TEETH  OF  THE  PLACODUS  LATICEPS. 
179 
(figs.  6,  7,  a),  is  a successional  one,  not  having  risen  into  place,  and  with  the  crown  showing 
the  fine  radiating  irregular  rugae:  its  breadth  much  exceeds  its  length,  being  as  10^ 
lines  to  6^  lines,  and  the  latter  dimension  is  along  the  inner  border,  which  is  double 
the  extent  of  the  outer  rounded  border ; the  grinding  surface  is  rather  triangular  than 
quadrate.  In  Placodus  cjicjas  the  corresponding  tooth  does  not  present  such  a dispro- 
portion between  the  outer  and  inner  borders,  but  preserves  more  of  the  transversely 
extended  quadrate  form.  By  the  comparison  of  the  above  three  principal  crushing 
teeth  of  the  lower  jaw,  therefore,  the  fossil  under  consideration  is  specifically  distinct 
from  PI.  gigas.  It  may  belong  to  the  species  called  Placodus  Andriani ; but,  if  distinct, 
it  is  most  probable  that  the  three  close-set,  large,  broad  and  flat-crowned  teeth  were 
opposed  to  three  similarly  shaped  and  aggregate  teeth,  such  as  are  seen  in  Placodus 
gigas  and  Placodus  Andriani. 
In  the  specimen  (Plate  X.  figs.  6 and  7),  the  symphysis  (s,  s)  has  been  broken  across 
immediately  in  front  of  the  first  of  the  three  teeth : its  breadth  at  this  part  is  2 inches 
3 lines;  its  depth  is  10  lines.  The  rami  of  the  jaw,  after  they  become  free,  expand  in 
depth,  retaining  a breadth  or  thickness  of  I-^-  inch ; are  flattened  or  broadly  rounded 
below ; are  thus  of  peculiar  massiveness  and  strength,  adapted  to  the  support  of  the  large 
crushing  teeth ; and  the  rami  diverge  at  an  open  angle  which  relates  to  the  character- 
istic breadth  of  the  skull  in  the  Placodi.  Only  the  dentary  elements  are  preserved ; the 
alveolar  border  extends  4 lines  beyond  the  last  of  the  grinding  teeth,  forming  a flat  ledge 
of  that  breadth ; the  corresponding  projection  in  the  larger  mandible  of  Placodus  gigas 
is  of  about  half  the  breadth.  The  inner  border  of  the  last  tooth  almost  overhangs  the 
vertical  inner  wall  of  the  ramus.  The  symphysis  seems  to  have  developed  a low  median 
ridge  along  its  under  surface. 
It  is  possible  that  the  specimen  here  described  may  be  part  of  the  under  jaw  of  a 
Placodus  Andriani ; but,  as  the  two  other  mandibular  rami  next  to  be  described  have 
the  same  claims  to  such  relationship  from  the  size  and  form  of  the  grinding  teeth,  and 
one  of  them  an  additional  claim  through  a structure  of  the  jaw-bone,  I am  induced  to 
indicate  the  species  presenting  the  peculiarly  thick,  massive  proportions  of  the  mandi- 
bular rami  above  described,  as  Placodus  pachygnathus. 
The  specimen  (Plate  IX.  figs.  3,  4,  6 and  6),  is  the  middle  portion  of  the  left  ramus 
of  the  mandible,  with  the  last  three  large  grinding  teeth  in  situ:  it  consists  of  the 
dentary  element,  showing  the  same  angular  notch  in  the  vertically  extended  back  part 
which  may  be  observed  in  Nothosaurus  and  other  Muschelkalk  Saurians*.  The  upper 
border  of  the  dentary  in  NotliosaAirus  rises  behind  the  last  tooth  into  a low  analogue  of 
the  coronoid  process,  and  a similar  modification  exists  in  the  present  mandible,  and 
ob\'iously  to  a greater  extent,  but  the  summit  of  the  process  is  broken  off ; the  fore-part 
of  its  base  is  continued,  as  in  Placodus  gigas  and  PI.  pachygnathus.,  forward  upon  a ledge 
of  bone  outside  the  alveoli  of  the  last  two  teeth : the  breadth  of  this  ledge  is  equal  to  that 
in  PI.  gigas^. 
* H.  V.  Meter,  Die  Saurier  des  Muschelkalkes,  tab.  3.  fig.  2.  f Agassiz,  tom.  cit.  tab.  70.  fig.  16 
2 B 2' 
