182 
PEOFESSOE  OWEN  ON  THE  SKELL  AXD 
harmonizes  with  the  expanse  of  the  temporal  fossa  and  the  size  of  the  zygomatic  arch, 
and  with  the  force  required  for  the  due  working  of  the  extraordinaiy  teeth  which  are 
developed  in  it. 
The  depth  of  the  ramus  behind  the  last  molar  is  1 inch  10  lines. 
It  may  be  concluded,  from  the  size  and  shape  of  the  two  molars  in  place  in  the 
present  jaw,  that  it  belonged  to  a species  which,  like  Placodus  gigas  and  PI.  Andiiani. 
had  similarly-proportioned  teeth  in  the  upper  jaw,  and  not  to  species  in  which,  as  in 
PI.  rostratus  and  PI.  laticeps,  the  last  tooth  is  considerably  larger  than  the  one  in  front 
of  it.  From  Placodus  pachygnathus  it  ditfers,  not  only  in  the  absence  of  the  ledge  out- 
side the  last  tooth,  but  by  the  commencement  of  the  symphysis  in  advance  of  the 
second  tooth.  The  species  which  it  indicates  may  be  named  Placodus  hathygnathus. 
The  last  fragment  of  a Placodus  from  the  Muschelkalk  of  Bapeuth,  which  will  be 
here  noticed,  is  a small  portion  of  jaw  containing  three  teeth  (Plate  X.  figs.  2,  3,  4 
and  5).  These  are  subequal,  and,  as  compared  with  those  last  described,  of  small  size : 
in  this  respect,  as  in  their  shape,  they  correspond  with  the  marginal  teeth  of  the  upper 
jaw  of  Placodus  gigas. 
The  bone  is  preserved  on  one  side  only,  and  for  the  extent  of  about  an  inch,  gradually 
thinning  off  into  the  mass  of  matrix  which  chiefly  constitutes  the  specimen. 
If  this  free  bony  surface  belongs,  as  I surmise,  to  the  upper  and  outer  part  of  the 
upper  jaw,  the  croAvus  of  the  teeth  project  further  beyond  the  free  outer  alveolar  mar- 
gin, as  in  fig.  4,  than  they  appear  to  do  in  the  figure  of  the  upper  jaw  oi Placodus  gigas. 
in  the  ‘ Poissons  Fossiles  ’ of  M.  Agassiz.  At  the  same  time,  the  crown  and  working  sur- 
face of  the  crowns  of  the  teeth  look  downward,  as  they  ought  to  do  on  the  above  sup- 
position of  the  natural  aspect  of  the  exposed  surface  of  the  bone. 
On  making  a vertical  section  of  one  of  the  three  teeth  in  situ  (fig.  5),  a caAty  of 
reserve,  f,  was  exposed  above  and  on  the  inner  side  of  the  tooth  in  place,  containing  half 
of  the  crown  of  a successional  tooth,  d^  with  the  well-defined  enamel  contrasting  ndth 
the  dentine. 
Heeman  V.  Meyee,  the  historian  of  the  extinct  Saurians  of  the  continental  Muschel- 
kalk, has  made  us  familiar,  by  means  of  his  exact  descriptions  and  beautiful  figures*, 
with  the  genera  Nothosaurus,  Pistosaurus  and  Simosaurus.,  the  precmsors  and  near  allies 
of  the  long-necked  Plesiosauri  of  a subsequent  mesozoic  age. 
The  evidence  which  has  been  adduced  in  the  foregoing  pages  establishes,  I trust,  an 
addition  to  this  series,  of  a Saurian  genus,  unique  in  its  order  for  the  singularity  of  its 
dentition.  It  is  remarkable  that,  hitherto,  no  vertebrae  or  other  bones  of  the  trunk  or 
limbs  have  been  found  so  associated  with  the  teeth  of  Placodus,  as  to  have  suggested 
their  belonging  to  the  same  species.  Usually,  after  the  indication  of  a reptile  by 
detached  teeth,  the  next  step  in  its  reconstruction  is  based  upon  detached  vertebrae. 
The  twelve  or  more  evidences  of  Placodus,  afibrded  by  bone  as  well  as  tooth,  are  all 
portions  of  the  skull. 
* Die  Saurier  des  Muschelkalkes.  Fol.  1847-55. 
