AND  PHYSIOLOGY  OP  THE  SPONOIAD^. 
319 
shaft.  From  both  terminations  of  the  shaft  a number  of  minute  radial  canals  pass  from 
the  centre  to  the  circumference  of  the  rotulae,  and  in  one  of  the  large  ones  I counted 
twenty  radial  canals.  The  rotulse  are  flat,  or  very  slightly  convex  outward  near  the 
centre,  and  the  margins  are  perfectly  entire. 
Boletifoem  (Plate  XXVI.  flg.  33). — The  form  of  this  spiculum  is  very  like  that  of 
the  common  edible  mushroom  when  fully  grown.  The  large  discal  end  is  convex 
externally,  and  has  the  margin  entire.  The  shaft  is  nearly  of  the  same  diameter 
throughout  its  length,  and  occasionally  it  has  one  or  two  large  spines  projected  from  it, 
near  the  middle  and  at  right  angles  to  its  axis. 
The  small  end  is  more  or  less  lentiform,  but  it  is  frequently  very  irregular  both  in 
size  and  shape. 
This  interesting  form  of  spiculum  is  from  the  gemmule  of  Spongilla  reticulata, 
Boweebaxe,  ms.  From  the  Kiver  Amazon. 
Boletifoem:  slexdee  (Plate  XXVI.  figs.  34  and  35). — This  graceful  and  elegant 
form  of  spiculum  occurs  at  the  inner  surface  of  the  crust  of  the  gemmule  of  Spongilla 
recurvata,  Boweebaek,  MS.,  fr’om  the  Eiver  Amazon.  The  shaft  is  exceedingly  slender, 
measuring  at  the  middle  ^ 5000th  of  an  inch  in  diameter.  The  large  discal  end  of  the 
spiculum  is  slightly  convex  externally,  has  the  margin  perfectly  entire,  and  is  l of 
an  inch  in  diameter.  The  small  lentiform  end  measured  g^g^th  of  an  inch  in  diameter, 
and  the  total  length  of  the  spiculum  is  y^th  of  an  inch. 
Umboxato-scutulate  (Plate  XXVI.  figs.  36  and  37). — This  spiculum  is  found  imme- 
diately beneath  the  outer  membrane  of  the  gemmule  of  a new  species  of  freshwater 
sponge,  from  the  River  Amazon,  Spongilla  Brownii,  Boweebaxk,  MS.  The  form  is 
truly  that  of  a little  shield,  the  lower  surface  being  concave,  while  the  upper  one  has  a 
corresponding  degree  of  convexity,  and  the  umbo  projects  from  its  centre  in  the  shape 
of  a small  cone.  The  diameter  of  an  average-sized  one  is  l ygoth  of  an  inch,  and  the 
height  very  nearly  equalled  the  diameter. 
Ixequi-teieotulate  (Plate  XXVI.  fig.  38). — HaAng  two  terminal  rotulse  of  equal 
size,  and  one  intermediate  of  greater  diameter  than  the  terminal  ones.  I have  no 
knowledge  of  the  sponge  from  which  this  spiculum  is  derived,  nor  in  truth  whether  it 
be  from  a sponge.  I found  two  of  them  among  a great  variety  of  sponge  spicula 
procured  by  washing  some  specimens  of  Oculina  rosea,  from  the  South  Sea.  The 
general  structure  of  the  spiculum  is  the  same  as  that  of  a Birotulate  one  from  a Spon- 
gilla, and  the  rotulse  exhibit,  rather  indistinctly,  the  same  description  of  radial  canals 
that  are  so  distinctly  portrayed  in  Plate  XXVI.  fig.  32.  It  may  possibly  have  been 
disengaged  by  decomposition  from  the  gemmule  of  some  unknown  species  of  Spongilla 
brought  down  the  rivers  which  discharge  their  contents  in  the  estuaries,  or  between  the 
islands  of  those  seas. 
2 X 
MDCCCLVIII, 
