  BRACHIOPODA. 
153 
On  the  Anatomy  of  Orbicula. 
I  have  not  been  able  to  find  any  account  of  the  anatomy  of  this  Brachiopod,  beyond 
the  statement  of  its  possessing  the  spiral  arms  peculiar  to  the  order.  Cuvier,  indeed, 
refers  the  soft  parts  to  Poll's  genus  Criopus^  ;  but  the  animal  so  denominated  belongs, 
as  Mr.  Sowerby  observes 2,  to  a  species  of  Crania  {Cran.  personata.  Sow.),  a  genus,  how- 
ever, which,  in  its  internal  organization,  is  without  doubt  closely  allied  to  Orbicula. 
Four  recent  and  well  preserved  specimens  of  the  species  which  Mr.  Broderip  in  the 
preceding  Memoir  has  termed  Orb.  lamellosa,  were  submitted  to  me  for  examination. 
The  margin  of  the  shell  is  of  a  soft  texture  and  thickened,  and  the  edges  of  all  the 
layers  of  increase  are  more  horny  than  calcareous.  The  layers  of  increase  are  large 
in  proportion  to  the  size  of  the  shell,  and  are  very  irregular  in  their  contour ;  the 
inside  of  the  shell  is  smooth  and  polished.  The  flattened  valve  is  perforated  by  a 
longitudinal  fissure,  measuring  nearly  three  lines  in  length,  and  about  half  a  line  in 
width,  and  situated  in  the  middle  of  an  oval  depression.  Through  this  fissure  the  organ 
of  adhesion,  or  the  foot,  passes,  and  immediately  expands  into  a  round  sucker  or  disk, 
which  fills  up  the  whole  of  the  depression,  and  conceals  the  margins  of  the  slit.  Im- 
mediately anterior  to  the  fissure  a  longitudinal  plate,  about  a  line  in  length,  projects 
into  the  interior  of  the  shell  for  the  extent  of  half  a  line ;  beyond  this  a  broader  ele- 
vated ridge  is  continued  to  within  two  lines  of  the  anterior  margin  of  the  valve 
Along  the  whole  circumference  of  the  valves  shining  cilia  are  seen  projecting  for  an 
extent  varying  from  two  to  four  lines.  These  arise  all  round  the  margins  of  both 
lobes  of  the  mantle ;  they  are  much  longer  than  in  Terebratula  and  Lingula  anatina, 
and  are  rather  longer  than  in  Ling.  Audebardii,  Brod.,  a  new  species  discovered  by 
Mr.  Cuming. 
On  carefully  removing  the  imperforate  valve,  the  vascular  mantle  is  seen  with  the 
margin  entire  in  the  whole  of  its  circumference.  The  muscles  and  viscera  form  a  rounded 
mass,  situated  in  the  posterior  half  of  the  shell.  First  are  seen  the  extremities  of  two 
muscles*,  of  an  oblong  figure,  converging  anteriorly,  and  measuring  two  lines  by  nearly 
one :  in  the  triangular  space  between  these  muscles  is  situated  the  green  liver'',  behind 
which  is  the  grey  ovary  ^ ;  and  at  the  posterior  part  of  the  circle  are  the  extremities  of 
two  smaller  muscles''.  The  four  impressions  of  these  muscles  are  observable  on  the 
interior  of  the  shelly  valve. 
On  removing  the  lower  valve,  which  should  be  cut  through  from  either  side  as  far  as 
*  Testacea  utriusque  Sicilise,  vol.  ii.  pi.  xxx.  Figg.  21-24.  -  Linn.  Trans.,  vol.  xiii.  p.  471. 
3  This  I  regard  as  a  rudimentary  form  cf  the  internal  calcareous  apparatus  of  Terebratula ;  it  represents  the 
central  process  of  support  (c.  Fig.  4.  Plate  xxiii.).    The  mantle-lobe  with  two  vessels,  and  the  position  of  the 
alimentary  canal,  prove  that  the  flattened  valve  of  Orbicula,  although  perforated  for  the  organ  of  adhesion,  is 
really  analogous  to  the  imperforate  valve  of  the  Terebratula. 
^  f.  f.  Figg.  5.  7.  8.  Plate  xxin.  ^  v.  Figg.  5.  11,  e  j^,.  pigg.  5.  n.  7  ^.     pigg.  5.  7.  8. 
