308 
DE.  J.  S.  BOWEEBAXK  OX  THE  AXATOilY 
Tethea  Ingalli,  Bowerbank,  MS.  In  this  sponge  and  in  other  species  this  foim  occasion- 
ally presents  a very  gradual  transition  from  the  purely  stellate  form  to  the  full  subsphero- 
stellate  one,  in  which  the  radii  and  the  spherical  centre  are  of  about  equal  length,  while 
in  the  fully  developed  sphero-stellate  forms  this  graduation  is  never  seen. 
Sphero-stellate  (Plate  XXV.  fig.  15). — HaVng  the  radii  acutely  conical  and  based 
on  a large  central  sphere  of  greater  diameter  than  the  length  of  the  radii.  Tethea  robiista, 
Bowerbank,  MS.,  a new  species  from  Australia,  in  the  British  Museum,  presents  an 
excellent  type  of  this  form  of  spiculum.  As  the  central  nucleus  appears,  imder  favour- 
able circumstances,  we  distinctly  trace  a central  canal  in  each  ray,  passing  from  the 
centre  of  the  sphere  to  near  the  distal  termination  of  each  of  the  radii,  as  represented  in 
Plate  XXV.  fig.  17.  These  canals  are  not  usually  apparent  in  the  perfect  spicula,  pro- 
bably in  consequence  of  the  fiuid  being  hermetically  sealed  -v^ithin  the  canals  of  the 
radii,  but  I could  not  determine  the  presence  of  the  fluid  by  polarized  light. 
Sphero-stellate  with  cylindro-subpoliate  radii  (Plate  XXV.  fig.  16). — Having  the 
cylindrical  radii  slightly  expanded  and  somewhat  foliated  at  the  distal  exti’emities. 
This  remarkable  form  was  obtained  by  washmg  some  specimens  of  Oculina  rosea,  from 
the  South  Sea,  and  there  is  little  doubt  of  its  being  from  an  unknown  species  of 
Tethea. 
Elongo-attenuato-stellate  (Plate  XXV.  fig.  18). — Having  the  radii  sprmging  fr-om 
an  elongated  instead  of  a central  base.  This  form  of  spiculum  occui's  abundantly  in 
Tethea  mwricata.,  Bowerbank,  MS. : from  Vigten  Island,  Norway. 
Arborescent  elongo-subsphero-stellate  (Plate  XXV.  fig.  19). — Ha'^ing  the  radii 
springing  from  a dilated  and  elongated  common  base  of  about  the  dimensions  of  two 
subsphero-stellate  spicula,  partially  fused  together. 
This  remarkable  form  occurs  abundantly  in  Geodia  carinata,  Bowerbank,  MS.,  from 
the  South  Sea.  The  nucleus,  whence  the  radii  proceed,  is  always  more  or  less  elongated, 
but  is  not  usually  so  much  dilated  as  in  the  specimen  figured.  The  arborescent 
character  of  the  distal  terminations  of  the  radii  is  also  very  variable. 
PiLEATED  CYLINDRO-STELLATE  (Plate  XXV.  figs.  20,  21,  22  and  23). — Ha'^ing  several 
recurved  spines  uniting  and  forming  a pileus  at  the  apex  of  the  ray,  shaped  like  that 
of  a young  mushroom.  These  singularly  variable  spicula  are  abundant  in  Sjjongilla 
pliimosa,  Carter.  They  are  remarkable  as  afibrding  a series  of  transitional  forms  fr-om 
a single  straight  spiculum  to  the  regular  multfradiate  stellate  one.  Fig.  20  represents, 
about  the  first  stage  of  variation  from  the  simple  elongate  spinous  spiculum,  a few 
rather  strongly  produced  cylindrical  spines  appearing  near  the  middle  of  the  shaft.  In 
fig.  21  two  of  these  spines  are  considerably  more  elongated  than  those  in  fig.  20,  and  the 
shaft  is  not  so  long  as  that  of  fig.  20.  In  fig.  22  the  axial  shaft  is  still  more  curtailed 
in  its  proportions,  and  the  central  radii  are  further  elongated  and  increased  in  number ; 
and  in  fig.  23  we  find  the  axial  spiculum  scarcely  distinguishable  from  the  lateral  rays. 
M hen  the  radii  projected  are  few  in  number,  they  are  usually  at  right  angles  to  the  axial 
spiculum ; but  when  they  are  produced  in  greater  numbers,  they  are  projected  at  various 
