174 
PEOFESSOE  OWEX  ON  THE  SKULL  AND 
the  length,  and  imparts  a triangular  form  to  the  whole,  viewed  from  above  or  below. 
The  name  laticejps  was  suggested  by  this  character ; but  it  would  apply  almost  as  well  to 
Placodus  rostratus  and  PI.  Miinsteri.  In  PL  Andriani  and  PI.  gigas,  the  length  of  the 
skull  plainly  exceeds  the  breadth. 
The  apex  of  the  triangle  is  formed  by  the  premaxillary  (Plate  IX.  fig.  1,  22) ; it  is 
rounded  off:  the  outer  surface  of  the  premaxillary  is  smooth,  and  the  convex  border 
projects  a little  way  beyond  the  alveoli  of  the  teeth.  It  is  a single  bone  sending  up- 
ward and  backward  a median  process  which  meets  and  articulates  ^^fith  the  nasals  15), 
and  so  forms  the  partition  between  the  external  nostrils. 
These  orifices  (Plate  IX.  n,  figs.  1 and  2)  are  of  an  oval  form  with  the  small  end  forward, 
rather  sharply  defined  above,  but  with  the  lower  border  rounding  ofi“  into  the  floor  of 
the  passage  below ; about  9 lines  by  6 lures  in  the  two  diameters.  The  suture  between 
the  premaxillary  arrd  maxillary  begins  about  the  middle  of  the  lower  border  of  the 
nostril ; that  between  the  premaxillary  and  nasal  is  behirrd  the  middle  of  the  upper- 
border  : the  hinder  border  is  formed  by  the  nasal  above,  by  the  maxillary  below,  and  by 
a bar  of  borre  half  an  inch  irr  breadth,  which  divides  the  nasal  from  the  orbital  cavity ; 
whether  any  or  what  proportiorr  of  a lacrymal  bone  enters  into  the  formation  of  this  bar, 
the  sutural  traces  are  too  obscure  to  enable  me  to  determine.  The  alveolar  border  of 
the  rrraxillary  {ib.  fig.  2, 21),  corrtaiiring  the  three  teeth,  c,  d,  e,  forms  a slight  convex  ciuve 
downward ; then  slightly  rises  and  bends  rapidly  outward  to  pass  into  the  malar  portion 
of  the  zygomatic  arch,  26.  The  orbit  (Plate  IX.  0,  figs.  1 and  2)  is  subcii’cular,  14  lines 
in  longitudinal,  by  12  lines  in  vertical,  diameter : like  the  nostril,  the  upper  boundary 
is  sharply  defined ; the  lower  border  rounds  in  to  the  floor  of  the  ca\ity ; the  fL’ont 
border  shows  a middle  prominence,  low  and  broad,  which  contracts  as  it  extends  back 
upon  the  floor  of  the  orbit. 
The  interorbital  space  on  the  summit  of  the  cranium  is  slightly  convex  both  across 
and  lengthAvise ; its  breadth  is  7 lines,  a little  increasing  backward. 
The  bony  wall  of  the  face  below  the  orbit  expands  rapidly  and  spreads  outward  as  it 
extends  backward,  dividing  about  1^  inch  behind  the  orbit  into  the  upper  and  lower 
zygomata.  It  is  by  analogy  that  I conclude  these  zygomata,  Avhich  are  peculiar  to  certain 
Sauria  and  a few  birds,  to  be  composed — the  upper  one  of  the  conjoined  postfrontal  and 
mastoid,  the  lower  one  of  the  malar  and  squamosal  elements.  Both  zygomata,  in 
Placodus,  arch  outward  with  a span  peculiar  for  its  extent  and  convexity,  the  lower 
arch  curving  outward  by  more  than  its  own  breadth  beyond  the  upper  one.  This  arch 
is  about  half  an  inch  in  breadth,  its  flat  surface  looking  almost  dfrectly  upAA  ard ; the 
margins  turned  inward  and  outward : the  former  is  sharply  defined.  The  lower  or 
proper  zygoma  was  evidently  of  greater  breadth  than  appears  in  the  specimen,  for  its 
outer  border  has  been  broken  aAvay : had  it  been  entire,  the  breadth  of  this  remarkable 
skull  would  have  been  even  greater  than  it  noAv  is.  The  temporal  fossa  {ih.  fig.  1,  t),  cir- 
cumscribed by  the  upper  zygoma  externally,  and  by  the  proper  cranial  paiietes  internally, 
is  an  ellipse,  3 inches  in  the  long  diameter,  which  is  in  the  axis  of  the  skull,  and  2 inches 
