288 
DE.  J.  S.  BOWEEBA^S'^K  OX  THE  AXATOI^TT 
furnished  with  acutely  conical  spines,  which  are  irregularly  dispersed  over  the  whole  or  a 
portion  of  the  spiculum,  and  they  are  therefore  said  to  be, — 
Entieely  seined. — When  the  spines  are  equally  dispersed  over  the  spiculum  from  the 
base  to  the  apex  (Plate  XXIII.  fig.  30).  Halichondria  incrustans,  Johnston. 
Basally  seined. — When  the  spines  do  not  occupy  more  than  about  one-third  of  the 
basal  portion  of  the  spiculum  (Plate  XXIII.  fig.  31).  Halichondria  Ingalli,  Boweebank. 
MS.,  a new  British  sponge. 
Medially  seined. — When  the  spines  occupy  only  about  one-thii’d  of  the  middle  of 
the  spiculum  (Plate  XXIII.  fig.  32).  Halichondria  Ingalli,  Boweebane,  MS. 
Aeically  seined. — When  the  spines  occur  only  at  and  near  the  apex  of  the  spiculum  : 
from  an  undescribed  sponge,  locality  unknown  (Plate  XXIII.  fig.  33). 
Teeminally  seined. — When  the  spines  occm’  near  both  the  base  and  apex  of  the  shaft 
of  the  spiculum,  but  not  at  the  middle : from  an  undescribed  species  of  sponge,  locality 
unknown  (Plate  XXIII.  fig.  34). 
Teieadiate  seicula. — These  forms  among  the  siliceous  sponges  are  occasionally  found 
imbedded  among  the  other  spicula  of  the  skeleton  at  the  angles  of  the  reticulation ; and 
the  radii  in  nearly  all  the  specimens  I have  seen  terminate  acutely.  But  it  is  in  the 
calcareous  sponges,  Grantia  of  Fleming,  they  are  essentially  the  spicula  of  the  skeleton. 
They  are, — 
Equiangulae  teieadiate. — Having  the  three  attenuating  rays  in  the  same  plane,  and 
the  intervening  angles  equal,  or  very  nearly  so  (Plate  XXIII.  fig.  35).  Grantia  corn- 
pressa,  Fleming. 
Eectangulae  teieadiate. — Having  the  three  attenuating  rays  in  the  same  plane,  two 
of  them  forming  a straight  line,  and  one  being  projected  from  the  middle  of  the  hue, 
forming  right  angles  to  it.  Abundant  in  the  base  of  the  ciliary  fringe  of  the  mouth  of 
the  cloaca  of  Grantia  tessellata^  Boweebank,  MS.  A new  British  species  (Plate  XXIII . 
fig.  36). 
Both  the  above  forms  are  subject,  even  in  the  same  species,  to  considerable  variations, 
arising  from  the  necessities  of  their  situation  in  the  skeleton.  The  radii  also  vary 
greatly  in  thickness  and  length  in  different  species : the  stoutest  form  I have  seen  occims 
in  a new  species  of  calcareous  sponge  from  Africa,  a spiculum  of  which  is  represented 
(Plate  XXIII.  fig.  37). 
Equiangulae  seiculated-teieadiate. — Having  the  three  rays  in  the  same  plane  uith 
the  intervening  angles  equal,  and  a fourth  ray  projected  from  the  basal  junction  of 
the  radii  at  right  angles  to  their  plane.  This  is  the  only  case  in  which  I have  found 
this  form  of  spiculum  in  a siliceous  sponge,  Halina  jBucklandi,  Boweebank,  MS.  A 
new  British  species.  But  this  form  is  very  common  in  the  calcareous  sponges,  where  it 
appears  as  a defensive  organ ; and  in  treating  of  it  as  such  I have  given  my  reasons  for 
not  designating  it  as  a quadriradiate  spiculum  (Plate  XXIH.  fig.  38). 
Biangulated  quadeieadiate. — Ha\dng  two  radii  projected  from  a common  basal  point, 
in  one  plane  forming  an  angle  of  about  90°,  and  the  other  two  projected  m a similar 
manner  in  an  opposite  direction  in  a second  plane  at  right  angles  to  the  first  one. 
