270 
MR.  R.  OWEN  ON  THE  ANATOMY  OF  CLAVAGELLA. 
remains  free,  or  is  connected  only  to  the  soft  parts  and  cardinal  ligament,  in  order  to 
assist  in  the  excavating  and  respiratory  actions. 
That  these  actions  are  of  a  powerful  kind  is  to  be  inferred  from  the  remarkable  de- 
velopment of  the  muscular  system  in  the  Clavagella.  The  impression  of  the  great  or 
posterior  adductor^  is  carried  2  lines  beneath  the  surface  of  the  chamber  posteriorly, 
but  gradually  rises  to  the  level  of  the  valve.  The  impression  of  the  smaller  anterior 
adductor^  is  fainter,  and  is  continued  into  the  sinuous  pallial  impression^,  which  fol- 
lows the  contour  of  the  anterior  margin  of  the  valve  at  about  2  lines'  distance  from  it. 
In  the  free  valve  *  the  last  two  muscular  impressions  are  separate. 
The  shelly  substance  of  the  fixed  valve  passes  without  interruption  into  that  of  the 
tube  :  a  slight  ridge  circumscribing  the  entry  of  the  tube  into  the  chamber  may  be  re- 
garded as  the  line  of  separation ;  unless  the  extent  of  the  valve  be  limited  to  that  of 
the  internal  nacreous  deposition. 
The  area  of  the  tube  is  of  an  oval  form,  in  diameter  7  lines  by  5.  The  calcareous 
parietes  are  -^th.  of  an  inch  in  thickness  at  the  outlet  of  the  tube,  and  about  -yVth  at 
the  opposite  extremity.  As  far  as  it  is  preserved  in  the  present  specimen  no  percep- 
tible increase  is  recognizable  as  it  approximates  the  chamber. 
The  free  valve  is  an  unequal  triangle,  with  the  angles  rounded  off,  about  the  thick- 
ness of  a  sixpence,  moderately  concave  towards  the  soft  parts,  striated  only  in  the  di- 
rection of  the  layers  of  increment  on  the  outer  surface,  as  in  most  of  the  Pyloridean 
Bivalves  of  M.  de  Blainville.  The  layers  of  increment  of  the  free  valve  gradually  in- 
crease towards  the  dorsal  edge  for  a  little  more  than  one  half  of  the  valve,  beyond  which 
the  layers  continue  of  almost  equal  breadth.  This  growth  of  the  valve  corresponds  to 
the  direction  in  which  the  chamber  is  enlarged,  which  is  principally  on  the  dorsal,  dex- 
tral,  and  anterior  sides  :  now  this  is  the  mode  of  enlargement  best  adapted  for  the  full 
development  of  the  ovary ;  so  that  it  would  seem  that  the  Clavagella  continues  for  a 
certain  time  to  work  its  way  into  the  rock  without  material  increase  of  size,  leaving 
behind  it  a  calcareous  tube,  which  marks  its  track ;  after  which  it  becomes  stationary, 
and  limits  its  operations  to  enlarging  its  chamber  to  the  extent  necessary  for  the  ac- 
complishment of  the  great  object  of  its  existence. 
The  mantle  envelopes  the  body  like  a  shut  sac,  but  is  perforated,  as  before  mentioned,, 
for  the  siphon  and  foot,  the  opening  for  the  latter  part  being  reduced  to  a  small  slit^. 
An  analogous  orifice  was  observed  by  M.  Riippell  in  the  corresponding  part  of  the 
mantle  of  Aspergillum,  viz.  that  which  is  next  the  sunken  sieve-like  extremity  of  the 
tube,  and  by  which  he  supposes  the  water  necessary  for  respiration  to  be  received  when 
the  retreating  tide  leaves  exposed  the  expanded  siphonic  extremity. 
This  cannot,  however,  be  its  use  in  such  species  of  Clavagella  as  reside,  like  the 
present,  at  depths  too  great  to  allow  of  their  being  ever  left  with  the  siphonic  aperture 
'/•  Figg.  8,  10.  2  g.  Figg.  8,  10.  '  h'.  Figg.  8,  10. 
*  Figg.  8,  10.  *  *.  Figg.  12,  13,  14. 
