AN  EXTINCT  GENES  OF  THE  STUEIONID^. 
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outer  and  an  inner  layer,  and  that  although  the  former  may,  through  the  irregularity  of 
its  configurations,  lead  to  erroneous  suppositions  as  to  the  coincidence  of  its  several  com- 
partments with  the  true  cranial  bones,  the  latter  is  sufficiently  constant  to  lead  to  reason- 
able conclusions  as  to  the  homologies  of  its  associated  members. 
The  only  portions  of  the  cranial  covering  of  the  fossil  Sturgeon  as  yet  discovered  are 
the  parietals,  the  mastoids,  the  frontals,  and  the  prefrontals.  A small  plate  observable 
in  one  specimen  may  perhaps  represent  the  paroccipital.  None  of  the  single  median 
plates  are  indicated  in  any  of  the  specimens.  As  a general  character,  all  the  head- 
bones  are  relatively  much  shorter  and  broader  than  those  of  the  recent  Sturgeon.  This 
feature  is  in  harmony  with  the  short  and  bidky  form  of  the  trunk.  The  parietal  plates 
(Plate  LXIX.  7)  are  oblong,  measuring  about  an  inch  and  a half  in  length  by  three- 
quarters  of  an  inch  in  breadth.  They  articulate  with  each  other  on  the  median  line, 
without  the  intervention  of  the  prolonged  process  of  a supraoccipital  plate.  Each  has 
a slight  central  umbo,  from  whence  the  fibres  radiate  outwards  to  the  periphery.  The 
mastoid  plates  (Plate  LXIX.  s)  are  of  larger  dimensions,  measuring  an  inch  and  three- 
quarters  in  length  by  an  inch  and  eight-tenths  in  breadth.  In  disjointed  specim.ens 
they  are  easily  distinguished  from  the  parietals  by  the  position  of  the  umbo  from  which 
the  bony  fibres  radiate,  this  being  situated  on  the  outer  margin  of  the  plate.  They 
articulate  by  their  inner  margins  with  the  parietals.  The  frontals  (Plates  LXVIII. 
fig.  1,  and  LXIX.  n ) are  remarkable,  in  comparison  with  then-  recent  homologues,  for 
great  width  and  contraction  of  the  antero-posterior  diameter.  They  measure  an  inch 
and  a half  in  length  by  one  inch  in  breadth.  The  corresponding  dimensions  of  these 
plates  in  the  recent  specimen,  from  which  former  comparative  measures  have  been  taken, 
are  two  inches  by  six-tenths  of  an  inch.  The  frontals  articulate  posteriorly  with  the 
parietals  and  mastoids,  and,  on  the  median  line,  with  each  other,  without  the  interposi- 
tion of  a midfrontal  plate  or  fontinelle.  The  centre  of  each  plate  is  slightly  elevated. 
A portion  of  a postfrontal  plate  (Plate  LXIX.  12)  is  visible  in  one  specimen,  articulating 
with  the  frontal  and  mastoid.  All  these  plates  are  united  and  interlocked  by  means  of 
coarse  sutures,  and  not  by  squamose  overlaps,  as  in  the  recent  Sturgeon.  On  the  outer 
surface  they  are  for  the  most  part  smooth,  and  seem  to  have  been  covered  by  a soft  inte- 
gument. A few  superficial  granules  are  scattered  irregularly  over  some  of  the  specimens 
(which  may  perhaps  be  a specific  character),  but  there  is  no  evidence  of  the  deep  pits  and 
coarse  radiating  ornament  so  characteristic  of  the  dermal  scales  of  the  Sturgeon.  The 
same  absence  of  surface  ornament  obtains  with  reference  to  the  opercular  apparatus.  The 
operculum  (Plates  LXVII.  and  LXIX.  35)  is  a broad  and  flat  plate,  almost  circular,  and 
exceedingly  thin.  It  has  no  articulating  facet  on  the  upper  margin,  and  was  probably 
suspended  in  the  tegumentary  tissues,  deriving  a certain  amount  of  additional  support 
from  the  broad  posterior  limb  of  the  epitympanic  bone.  The  outer  surface  is  smooth  and 
very  compact.  The  inner  surface  is  characterized  by  the  occurrence  of  numerous  minute 
punctures  passing  obliquely  into  the  substance  of  the  plate.  The  posterior  outline  is 
semicu’cular,  adapting  itself  to  the  anterior  curve  of  the  thoracic  cincture.  The  lower 
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