880 
SIE  P.  G.  EGEETON  OX  CHOXDEOSTEUS, 
sidered  tlie  representative  of  the  parietal  bone,  a misnomer  -which  mast  of  necessity 
invalidate  the  implied  relations  of  all  the  surrounding  parts.  The  knowledge  of  the 
true  demarcation  of  this  bone  leads  to  the  exposm’e  of  another  eiTor  which  has  embar- 
rassed the  solution  of  this  question,  namely,  the  idea  that  the  large  posterior  median 
plate  represents  the  supraoccipital  bone,  whereas  it  must  be  considered  as  belonging  to 
the  series  of  dorsal  plates,  and  may  be  designated  as  the  nuchal  scale.  In  front  of  this 
a smaller  median  plate  is  situated,  which  may  with  more  probability  be  refeined  to  the 
supraoccipital  region.  This  is  flanked  on  either  side  by  a small  lateral  plate  in  the 
position  of  the  paroccipitals,  to  which  and  the  exoccipital  the  upper  extremities  of  the 
suprascapular  bones  are  articulated.  Beneath  and  behind  the  paroccipital  two  plates 
are  attached,  ha-\dng  broad  alee  descending  into  the  cranial  ca-vity.  These  may  be 
considered  the  exoccipitals,  or  lateral  elements  of  the  neural  arch  of  the  first  occipital 
vertebra.  The  upper  extremity  of  the  suprascapular  and  the  exoccipital  bone  are 
covered  externally  by  a single  scale,  concealing  the  junction  between  the  two.  The 
arch  of  the  second  cranial  segment  is  composed  of  two  pairs  of  large  elongated  plates. 
The  outer  pair  are  characterized  by  well-developed  processes,  extending  do-^mwards  and 
inwards  to  meet  the  corresponding  extremities  of  the  alisphenoids,  a character  which 
aflbrds  strong  reasons  for  considering  them  the  true  homologues  of  the  mastoid  bones. 
They  extend  backwards  as  far  as  the  par-  and  ex-occipitals,  and  are  united  in  front  -with 
the  frontals  and  postfrontals.  The  centre  pair  of  plates  are  the  parietals.  They  arti- 
culate with  the  paroccipitals  and  the  supraoccipital.  The  latter  sends  out  a long 
median  process  between  the  parietals,  in  advance  of  which  a small  mid  frontal  plate 
occurs,  occupying  the  position  of  the  fontinelle,  observable  in  the  young  Sturgeon,  and 
sometimes  present  in  the  bony  fishes,  both  recent  and  fossil.  The  parietals  are  con- 
nected laterally  with  the  mastoids,  and  anteriorly  with  the  inner  posterior  margins  of 
the  frontals.  The  third  segment  of  the  cranium  is  likewise  composed  of  two  pairs  of 
plates,  representing  the  frontals  and  postfrontals.  The  former  are  wedged  posteriorly 
into  the  angles  between  the  extremities  of  the  mastoid  and  parietal  plates,  and  extend 
forwards  beyond  the  nasal  aperture ; they  are  flanked  by  the  outer  pair  of  bones,  the 
postfrontals.  These  are  united  behind  to  the  mastoids  by  strong  squamose  overlaps, 
from  which  points  the  tympano-mandibular  arch  is  suspended. 
The  truth  or  fallacy  of  the  homologies  I have  attempted  to  trace  in  the  indui'ated 
portions  of  the  cranial  cartilages,  must  depend  upon  more  extended  investigations  of  the 
softer  tissues  entering  into  the  cranial  oeconomy,  both  in  the  embryonic  and  adult  condi- 
tions ; in  the  meantime,  by  adopting  (if  only  provisionally)  the  analogies  now  proposed, 
one  object  I have  in  view  will  be  obtained,  namely,  the  use  of  intelligible  terms  for 
comparison  between  the  recent  and  fossil  Sturgeons.  The  description  given  by  Professor 
Owen,  in  his  ‘ Lectures  on  Comparative  Anatomy,’  of  the  cartilaginous  elements  of  the 
cranial  bones  of  the  Sturgeon,  tends  to  verify  the  positions  I have  assigned  to  the  indu- 
rated parts  in  the  epicranial  table ; the  difliculties  which  have  hitherto  embarrassed  the 
subject  I have  attempted  to  overcome,  by  showing  that  these  parts  are  composed  of  an 
