176 
PEOFESSOE  O^VEX  ON  THE  SKULL  AND 
the  labyrinthodont  Reptilia,  with  genera  of  lacertian  affinities,  for  the  knowledge  of 
which  we  are  chiefly  indebted  to  M.  Heeman  v,  Meyee*. 
Nothosaurus,  Simosaurus  and  Fistosaurus^  present  the  same  e-^idences  of  lacertian 
affinities  in  the  division  of  the  nostrils  by  the  median  extension  of  the  premaxillary 
backward  to  the  nasals,  the  same  thecodont  dentition,  and  the  same  cu’cumsciiption  of 
the  orbits  and  temporal  fossae  as  in  Placodus : there  is  also  a general  family  likeness  in 
the  upward  aspect  of  these  apertures,  accompanjdng  an  extreme  depression  of  the  skull. 
The  muzzle,  though  varying  greatly  in  length  in  these  genera,  presents  the  same  obtuse- 
ness ; and  the  alveolar  border  of  the  jaws  the  same  smooth  outward  convexity  which 
we  observe  in  the  Placodus.  The  peculiar  confluence  of  the  elements  of  the  upper  and 
lower  zygomatic  arches,  i.  e.  of  postfrontal  and  malar,  forming  the  broad  wall  of  bone 
behind  the  orbit,  is  contmued  still  further  backward  in  Simosaurus^.  In  Pistosaurus, 
the  elongated  postfrontal,  malar  and  squamosal  are  united  together  in  one  deep  zygo- 
matic arch,  which  has  the  mastoid  and  tympanic  for  its  hinder  abutment.  The  lower- 
articular  surface  of  the  tympanic  bone  presents  the  same  trochlear  form  in  Pistosaurus 
and  Simosaurus  as  in  Placodus^. 
The  dentition  of  the  triassic  Saurians  described  by  v.  Meyee, 
although,  like  Placodus.^  thecodont  in  respect  of  implanta- 
tion, is  of  the  ordinary  lacertian  or  crocodilian  type  in 
respect  of  form,  the  crown  of  every  tooth  being  long  and 
sharp-pointed,  adapted  to  the  prehension  of  fishes  or  other- 
active  vertebrate  animals : moreover,  they  are  developed,  as 
in  Crocodiles  and  Enaliosarrrs,  exclusively  in  the  premaxillary, 
maxillary  and  mandibular  bones,  the  palatal  bones  being- 
edentulous.  Besides,  however,  the  instances  in  modern  lacer- 
tian genera  of  palatal  teeth,  the  triassic  Labyrinthodonts  also  '' 
exhibit  the  same  superaddition,  and  the  unequal  magnitude 
of  some  of  these  teeth  may  be  noticed  as  preserrting  an  arra- 
logy  -with  the  dentition  of  Placodus. 
The  structure  of  the  teeth,  of  Placodus,  like  that  in  Notho- 
saurus  and  Simosaurus,  conforms  to  the  ordinary  crocodilian 
and  lacertian  type.  The  dentine  (fig.  1,  d)  is  of  the  hard 
unvascular  kind,  and  the  crown  of  the  tooth  is  covered  by  a 
moderately  thick  well-defined  layer  of  true  enamel  (fig.  1,  e). 
This  enamel,  in  the  newly-formed  tooth,  presents  numerous, 
close-set,  fine  irregular  striae  or  rugae,  radiating  from  a central 
groove  or  pit  on  the  summit:  the  teeth  are  subject  to  the 
same  succession  and  displacement  as  in  the  Reptilia  generally. 
* Die  Saurier  des  Muschelkalkes.  Eol.  1847-1855. 
t Ib.  tab.  20.  fig.  1,  and  tab.  65.  fig.  2. 
r.  at.  size. 
Dentine  and  Enamel, 
Placodus  laticeps. 
t Tab.  21.  fig.  3. 
