172 
PEOFESSOE  OWEN  ON  THE  SKULL  AND 
specific  differences  will  be  pointed  out  after  the  description  of  the  teeth  as  they  appear 
in  Placodus  laticeps. 
The  teeth  of  the  upper  jaw,  in  this,  as  in  the  other  known  species  of  Placodus.  con- 
sist of  an  external  or  maxillary  series  (Plate  X.  fig.  1,  a — e),  and  an  internal  or  palatal 
series,  ib./*  g.  The  maxillary  series  are  supported  in  a marginal  row  of  alveoli  by  the 
premaxillary  and  maxillary  bones : the  palatal  series  appear  to  be  implanted  in  the 
palatine  and  pteiygoid  bones. 
The  maxillo-premaxillary  teeth  are  five  in  number  on  each  side,  two,  a and  b,  im- 
planted in  the  premaxillary,  and  three,  c,  d and  e,  in  the  maxillary.  The  premaxillary 
teeth  are  subequal,  smaller  than  the  maxillary  teeth ; their  crowns  are  subhemispheric, 
that  of  the  first  being  4 lines,  that  of  the  second  3 lines  in  diameter : the  enamel  is  worn 
off  from  the  back  or  inner  side  of  the  crown,  showing  that  the  lower  jaw'  was  shorter 
than  the  upper  one.  On  the  inner  side  of  the  right  premaxillary  teeth  the  crown  of  a 
small  successional  tooth  has  begun  to  protrude  from  the  bone:  it  presents  rather 
strong  irregular  rugse  diverging  from  a small  central  pit : this  character  has  been  worn 
away  from  the  crowns  of  the  teeth  in  place.  The  premaxillary  teeth,  in  proportion  to 
their  breadth,  are  longer  or  project  further  from  their  sockets  than  the  other  teeth ; but 
none  of  them  present  the  bent,  pointed,  prehensile  character  of  the  incisive  tooth  figured 
in  tab.  70,  fig.  21  of  the  ‘ Recherches  sur  les  Poissons  Fossiles,’  and  referred  by  Agassiz 
to  Placodus  gigas. 
The  maxillary  part  of  the  marginal  series  is  not  continued  directly  from  the  premaxil- 
lary row,  but  begins  on  a plane  internal  to  it,  by  the  breadth  of  the  second  premaxillary 
tooth,  h.  From  this  point  the  three  maxillary  teeth  extend  outward  and  backward  in 
a line  parallel  with  that  formed  by  the  premaxillary  teeth. 
The  first  maxillary  tooth,  c,  has  a full  oval  crown,  4^  lines  by  4 in  diameter.  The 
second  maxillary  tooth,  d,  measures  5^  lines  by  4^  lines  in  diameter : the  longer  axis  in 
both  teeth  is  in  the  line  of  the  series.  The  third  maxillary  tooth,  e,  is  subcii’cular.  8 
lines  in  diameter,  on  the  right  side ; but  on  the  left  side  the  transverse  diameter  is  9 lines, 
the  other  diameter  the  same  as  the  left  tooth. 
The  palatal  series  begins  on  the  inner  side  of  this  tooth,  and  consists  of  two  teeth  on 
each  side. 
The  first  tooth,/,  apparently  developed  in  the  true  palatine  bone,  20,  has  a full  ellip- 
tical crown,  10  lines  by  8 lines:  the  second  tooth,  g,  developed  in  the  broad  pterygoid 
bone,  presents  a full  oval  shape,  1 inch  9 lines  by  1 inch  3 lines  in  diameter.  The 
longer  axis  of  both  teeth  is  in  the  line  of  the  series,  and  this  line  is  on  nearly  the  same 
parallel — from  within  outward  and  backward — as  the  premaxillary  and  maxillary  roAvs. 
The  last  large  tooth  is  slightly  hollowed,  by  attrition,  at  the  middle  of  the  grinding  sur- 
face ; the  primitive  radiating  striae  of  the  enamel  are  visible  only  round  the  margin  and 
sides  of  the  tooth. 
All  these  teeth  are  implanted  by  short  simple  bases  in  distinct  shalloAv  sockets,  accord- 
ing to  the  ‘ thecodont  ’ type  of  dentition  in  the  Lacertian  order.  EAidence  has  already 
