AN  EXTINCT  GENUS  OE  THE  STUEIONIDAi. 
879 
caudal  region  the  processes  both  above  and  below  the  vertebral  column  were  bony  and 
persistent.  In  the  anterior  parts  of  the  trunk,  the  former,  or  neurapophyses,  are  present, 
but  the  heemapophyses  are  in  no  case  preserved ; the  latter,  together  with  the  vertebral 
centra,  were  composed  of  perishable  material  unable  to  resist  the  process  of  decomposi- 
tion, which  the  harder  parts  successfully  withstood.  Professor  Owen  describes  the 
neural  arch  of  the  vertebra  in  the  Sturgeon  as  composed  of  “ two  superimposed  pieces 
on  each  side,  the  basal  portion  bounding  the  neural  canal,  the  apical  portion  the  parallel 
canal  filled  by  fibrous  elastic  ligament  and  adipose  tissue ; above  this  is  the  single  carti- 
laginous neural  spine*.”  The  corresponding  parts  in  the  fossil  connected  with  the 
anterior  portion  of  the  column  are  preserved  in  one  specimen.  The  neurapophyses 
(Plate  LXIX.  n a)  are  expanded  at  each  extremity,  and  slightly  constricted  in  the  middle 
region.  They  appear  to  be  composed  of  two  elements,  corresponding  to  the  basal  and 
apical  cartilages  in  the  Sturgeon ; but  whether  the  latter  embraced  a second  canal 
parallel  to  and  above  the  spinal  chord,  I am  unable  to  determine.  Probably  the  substi- 
tution of  bone  for  cartilage  in  the  extinct  genus  may  have  rendered  the  support  of  the 
fibro-cartilaginous  substance  contained  in  this  canal  superfiuous.  The  neural  spines 
(Plate  LXIX.  n s)  are  higher  than  those  of  the  Sturgeon.  The  highest,  although  not 
perfect,  measures  three-quarters  of  an  inch.  These,  together  with  the  apophyses  which 
support  them,  slope  backwards  at  a considerable  inclination ; but  by  a curve  forwards  at 
a little  distance  from  the  articulation,  the  former  show  a tendency  to  assume  a perpen- 
dicular position.  The  breadth  and  proximity  of  the  neural  arches  must  have  given 
great  support  and  protection  to  the  weaker  parts  of  the  spinal  column. 
Dermal  Bones. — The  roof  of  the  cranial  cavity  of  the  extinct  Sturgeon  was  composed, 
as  in  the  recent  fish,  of  indurated  plates  united  to  each  other  by  close  sutures.  Before 
entering  upon  the  description  of  these  several  parts,  it  is  advisable  to  premise  a few 
observations  on  the  corresponding  parts  of  the  existing  Sturgeon.  Many  attempts  have 
been  made  by  comparative  anatomists  to  recognize  in  these  cranial  plates  the  homo- 
logues  of  the  true  epicranial  bones  of  the  bony  fishes,  but  no  conclusions  have  been  as 
yet  received  as  satisfactory.  The  discrepancy  and  irregularity  observable  in  the  size, 
form,  and  number  of  the  head-plates,  not  only  in  the  different  species,  but  even  in  indivi- 
duals of  the  same  species,  have  been  considered  good  and  sufficient  grounds  for  demur- 
ring to  the  several  theories  advanced  as  to  their  special  and  general  homologies. 
Ha\fing  had  occasion  to  examine  a considerable  number  of  recent  crania  while  investi- 
gating the  affinities  of  the  fossil  Sturgeon,  I have  observed  a great  want  of  uniformity 
between  the  outer  and  inner  demarcations  of  the  principal  cranial  plates,  and  that, 
although  the  former  may  vary  in  number  and  relative  proportions,  the  latter  are  con- 
stant and  symmetrical.  This  nonconformity  is  frequently  so  marked,  that  a bone,  single 
and  well-defined  on  the  inner  table  of  the  skull,  may  be  invested  with  two  or  more 
scutes  on  the  dermal  surface.  The  suprascapular  bone,  for  instance,  usually  carries 
tw'o  scales  externally,  in  consequence  of  which  deception  the  upper  one  has  been  con- 
* Hunterian  Lectures,  8vo,  1846,  vol.  ii.  p.  53. 
MDCCCLVIII.  5 Y 
