180 
PEOFESSOE  OWEX  OX  THE  SKULL  AXD 
The  three  grinding  teeth  are  relatively  much  smaller,  as  the  foUo't\’ing  dimensions 
show : — 
Vertical  diameter  of  the  jaw  behind  the  last  tooth  . 
PZ.  gigas. 
in.  lines. 
. I 3 
PZ.  horabidens. 
in.  lines. 
I 9 
Antero-posterior  extent  of  the  three  grinding  teeth  . 
. 2 9 
2 
2 
Antero-posterior  extent  of  the  last  grinding  tooth  . 
• 1 
0 
10 
Antero-posterior  extent  of  the  middle  grinding  tooth  . 
. 0 101 
0 
Transverse  extent  of  middle  grinding  tooth  .... 
. 0 14 
0 
10 
Antero-posterior  extent  of  first  grinding  tooth 
. 0 71 
0 
6 
Transverse  extent  of  first  grinding  tooth 
. 0 Ilf 
0 
9 
The  inner  half  of  the  last  tooth  of  Placodus  bomhidens  (Plate  IX.  fig.  5,  c)  is  broken 
away,  but  its  relatively  smaller  dimensions  in  the  remaining  part  are  decisive  against 
its  specific  identity  with  the  PI.  gigas. 
The  crown  of  the  penultimate  grinder  (figs.  3,  4 and  5,  b)  is  peculiar  for  its  great 
degree  of  convexity,  especially  in  the  transverse  direction.  The  median  transverse  fuiTow 
from  which  the  rugse  of  the  enamel  have  radiated  is  still  discernible  on  the  worn  cro^vn. 
as  are  the  rugee  themselves  on  the  inner  part  of  the  crown. 
Beneath  the  anterior  tooth,  a,  the  crown  of  its  successor,  is  exposed  in  figs.  4 and  6. 
the  enamelled  summit  of  which  is  complete ; it  is  of  the  same  size  as  the  tooth  it  is  about 
to  displace,  and  this  is  important  in  estimating  the  value  of  dental  characters  in  regard 
to  difference  of  size.  For,  had  the  present  fossil  belonged  to  a young  animal,  not  fully 
grown,  the  successional  teeth  would  probably  have  been  larger  than  their  predecessors ; 
or  if,  as  in  the  case  of  most  fishes,  growth  of  body  had  continued  throughout  life,  some 
indication  of  a maintenance  of  proportion  with  augmenting  general  bulk  might  have  been 
expected  in  the  successional  teeth.  I conclude,  therefore,  that  we  have  in  the  portion 
of  jaw  here  described,  a part  of  a full-grown  animal  of  its  species,  and  that  the  dental 
characters  exhibited  are  specific. 
The  mandible  begins  in  advance  of  the  first  grinding  tooth  to  bend  a little  outward, 
and  this  indicates  an  expansion  of  the  fore  or  symphysial  part  of  the  jaw,  analogous  to 
that  of  the  premaxillary  part  of  the  jaw  in  Placodus  Andriani*.  The  fore-part  of  the 
angular  element  of  the  jaw  appears  below  the  back  part  of  the  dentary,  to  which  it  arti- 
culates by  suture ; and  here  the  under  part  of  the  angular  shows  a sutural  surface  for  the 
splenial  element,  indicative  that  this  element  appeared  in  a slight  degree  upon  the  outer 
surface  of  the  ramus. 
All  these  indications  of  the  compound  structure  of  the  mandible  concur  with  the  cha- 
racters of  that  part  in  Nothosaurus  and  some  other  triassic  Saurians. 
It  is  very  probable  that  the  fossil  just  described  may  belong  to  the  species  called  Pla- 
codus Andriani.  Provisionally  it  has  been  entered  under  that  name  in  the  ‘ Catalogue 
of  the  British  Museum  Series  of  Fossils.’  Should  ulterior  acquisitions  prove  it  to  belong 
* Agassiz,  tom.  cit.  tab.  70.  fig.  8.  • 
