TEETH  OE  THE  PLACODUS  LATICEPS. 
171 
^luschelkalk  near  Bayi’euth.  One  of  these  included  a larger  proportion  of  the  cranial 
structure  than  in  any  previously  known  specimen : it  also  exhibited  the  upper  surface, 
which  in  Count  Munster’s  specimen  of  Placodus  rostratus  was  buried  in  the  matrix, 
and  this  surface  in  the  fossil  in  question,  now  acquired  for  the  Geological  Department 
of  the  British  Museum  (Plates  IX.  and  X.  fig.  1),  showed  the  character,  viz.  the  external 
nostrils  (w),  so  surrounded  as  to  be  decisive  of  the  genus  belonging  to  an  air-breathing 
class  of  the  Vertebrate  series.  But,  besides  the  external  bony  nostrils,  the  specimen 
presented  the  orbits,  ib.  o,  with  a similar  continuous  border  of  bone,  together  with  one 
of  the  temporal  fossse,  C bounded  externally  by  an  arch  of  bone  continued  from  the  post- 
frontal, 12,  to  the  mastoid,  s ; below  which  was  the  true  zygomatic  arch  {ib.  fig.  2)  formed 
by  the  malar,  26,  and  squamosal,  27,  elements,  extending  from  the  upper  maxillary,  21,  to 
abut  against  a short  thick  but  vertically  descending  tympanic  bone,  23.  The  articular 
surface  for  the  lower  jaw,  convex  from  behind  forward,  was  concave  transversely  at  the 
middle  part  between  two  convexities;  this  strong  trochlear  form  of  joint  being  unknown 
in  any  recent  or  fossil  fish'.  The  bony  palate  (Plate  X.  fig.  1)  showed  the  same 
absence  of  median  vomerine  teeth  as  in  Count  Munster’s  Placodus  rostratus  above 
referred  to ; and  it  also  showed  a broad  pterygoid  plate,  24,  abutting  against  the  antero- 
internal  side  of  the  fixed  tympanic,  28.  The  sum,  therefore,  of  the  characters  presented 
by  the  skull,  as,  e.  g.  the  nostrils,  divided  by  an  ascending  process  of  the  premaxillary,  22, 
and  bounded  by  that  bone,  the  maxillaries,  21,  and  nasals,  15 ; the  size,  shape,  and  sur- 
rounding of  the  orbits ; the  magnitude  of  the  temporal  fossse,  with  their  complete  double 
zygomatic  outer  arches,  the  condition  of  the  single  tympanic  bone,  and  the  structure  of 
the  bony  palate,  left  no  hesitation  as  to  the  reference  of  the  genus  Placodus  to  the 
class  Reptilia,  and  herein  with  nearest  affinities  to  the  Lacertian  order,  and  more  espe- 
cially wdth  that  modification  thereof  exemplified  by  the  extinct  genus  Simosaurus.,  from 
the  Muschelkalk*. 
The  generic  characters  of  the  fossil  were  unequivocally  shown  by  the  shape  and  rela- 
tive size  of  the  crowns  of  the  teeth,  covered,  as  in  the  previously  figured  specimens,  by  a 
black  enamel,  contrasting  strongly  with  the  light  grey  colour  of  the  fossil  bone  support- 
ing them.  Two  slight  rectifications  of  the  generic  character  originally  proposed  by 
M.  Agassiz  are  necessitated  by  the  subsequent  accession  of  different  species.  The  teeth 
may  present  a rounded  contour,  which  is  the  case  in  all  those  of  the  specimens  here 
described ; and  the  surface  of  the  crown  becomes  quite  smooth  only  by  attrition,  the 
enamel  of  the  newly-risen  tooth  presenting  fine  striae  diverging  from  a central  point  or 
fissure.  The  character  on  which  Placodus  impressus,  Agassiz,  is  differentiated,  “ une 
impression  ou  une  sorte  de  sillon  longitudinal  qui  se  voit  au  milieu  de  la  couronne,”  is 
one  common  to  the  newly-formed  crushing  teeth  of  all  Placodi. 
The  species  which  the  specimen  under  description  most  nearly  resembles  in  the 
shape,  proportions  and  arrangement  of  the  teeth,  is  Placodus  rostratus  of  MtiNSTERf : the 
* H.  V.  Mevee,  Die  Saurier  des  Muschelkalkes,  tab.  20,  Simosaurus  Ougliehni. 
t Beytrage,  loc.  cit.  Taf.  xv.  fig.  1 a (Plate  XI.  fig.  4). 
2 A 2 
I 
