IGO  MR.  R.  OWEN  ON  THE  ANATOMY  OF  THE  BRACHIOPODA. 
termediate  to  the  Lamellibranchiate  and  Tunicate  orders ;  not,  however,  possessing,  so 
far  as  they  are  at  present  known,  distinctive  characters  of  sufficient  importance  to 
justify  their  being  regarded  as  a  distinct  class  of  Mollusks,  but  forming  a  separate 
group  of  equal  value  with  the  Lamellibranchiata. 
PLATE  XXII. 
Anatomy  of  Terebratula. 
Fig.  1.  Terebratula  Chilensis,  Brod.,  natural  size. 
2.  Terebratula  Uva,  Brod.,  natural  size. 
3.  Perforate  valve  of  Terebratula  Chilensis. 
a.  The  perforation,  or  groove,  through  which  the  tendons  of  the  muscles 
pass  to  form  the  pedicle. 
b.  The  teeth  of  the  hinge,  which  are  locked  in  the  depressions  b.  b.  of  fig.  4. 
c.  The  muscular  impressions. 
4.  Imperforate  valve  of  the  same  specimen. 
a.  The  middle  depression  of  the  hinge. 
6.  b.  The  lateral  depressions. 
c.  The  mesial  process,  or  ridge  continued  from  the  hinge. 
d.  The  lateral  processes  of  the  same,  which  are  attached  to  the  bends  of 
e.  e.  The  elastic  calcareous  loop. 
/.  /.  Small  processes  at  the  origins  of  the  crura  of  the  loop. 
g.  g.  Impressions  of  the  anterior  pair  of  muscles. 
Ji.  h.  Impressions  of  the  posterior  pair. 
Figg.  5.  to  9.  are  dissections  of  a  smaller  specimen  of  Ter.  Chilensis  magnified  two 
diameters. 
5.  The  soft  parts  corresponding  to  the  perforate  valve. 
6.  The  soft  parts  corresponding  to  the  imperforate  valve.    The  branchial  ves- 
sels, brachial  filaments,  and  liver,  may  be  distinguished  through  the  serai- 
transparent  mantle. 
7.  The  soft  parts  viewed  from  the  same  aspect  as  in  Fig.  5,  with  the  mantle- 
lobe  reflected,  showing  more  distinctly  the  branchial  vessels,  and  ex- 
posing the  arms  in  their  natural  position. 
8.  The  soft  parts  viewed  from  the  same  aspect  as  in  Fig.  6.,  with  the  mantle- 
lobe  reflected,  exposing  part  of  the  visceral  mass,  and  the  bend  of  the 
arms  following  the  bend  of  the  calcareous  loop. 
9.  The  mantle-lobes  further  separated,  the  calcareous  loop  broken  through  and 
removed,  and  the  arms  dissected  off  and  displaced,  to  show  the  decus- 
sation of  the  muscles,  and  the  small  visceral  mass. 
