876 
SIE  P.  G.  EGEETON  OX  CHOXDEOSTErS, 
distal  parts  than  in  the  vicinity  of  the  base.  The  transverse  joints  are  very  frequent 
in  all  the  rays.  The  upper  lobe  is  invested,  from  the  base  to  the  extremity,  -svith 
numerous  rows  of  thick  narrow  imbricated  scales,  lozenge-shaped  near  the  base,  but 
elongating  more  and  more  until  they  become  almost  acicrdar  at  the  distal  termination 
of  the  fin.  The  pedicle  of  the  tail  is  robust,  in  proportion  to  the  length  and  spread  of 
the  organ  it  supports,  which,  both  as  to  size  and  mechanism,  is  the  most  powerful  instru- 
ment of  progression  yet  met  with  amongst  extinct  fishes. 
Cranial  Anatomy. — The  skull  of  the  extinct  Stogeon  (so  far  as  one  can  judge  from 
the  confused  and  dislocated  condition  of  its  component  parts  in  the  specimens  examined) 
corresponds  with  that  of  the  recent  Sturgeon  in  being  composed  of  a semi-ossified  carti- 
laginous box,  protected  by  a series  of  bony  plates.  Of  the  bones  composing  the  maxil- 
lary arch,  the  palatals  (Plate  LXIX.  20)  are  most  easily  recognized.  They  are  situated 
immediately  in  front  of  the  epitympanic  bones,  and  spread  upwards,  downwards,  and 
backwards,  occupying  a large  area,  and  having  their  broadest  expanse  directed  for- 
wards. They  are  thick  and  of  compact  structure,  and  ha\ing  consequently  resisted  the 
effects  of  time  more  successfully  than  the  slighter  and  more  destructible  parts,  are  of 
more  frequent  occurrence  in  the  fossil  specimens.  The  posterior  process  of  the  palatine 
bone  is  more  prolonged  than  the  corresponding  part  of  the  recent  Sturgeon,  and  the 
lower  anterior  process  to  which  the  maxillary  and  premaxillary  bones  attach,  is  more 
expanded.  The  result  of  this  character  would  be  a more  complete  closm-e  of  the 
anterior  roof  of  the  mouth  by  the  substitution  of  bone  for  cartilage  than  is  the  case  in 
the  existing  Sturgeons.  The  upper  anterior  process  is  also  broad  for  the  attachment  of 
the  corresponding  bone  of  the  opposite  side. 
On  comparing  the  fossil  palatal  bone  with  the  recent  one,  the  former  so  much 
resembles  the  latter  inverted,  that  were  it  not  for  the  conclusive  evidence  of  several 
specimens  having  these  bones  in  their  proper  position,  the  error  might  have  been  com- 
mitted of  describing  the  longer  process  as  the  anterior  one,  and  vice  versd.  That  por- 
tion of  the  premaxillary  bone  (Plate  LXIX.  22)  forming  the  trenchant  margin  of  the 
upper  jaw,  resembles  the  homologous  part  of  the  recent  Sturgeon,  but  the  long  posterior 
process  which  bends  backwards  and  downwards  in  the  latter,  tuims  upwards  in  the 
former.  The  anterior  plate  is  triangular,  and  is  composed  of  hard  bony  material, 
having  a rugged  outward  surface  occasioned  by  the  radiation  of  bony  fibres  from  a 
central  point  of  ossification.  It  is  attached  to  the  anterior  lower  process  of  the  palatine 
bone,  and  connected  with  its  fellow  on  the  median  line.  The  upward  direction  of 
the  long  process  is  a material  deviation  from  the  modern  type ; in  the  latter  this  pro- 
cess overlaps  the  articulation  of  the  lower  jaw,  whereas  in  the  fossil  genus  it  rather 
simulates  a nasal  process.  The  distal  extremity,  instead  of  bemg  fiattened  as  in  the 
Sturgeon,  has  a rounded  termination  as  if  for  articulation  with  the  nasal  cartilages,  or 
with  the  corresponding  process  of  the  opposite  side. 
There  is  no  satisfactory  evidence  in  any  of  the  specimens  examined  of  the  character 
of  the  maxillary  bone.  A small  bifurcate  bone  (Plate  LXIX.  21)  is  occasionally  preserved 
