AJf  EXTINCT  GENUS  OF  THE  STUEIONIDiE. 
875 
commence  near  the  insertions  of  the  rays,  and  are  repeated  at  shorter  intervals  on  the 
posterior  parts  of  the  fin.  The  form  of  the  dorsal  fin,  when  expanded,  represents  a 
right-angled  triangle,  the  rectangle  occupying  the  apex,  and  the  posterior  angle  being 
rather  more  acute  than  the  anterior  one,  if  the  fulcral  rays  are  not  taken  into  account. 
Compared  with  the  homologous  organ  of  the  Sturgeon,  it  is  remarkable  for  its  greater 
proportional  expansion  and  strength.  The  comparison  with  the  recent  specimen  em- 
ployed in  all  the  comparative  measurements,  gives  the  dimensions  of  2 inches  by  inch 
for  the  Sturgeon,  and  4^  inches  by  3^  inches  for  the  fossil.  The  former  contains  thirty- 
five  rays,  the  latter  probably  eighty. 
Anal  Fin  (Plates  LXVII.  and  LXX.  A). — The  anal  fin  is  placed  nearer  to  the  base  of 
the  caudal  fin  than  to  the  ventral  fins.  It  is  the  smallest  of  the  natatory  organs  in  the 
fossil  genus,  whereas  the  ventral  fins  are  the  smallest  in  the  recent  Sturgeon.  The  inter- 
haemal  spines  supporting  the  fin  are  ten  or  twelve  in  number,  elongated,  slightly  arched, 
and  flattened  at  the  articular  extremities  (Plate  LXX.  i h).  The  fin-rays  are  above  thirty 
in  number,  and  more  slender  than  those  of  the  dorsal  fin.  The  lateral  surfaces  are  sub- 
angular  and  not  rounded,  as  in  the  dorsal  and  ventral  fins.  The  transverse  joints  are 
frequent.  The  decrement  in  the  length  of  the  rays,  from  the  anterior  to  the  posterior 
limit  of  the  fin,  is  very  rapid,  the  posterior  rays  being  short,  slender,  and  finely  fimbriate. 
The  form  of  the  fin  is  that  of  an  isosceles  triangle,  broader  at  the  base  and  less  acute  at 
the  apex  than  the  corresponding  fin  of  the  recent  Sturgeon. 
Caudal  Fin  (Plate  LXX.  C). — In  all  essential  structural  points  the  caudal  fin  of  Chon- 
drosteus  corresponds  closely  with  that  of  the  recent  Sturgeons.  It  differs,  however,  in 
form  and  relative  proportions.  It  is  considerably  broader  and  more  massive ; the  lobes 
are  more  divergent ; the  upper  lobe  is  less  acutely  prolonged,  and  the  lower  one  more 
nearly  the  length  of  the  upper  one.  The  point  of  divergence  of  the  lobes  is  on  a line 
with  the  axis  of  the  body,  so  that  the  two  halves  are  more  symmetrical  than  in  the  tail 
of  the  Sturgeon.  The  upper  border  of  the  fin  is  composed  of  a single  row  of  strong 
chevron-shaped  fulcral  scales,  elongating  in  succession  as  they  extend  towards  the  ex- 
tremity of  the  tail.  These  scales  are  imbricated,  and  each  had  a deep  median  sulcus 
at  its  base  for  the  attachment  of  the  preceding  scale  of  the  series.  This  portion  of 
the  caudal  organ  is  further  strengthened  by  a set  of  short  spatulate  neurapophyses 
(Plate  LXX.  C,  n a),  extending  some  distance  into  the  lobe  of  the  tail  and  support- 
ing the  ridge  scales,  but  not  giving  attachment  to  any  of  the  fin-rays.  The  lower  lobe 
has  a similar  apparatus  of  heemapophyses  (Plate  LXX.  C,  h a),  considerably  longer 
than  the  former.  They  afford  attachment  to  a numerous  group  of  interhsemal  ossicles, 
having  pointed  bases  and  broad  flattened  extremities,  to  which  the  fin-rays  are  attached. 
The  rays  are  from  seventy  to  eighty  in  number.  The  inferior  ones  are  the  strongest, 
and  they  decrease  in  size  as  they  succeed  each  other  to  the  furthest  extremity  of  the  fin. 
The  lower  margin  of  the  fin  is  destitute  of  fulcral  scales,  and  is  bounded  by  true  fin-rays, 
elongating  rapidly  in  succession  until  the  maximum  length  of  the  lobe  is  attained.  The 
bifurcations  of  the  rays  occur  at  very  irregular  intervals ; they  are  more  frequent  in  the 
