TEETH  OE  THE  PLACODUS  LATICEPS. 
175 
across.  The  blow  which  severed  the  portion  of  rock,  containing  the  fossil,  extended 
obliquely  from  behind  the  right  orbit  to  the  back  part  of  the  left  temporal  fossa,  and 
destroyed  the  upper  wall  of  the  cranium  proper.  The  facial  part  of  the  skull  thus  j)re- 
served,  including  the  nostrils  and  orbits,  gives  the  direction  of  the  mid-line  of  the  skull, 
and  demonstrates  the  reptilian  contraction  of  the  cranial  cavity ; which  appears  to  have 
measured  only  an  inch  across  at  the  middle  of  the  temporal  fossse.  The  total  breadth 
of  the  skull  here  exceeded  8 inches ; the  total  length  being  somewhat  under  8 inches. 
The  small  degree  of  height  of  the  skull,  tig.  2,  contrasts  strongly  with  these  dimensions, 
especially  with  the  breadth.  The  vertical  diameter  of  the  skull  between  the  orbits  is 
1|-  inch : from  the  very  gradual  slope  from  this  part  to  the  end  of  the  mouth,  it  would 
seem  that  the  height  of  the  cranium  was  not  greatly  increased  as  it  extended  backward. 
The  occipital  portion  of  the  skull  is  wanting ; and  the  structure  of  the  base  of  the  skull 
cannot  be  satisfactorily  made  out ; the  bone  and  matrix  are  so  blended  on  the  palate 
and  so  nearly  alike  in  tint.  I believe  the  inner  nostrils  to  have  opened  between  the 
two  series  of  maxillary  teeth : the  matrix  can  be  traced  uninterruptedly  from  the  outer 
nostrils  downward  and  a little  backward  to  this  part  of  the  palate ; the  pterygoid  part 
of  the  palate  behind  the  last  great  tooth  is  3-g-  inches  across,  slightly  arched,  with  the 
outer  angles  abutting  against  the  short  and  thick  tympanic,  the  structure  of  which  has 
been  already  described. 
For  the  purposes  of  future  comparison  the  following  dimensions  (English,  and  French 
metrical)  are  added  to  those  mentioned  in  the  text : — 
Placodus  laticeps. 
From  the  first  premaxillary  to  the  last  pterygoid  tooth,  in  a straight  line,  in.  lines.  French. 
including  both  teeth  
..  4 
4 
0*11 
Extent  of  the  two  premaxillary  teeth  
..  0 
9 
0-018 
Extent  of  the  three  maxillary  teeth  
..  1 
8 
0-042 
Extent  of  the  two  palato-pterygoid  teeth 
..  2 
9 
0-07 
Breadth  between  the  hindmost  premaxillary  teeth,  including  those  teeth .... 
..  1 
5 
0-035 
Breadth  between  the  hindmost  maxillary  teeth,  including  those  teeth  ....... 
..  3 
1 
0-08 
Breadth  between  the  hindmost  pterygoid  teeth,  including  those  teeth 
..  3 
6 
0-09 
Breadth  of  trochlear  articular  surface  of  the  tympanic  
..  1 
3 
0-03 
From  the  fore-end  of  the  skuU  to  the  back  part  of  the  tympanic  
..  6 
10 
0-175 
From  the  fore-end  of  the  skull  to  the  fore-part  of  the  external  nostril 
..  0 
9 
0-018 
From  the  fore-end  of  the  skull  to  the  fore-part  of  the  orbit  
..  2 
2 
0-05 
From  the  fore-end  of  the  skull  to  the  fore-part  of  the  temporal  fossa  
..  3 
6 
0-09 
From  the  outer  margin  of  one  nostril  to  that  of  the  other  
..  1 
3 
0-03 
From  the  outer  margin  of  one  orbit  to  that  of  the  other 
..  2 
10 
0-073 
Breadth  of  the  skull  across  the  back  part  of  the  nostrils 
..  2 
4 
0-055 
Breadth  of  the  skull  across  the  back  part  of  the  orbits  
..  6 
3 
0-158 
(The  latter  admeasurement  gives  the  peculiarly  rapid  expansion  of  the  facial  part  of 
the  skull.) 
In  the  German  members  of  the  Trias,  from  which  remains  of  Placodus  have  been 
obtained,  other  well-marked  forms  of  the  Reptilian  class  have  been  discovered,  including 
