TEETH  OF  THE  PLACODUS  LATICEPS. 
173 
been  given  of  their  being  subject  to  the  same  law  of  displacement  and  succession  as  in 
other  reptiles ; but  this  is  common  to  both  reptiles  and  fishes. 
The  total  number  of  teeth  in  place  in  the  upper  jaw  of  Placodus  latice'ps  is  four- 
teen. 
In  Placodus  rostmtiis  (Plate  XI.  fig.  4),  as  in  PI.  Milmteri,  PL  Andriani^  and 
PL  gigas,  the  internal  or  palatal  teeth  are  six  in  number,  three  on  each  side.  In  PL 
rostratus  and  PL  Milnsteri.,  the  first  two  palatal  teeth  are  subequal  and  much  smaller 
than  the  last  great  tooth : they  are  situated  on  the  inner  side  of  the  last  two  maxillary 
teeth  in  PL  Mwisteri ; but  are  both  posterior  to  the  maxillary  series  in  PL  rostratus. 
In  Placodus  gigas  and  PI.  Andriani.,  the  palatal  teeth  (called  ‘ vomerine  ’ by  Mtix- 
STER  and  ‘ internal  ’ by  Agassiz),  three  in  number  on  each  side,  are  all  of  large  size, 
shghtly  increasing  from  before  backward ; they  are  situated  close  together,  forming  on 
each  side  a series  a little  curved  with  the  convexity  outward,  and  the  interspace 
between  the  two  series  is  very  narrow.  The  first  tooth  is  triangular,  the  second  and  third 
are  quadrangular ; each  uuth  the  angles  rounded,  and  the  transverse  diameter  exceeding 
the  fore  and  aft  or  longitudinal  one.  The  maxillary  teeth  are  much  smaller  than  the 
palatal  ones ; have  a rounded  or  subquadrate  crown ; are  four  in  number ; and  of  sub- 
equal dimensions : the  series  is  somewhat  curved,  with  the  convexity  next  the  outer 
alveolar  margin ; commencing  a little  in  advance  of  the  first  palatal  tooth,  and  ter- 
minating opposite  the  interspace  between  the  penultimate  and  last  palatal  teeth.  The 
premaxillary  teeth,  three  in  number  on  each  side,  are  more  remote  and  distinct  from 
the  maxillary  teeth  than  in  Placodus  rostratus  and  PL  laticeps.  They  form  an  almost 
transverse  series  at  the  fore-part  of  the  upper  jaw;  and  their  crowns  are  more  elongated 
and  conical  than  in  PL  laticeps ; the  prehensile  power  of  the  prolonged  premaxillary 
part  of  the  jaw  being  obviously  greater  in  Placodus  gigas  than  in  PL  laticeps  or  PL 
rostratus.  The  size  of  the  last  tooth  in  PL  laticejps  surpasses  that  of  any  of  the  teeth  in 
the  previously  discovered  species.  In  proportion  to  the  entire  skull,  it  is  the  largest 
grinding  tooth  in  the  animal  kingdom,  that  of  the  Elephant  itself  not  excepted. 
The  specific  distinction  of  the  fossil  exhibiting  the  true  reptilian  characters  of  the 
genus,  and  for  which  I propose  the  name  Placodus  laticeps.,  is  satisfactorily  established 
by  the  dental  distinctions  pointed  out  in  the  foregoing  comparisons.  By  some  system- 
atists  it  may  even  be  deemed  requisite  to  separate  generically  the  Placodi  with  two  teeth, 
from  those  with  three  teeth,  in  each  palatal  series:  but  Placodus  rostratus  offers  a 
transitional  condition  in  the  small  relative  size  of  the  first  two  palatal  teeth,  and  in  the 
rounded  form  of  all  the  teeth,  from  PL  Andriani  to  PL  laticeps ; and  I do  not  perceive 
any  advantage  that  would  compensate  for  an  additional  generic  term  in  the  present 
state  of  knowledge  of  the  Placodont  reptiles. 
The  chief  distinctive  feature  of  the  cranium  of  Placodus  laticeps,  is  the  great  pro- 
portional size  of  the  temporal  fossae  and  wide  span  of  the  zygomatic  arches.  The  great 
muscular  force  applied  to  work  the  jaws  and  their  crushing  machinery  adequately  is 
indicated  by  this  modification.  The  breadth  of  the  skull  at  this  part  equals,  at  least. 
