TEETH  OF  THE  PLACODHS  LATICEPS. 
181 
to  a different  species,  the  term  hombidens  would  best  express  what  seems  to  be  the  spe- 
cific peculiarity  in  the  shape  of  the  surface  of  the  grinding  teeth. 
The  third  portion  of  the  lower  jaw  (Plate  XI.  figs.  1,  2 and  3)  differs  from  both  of  the 
last  described,  and  from  that  of  Placodus  gigas^  in  the  absence  of  any  ledge  outside  the 
last  molar  tooth ; the  outer  alveolar  wall  of  which,  as  well  as  that  of  the  next  tooth, 
descends  sheer  upon  the  outer  surface  of  the  ramus. 
The  present  specimen  is  chiefly  instructive  for  the  extent  and  height  of  the  coronoid 
process,  29',  and  for  the  additional  evidence  it  yields  of  the  compound  structure  of  the 
mandibular  ramus. 
The  angular  element,  30,  is  preserved  for  an  extent  of  nearly  4 inches  behind  the  last 
molar  tooth : the  osseous  substance  is  plainly  traceable  in  the  matrix  midway  between 
the  tooth  and  the  broken  angle  of  the  jaw,  so  as  to  give  a vertical  extent  of  3^  inches 
to  this  part  of  the  ramus,  which  much  resembles  in  general  shape  what  is  called  the 
ascending  ramus  of  the  jaw  of  a carnivorous  quadruped.  At  its  upper  half  the  outer 
surface  is  flat,  the  lower  half  is  convex,  and  this  undulates  by  a slight  concavity  as  it 
extends  forward  into  the  again  convex  outer  surface  of  the  horizontal  ramus  supporting 
the  teeth.  The  fore-part  of  the  angular  piece,  30,  is  wedged  between  the  dentary,  32,  and 
splenial,  31 ; the  surangular,  29,  is  interposed  between  the  angular  and  the  upper  and 
back  part  of  the  dentaiy,  29-;  the  summit  of  the  coronoid  process  appears  to  be  contri- 
buted by  the  surangular.  The  joint-part  of  the  articular  element  seems  to  be  acci- 
dentally lost ; it  was  not  brought  out  in  the  extent  to  which  the  matrix  could  be 
safely  removed  from  the  present  specimen.  The  concave  inner  surface  of  the  angular 
element,  30,  and  the  suture  between  it  and  what  I take  to  be  the  fore-part  of  the  sur- 
angular element,  29,  are  well  shown.  The  inner  plate  of  the  splenial  has  been  removed : 
the  articular  surface  for  the  splenial  element,  and  a vascular  longitudinal  groove  which 
that  element  would  have  covered,  are  exposed  on  the  inside  of  the  dentary  part  of  the 
hoiizontal  ramus. 
In  this  part  of  the  jaw  two  teeth — the  last  (c)  and  penultimate  (b)  grinders — are  in 
place : both  are  of  subquadrate  form,  and  in  this  respect,  as  well  as  size,  resemble  the 
same  teeth  in  Placodus  pachygnathus^  from  which  species,  however,  the  present  much 
differs  in  the  shape  of  the  jaw. 
The  last  tooth  (c,  fig.  2)  has  been  but  recently  acquired ; the  rugae  being  obliterated 
from  only  a small  proportion  of  the  middle  of  the  crown,  where  there  is  a depression  of 
a somewhat  crucial  figure.  This  tooth  measures  1 inch  along  the  inner  border,  and  the 
same  across  the  crown ; the  outer  border  is  9-|-  lines  in  extent. 
The  next  tooth  (^,  fig.  2)  has  been  worn  smooth  and  flat ; its  outer  part  is  broken 
away;  the  extent  of  its  inner  border  is  10  lines;  its  breadth  11  lines.  Beneath  it  on 
the  surface  of  the  dentary,  left  uncovered  by  the  missing  splenial  piece,  the  inner  side 
of  the  enamelled  crown  of  a successional  tooth  {b\  fig.  3)  is  exposed.  The  inner  border 
of  the  crown  of  both  teeth  a little  overhangs  the  inner  wall  of  the  jaw. 
The  development  of  the  parts  of  this  compound  jaw  for  the  attachment  of  muscle 
