878 
SIE  P.  Gr.  EGEETOiS"  OX  CHOKOEOSTEUS, 
ceratohyals,  occurs  in  several  specimens  in  a position  which  leads  to  that  conclusion.  It 
has  flattened  extremities  and  a constricted  shaft,  resembling  in  outline  two  isosceles 
triangles  united  at  the  apices.  The  corresponding  bone  in  the  Sturgeon  is  shoi*ter, 
broader,  and  thicker  in  all  its  dimensions.  In  one  specimen  a series  of  eight  or  nine 
plates  (Plate  LXIX.  44)  is  preserved,  indicating  the  existence  of  the  appendages  of  the 
hyoid  arch,  which  are  wanting  in  the  recent  Sturgeon.  These  homologues  of  the 
branchiostegous  rays  of  bony  fishes  are  flat,  elongated  bony  laminge,  of  which  the  upper 
one  is  the  longest.  They  are  seen  in  their  natural  position,  behind  the  hyoid  bones  and 
immediately  below  the  opercular  flap.  They  have  the  ganoid  character  of  the  coiTe- 
sponding  parts  in  the  fossil  genus  Pachycormus,  but  are  fewer  in  number  than  in  that  and 
the  allied  genera.  The  only  portions  of  the  branchial  arches  I have  been  able  to  deter- 
mine are  the  epibranchials  (Plate  LXVIII.  fig.  2, 48  a,  5,  c,  d).  These  are  four  in  number. 
The  first  or  anterior  bone  is  the  longest.  The  upper  extremity  is  broad  and  thin,  the 
shaft  and  lower  extremity  slender  and  cylindrical.  The  two  succeeding  bones  are  less 
expanded  above  and  are  rather  shorter.  The  last  of  the  series  difiers  from  the  others 
in  having  its  broader  extremity  directed  downwards.  It  is  in  fact  very  similar  in  form 
to  the  ceratohyal  bone,  but  difiers  in  having  the  upper  extremity  and  the  shaft  rather 
more  cylindrical.  The  number  corresponds  with  that  of  the  homologous  parts  of  bony 
fishes,  if  we  limit  the  term  branchial  arch  to  the  gill  supporting  bones,  to  the  exclusion 
of  the  pharyngeal  arch.  The  proportions  of  the  epihyals  in  the  fossil  correspond  with 
those  of  the  same  parts  in  most  of  the  bony  fishes,  the  fii’st  being  the  longest,  and  the 
fourth  the  shortest,  with  a broad  inferior  articulation. 
The  sphenoid  bone  (Plate  LXVIII.  fig.  1, 9),  constituting  the  base  or  platform  of  the 
cranial  cavity,  is  more  completely  ossified  in  the  extinct  than  in  the  recent  Stiu’geon : 
hence  its  preservation  in  the  fossil  state.  The  anterior  or  presphenoid  portion  of  the 
bone  (Plate  LXVIII.  fig.  1,  9)  is  extended  forwards  beyond  the  palatal  bones,  and  the 
basisphenoid  (Plate  LXVIII.  fig.  1,  s)  projected  backwards  to  the  point  of  junction  of 
the  trunk  with  the  head.  The  posterior  termination  of  the  bone  was  deeply  notched, 
and  so  thin,  that  the  non-existence  of  a distinct  basioccipital  bone  is  made  e^ident  fr-om 
the  absence  of  any  articulating  surface  for  its  attachment.  The  alisphenoid  processes 
(Plate  LXVIII.  fig.  1, 9)  are  given  off  at  a point  considerably  in  advance  of  the  position 
they  occupy  in  the  generality  of  bony  fishes ; they  correspond  very  nearly  in  this  respect 
with  the  homologous  parts  of  the  Sturgeon.  They  are  situate,  in  the  fossil,  3^  inches 
from  the  posterior,  and  2 inches  from  the  anterior  extremities  of  the  bone.  This 
extension  of  the  bone  backwards  in  both  the  recent  and  fossil  Sturgeon,  so  remarkable 
in  comparison  with  the  Relative  dimensions  of  the  elements  of  the  sphenoid  bone  in  the 
Teleostei,  is  considered  by  Dr.  Eudolph  Wagner  to  be  efiected  by  the  calcification  of 
the  fibrous  sheath  of  the  basioccipital  and  postsphenoidal  regions  of  the  chorda  dorsahs ; 
Professor  Owen,  however,  considers  that  its  extension  upon  the  neck,  the  absence  of  the 
articular  concavity,  and  the  persistence  of  the  cartilaginous  basis  of  the  skull,  are  opposed 
to  this  view  of  its  homological  relations. 
It  has  already  been  stated,  in  the  description  of  the  organs  of  locomotion,  that  in  the 
