238  ME.  E.  WETHERED  ON  THE  MICROSCOPIC  [May  1 893, 
Girvanellci  can  be  detected  leads  one  to  infer  that  some  at  least 
have  originated  from  these  tubules.  Nevertheless  I  may  state, 
though  I  shall  refer  to  the  subject  again,  that  Girvanellci- tubules 
are  clearly  defined  only  when  the  interior  has  been  filled  in  with 
calcite.  The  walls  of  the  tubules  usually  present  the  same 
minutely-granulated  appearance  as  that  noticed  in  the  structureless 
masses  just  referred  to ;  and  this  constitutes  further,  though  not 
conclusive  evidence  in  support  of  the  view  that  the  masses  represent 
original  aggregations  of  tubules,  the  outlines  of  which  have  not 
been  nreserved  by  the  infilling  of  calcite.  If  some  of  the  tubules 
had  been  partially  filled  in  with  calcite,  that  infilling  would  account 
for  the  streaks  and  spots  of  the  mineral  which  the  masses  so  fre¬ 
quently  contain. 
Slide  No.  3. — Fragments  of  polyzoa,  ossicles  of  crinoids,  aggrega¬ 
tions  of  Girvanella  'problematical  and  granulated  masses  in  which 
occasional  tubules  appear. 
(ii)  Slides  f  rom  the  Thin-bedded  Limestones. 
Slide  No.  1. — :Some  of  the  organic  fragments  are  so  crystalline 
that  their  true  nature  cannot  be  determined,  but  among  them  are 
polyzoa  and  ostracoda.  The  slide  is  remarkable  for  the  number  of 
aggregations  of  Gvnjanella  which  occur.  Of  these  at  least  two 
types  can  be  distinguished  :  (a)  that  of  G.  problematical  the  tubules 
measuring  *017  millim.  in  diameter,  and  sometimes  encrusting 
foreign  objects ;  (b)  a  form  of  Girvanella- tubules  of  irregular  size, 
with  a  maximum  diameter  of  *03  millim.  These  latter  also  grow 
around  fragments  of  organisms  which  occur  in  the  limestone. 
Slide  No.  2. — In  this  the  remains  of  organisms  are  for  the  most 
part  obscure.  They  include  those  of  crinoids,  aggregations  of 
Girvanella  problematical  spicular  objects,  and  inorganic  material  (con¬ 
sisting  chiefly  of  very  small,  angular  grains  of  quartz  and  mica, 
apparently  of  detrital  origin).  There  are  also  numerous  minute 
spherules  of  pyrites,  which  are  presumably  of  organic  origin. 
Slide  No.  3. — In  this  we  see  fragments  of  polyzoa  and  ostracoda, 
but  the  limestone  is  chiefly  made  up  of  remains  of  crinoids.  Tubules 
of  Girvanella  problematica  occur,  and  in  one  instance  are  most 
beautifully  preserved.  The  tubules  form  a  granule,  having  a 
nucleus  in  the  centre  which  has  passed  into  calcite,  and  their 
preservation  is  due  to  the  interiors  having  been  filled  in  with  the 
same  mineral,  against  which  the  wails  of  the  tubules  stand  out  in 
relief. 
Slide  No.  4. — The  organic  fragments  here  are  similar  to  those 
noticed  in  previous  slides.  Two  types  of  Girvanella  are  represented, 
the  tubules  of  the  largest  having  a  maximum  diameter  of  *07  millim. 
In  all  cases  they  collect  round  a  nucleus,  forming  a  spherule.  There 
are  also  spherules  which  show  no  structure. 
Slide  No.  5. — Fragments  of  polyzoa  and  obscure  masses  of 
Girvanella.  In  some  of  the  latter  the  tubules  are  seen  passing 
into  the  minutely-granulated  structure,  so  characteristic  of  the 
spherules  in  which  tubules  are  absent  or  the  outlines  obscured. 
