With  it  have  been  found  several  other  genera  of  the  same
family,  such  as  Sphceronites,  Hemicosmites,  &c.  Among  the
mollusca  are  Pteropods  of  the  genus  Conularia  of  large  size  (for
genus,  see  fig.  518,  p.  435).  About  11  species  of  Graptolite  are
reckoned  as  belonging  to  this  formation  ;  they  are  chiefly  found
in  peculiar  localities  where  black  mud  abounded.  The  forma-
tion, when  traced  into  South  Wales  and  Ireland,  assumes  a
greatly  altered  mineral  aspect,  but  still  retains  its  characteristic
fossils.  The  known  fauna  of  the  Bala  group  comprises  56«?
species,  352  of  which  are  peculiar,  and  93,  as  before  stated,
are  common  to  the  overlying  Llandovery  rocks.  It  is  worthy
of  remark  that,  when  it  occurs  under  the  form  of  trappean  tuff'
(volcanic  ashes  of  De  la  Beche),  as  in  the  crest  of  Snowdon,
the  peculiar  species  which  distinguish  it  from  the  Llandeilo  beds
are  still  observable.  The  formation  generally  appears  to  be  of
shallow-  water  origin,  and  in  that  respect  is  contrasted  with  the
group  next  to  be  described.  Professor  Ramsay  estimates  the
thickness  of  the  Bala  Beds,  including  the  contemporaneous  vol-
canic rocks,  stratified  and  unstratified,  as  being  from  10,000  to
12,000  feet.
Llandeilo  Flags.  —  The  Lower  Silurian  strata  were  originally
divided  by  Sir  R.  Murchison  into  the  upper  group  already  de-
scribed, under  the  name  of  Caradoc  Sandstone,  and  a  lower  one,
called,  from  a  town  in  Caermarthenshire,  the  Llandeilo  flags.
The  last-mentioned  strata  consist  of  dark-coloured  micaceous
nags,  frequently  calcareous,  with  a  great  thickness  of  shales,
generally  black,  below  them.  The  same  beds  are  also  seen  at
Builth  in  Radnorshire,  where  they  are  interstratified  with
volcanic  matter.
