178 
PROrESSOE  OWEX  OX  THE  SKHEL  AXH 
France,  presents  round  obtuse  teeth,  of  which  the  last,  in  the  lower  jaw,  is  suddenly  and 
considerably  larger  than  the  rest 
Mandible,  or  loioer  jaw. — Count  Muxstee  obtained  from  the  Muschelkalk  of  Bap’euth 
a portion  of  one  ramus  of  a lower  jaw  with  three  teeth  in  place,  which  M.  Agassiz 
has  figured  in  tab.  70,  figs.  15  and  16,  of  his  great  work  on  Fossil  Fishes f;  referring  it 
to  Placodus  gig  as.  From  this  specimen  M.  Agassiz  concludes  that  there  was  but  one 
row  of  teeth  in  each  ramus  of  the  jaw  J.  The  three  teeth  in  place  are  of  large  size 
and  subquadrate  form : a fourth  smaller  tooth  appears  on  the  smTace  of  the  bone,  at  a 
lower  level.  This  M.  Agassiz  considers  an  incisor;  but  certain  appearances  in  the 
specimens,  presently  to  be  described,  Fad  me  to  regard  it  as  a successional  anterior 
molar,  exposed  in  its  formative  cavity  by  fracture  or  abrasion  of  the  bone  at  that  part. 
M.  Agassiz,  indeed,  remarks  on  the  peculiarity  of  its  implantation  in  the  outer  border 
of  the  jaw§.  Its  crown  is  obtuse,  and  of  half  the  size  of  the  foremost  of  the  thi-ee  teeth 
in  place. 
There  is  no  character  adduced  in  the  text  or  plate  in  proof  of  the  piscine  affinities  of 
the  above  fossil  jaw. 
Four  specimens  of  portions  of  lower  jaw  form  part  of  the  collection  of  Muschelkalk 
fossils  from  Bayreuth,  in  addition  to  the  cranium  of  the  Placodus  laticeps,  now  acquu'ed 
by  the  British  Museum. 
The  first  of  these  mandibular  specimens  (Plate  X.  figs.  6 and  7),  includes  part  of  the 
left  ramus  and  a smaller  portion  of  the  right  ramus  united  by  the  hinder  portion  of  the 
symphysis,  s,  s.  In  the  left  ramus  the  first  (a)  and  third  (c)  teeth  are  preserved,  in  the 
right  ramus  the  first  (a)  and  second  (b)  of  the  three  large  subquadrate  teeth  which  are 
figured  in  the  mandibular  fossil  described  by  M.  Agassiz. 
These  teeth  are  somewhat  smaller  in  the  present  specimen  and  are  differently  shaped. 
The  grinding  surface  of  the  last  (fig.  7 c)  is  almost  a complete  square  with  the  angles 
rounded  off,  measuring  1 inch  in  length,  and  1 inch  ^ a line  in  breadth ; the  inner  border 
is  one  line  longer  than  the  outer  border.  In  the  Placodus  gigas  the  corresponding  tooth 
is  I inch  2^  lines  in  length,  and  1 inch  3 lines  in  breadth : the  inner  border  is  4 hues 
longer  than  the  outer  one,  and  the  angles  are  more  rounded  off  than  in  the  present 
specimen.  The  second  (penultimate)  tooth  in  the  present  specimen  (fig.  7,  b)  resembles 
in  shape  the  last : the  length  and  breadth  are  each  II  lines.  In  Placodus  gigas  the 
corresponding  tooth  is  notably  broader  than  it  is  long,  and  the  inner  border  is  longer 
than  the  outer  one.  The  first  tooth  on  each  side,  in  the  specimen  under  description 
* GtErvais,  Zoologie  et  Paleontologie  Erancaises,  pi.  64,  figs.  5-7. 
t Eeclierches  sur  les  Poissons  Eossiles,  4to.  tom.  ii. 
X “ II  est  evident  d’apres  cela,  que  chaque  branche  de  la  luaclioire  u’avait  qu’mie  raugee  de  larges 
molaires,  et  que  les  rangees  externes  inanquaient  completement.” — Eeclierches  sur  les  Poissons  Eossiles,  4tn. 
tom.  ii.  p.  219. 
§ “ Ce  que  est  remarquable,  c’est  que  cette  dent  soit  implantee  en  quelque  sorte  au  horde  exterue  de  la 
machoire,”  ib.  p.  219. 
