264 
MR.  BRODERIP  ON  CLAVAGELLA, 
In  the  first-mentioned  specimen,  at  the  anterior  or  greater  end  of  the  ovate  chamber, 
an  insulated  shelly  plate  has  been  secreted  with  the  tubular  perforations^  ;  that  part  of 
the  chamber  having  afforded  (apparently  at  a  former  period)  the  best  communication 
with  the  ambient  fluid :  but  a  calcareous  deposit  having  almost  entirely  cut  off  that 
communication,  the  animal  appears  to  have  been  compelled  to  secrete  a  second  shelly 
plate  towards  the  anterior  ventral  edge  of  the  fixed  valve,  where  the  perforation  of  some 
other  shell  (a  Ldthodomus  probably)  secured  the  necessary  influx  of  the  water^.  Nor  is 
this  the  only  instance  of  the  secretion  of  a  second  tubular  plate  which  has  fallen  under 
my  notice. 
In  the  last-mentioned  or  smaller  specimen,  the  perforated  shelly  plate  joins  the  ante- 
rior ventral  edge  of  the  fixed  valve  laterally^,  that  point  of  the  chamber  being  evidently 
the  most  practicable  for  communicating  with  the  water  by  means  of  the  tubules  :  the 
rest  of  the  anterior  edge  of  the  fixed  valve  is  surrounded  by  the  coral  wall. 
In  Mr.  Cuming's  specimen  the  fixed  valve  is  continued  on  to  the  tube"*.  The  anterior 
edge  of  this  valve  is  surrounded  by  the  naked  wall  of  the  chamber,  and  the  greater  end 
of  the  chamber,  or  that  part  of  it  which  is  opposite  to  this  anterior  edge,  being  imprac- 
ticable, from  its  thickness,  as  a  water  communication,  (with  a  small  exception^,  which, 
not  improbably,  had  ceased  to  be  available,)  the  animal  has  been  driven  to  secrete  the 
perforated  shelly  plates  not  far  from  the  throat  of  the  tube  on  either  side,  where  the 
chambers  of  Petricolce  or  Lithodomi  opened  a  passage  to  the  surrounding  water^.  As  a 
further  proof  of  this,  Mr.  Owen  informs  me  that  the  mantle  is  torn  at  these  particular 
points. 
I  feel  the  difficulty  of  laying  down  specific  characters  from  the  specimens  belonging 
to  Mr.  Goldsmid  and  Mr.  Cuming.  The  tubes,  or  siphonic  sheaths,  of  each  of  them  are 
broken,  and  nothing  is  left  sufficiently  distinct  to  show  the  form  of  the  aperture  when 
it  was  perfect.  The  valves  being  nearly,  perhaps  altogether,  excluded  from  the  fight, 
colour,  at  best  but  a  treacherous  guide,  is  absent  entirely.  I  cannot  conceal  from  my- 
self that  the  shape  of  the  chamber  and  of  the  valves,  together  with  the  comparative 
roughness  or  smoothness  of  their  outer  surfaces,  may  depend  upon  the  greater  or  less 
degree  of  hardness  of  the  material  in  which  the  chamber  is  formed.  With  such  data, 
however,  as  these  specimens  afford,  I  shall  endeavour  to  characterize  them ;  and  if, 
hereafter,  they  should  prove  to  be  mere  varieties,  the  descriptions  and  drawings  may 
at  all  events  assist  in  elucidating  the  natural  history  of  the  genus. 
'  Plate  XXXV.  Fig.  1.  e. 
*  Plate  XXX.  Fig.  8.  d. 
2  Ibid.  Fig.  2.  e. 
5  Ibid.  e". 
"  Ibid,  Fig.  2.  x. 
6  Ibid,  e',  e\ 
