TEETH  OP  THE  PLACODUS  LATICEPS. 
183 
It  is  possible  that  some  of  the  singularly  modified  vertebrge  from  the  Muschelkalk, 
indicated  by  v.  Meter  under  the  generic  name  of  Tanistropheus  and  the  family  one  of 
Macrotraclielien&^ ^ may  belong  to  the  Placodus ; and  the  same  surmise  suggests  itself 
in  reference  to  some  of  the  limb-bones  from  the  Muschelkalk  that  cannot  be  assigned  to 
other  known  Saurian  genera.  I shall  be  amply  repaid  if  the  present  labour  which  has 
been  devoted  to  the  rectification  of  the  class  affinities  of  the  Placodus,  should  tend  to 
accelerate  the  acquisition  of  fossils  fui’ther  advancing  our  knowledge  of  its  Reptilian 
structiu’e. 
The  obvious  adaptation  of  the  dentition  of  Placodus  to  the  crushing  of  very  hard 
kinds  of  food,  its  close  analogy  to  the  dentition  of  certain  fishes  known  to  subsist  by 
breaking  the  shells  of  whelks  and  other  shell-clad  Mollusks,  and  the  characteristic 
abundance  of  fossil  shells  in  the  strata  to  which  the  remains  of  Placodus  are  peculiar, 
concur  in  producing  the  belief  that  the  species  of  this  genus  were  reptiles  frequenting 
the  sea-shore,  and  probably  good  swimmers  But  as,  at  present,  we  have  got  no 
further  than  the  head  and  teeth  in  the  reconstruction  of  this  mesozoic  form  of  conchi- 
vorous  Reptile,  I will  conclude  with  a remark  suggested  by  the  disposition  and  form  of 
the  teeth.  In  all  the  species,  under  the  rather  wide  range  of  specific  varieties  of  the 
dentition,  there  are  two  rows  of  the  crushing  teeth  in  the  upper  jaw,  and  only  one  row 
in  the  lower  jaw,  on  each  side  of  the  mouth ; and  the  lower  row  plays  upon  both  upper 
rows,  with  its  strongest  (middle)  line  of  force  directed  against  their  interspace.  Thus 
the  crushing  force  below  presses  upon  a part  between  the  two  planes  or  points  of  resist- 
ance above,  on  the  same  principle  as  that  by  which  we  break  a stick  across  the  knee ; 
only  here  the  fulcrum  is  at  the  intermediate  point,  the  moving  powers  at  the  two  parts 
grasped  by  the  hands.  It  is  ob\ious,  that  a portion  of  shell  pressed  between  two  opposite 
fiat  surfaces  might  resist  the  strongest  bite,  but  subjected  to  alternate  points  of  pressure 
its  fracture  would  be  facilitated. 
Description  of  the  Plates. 
PLATE  IX. 
Fig.  I.  Upper  surface  of  the  skull  of  Placodus  laticeps,  Owen. 
Fig.  2.  Side  -view  of  the  same  skull. 
Fig.  3.  Outside  view  of  part  of  the  lower  jaw  of  Placodus  bomhidens,  Owen. 
Fig.  4.  Inside  view  of  the  same  jaw  of  Placodus  bomhidens,  showing  a successional  tooth. 
Fig.  6.  Upper  view  of  the  same  jaw  of  Placodus  bomhidens. 
Fig.  6.  End  view  of  the  same  jaw  of  Placodus  bomhidens,  showing  the  formative  cavity 
of  the  successional  tooth,  d. 
* Die  Saurier  des  Muschelkalkes.  Eol.  1847-55,  pp.  42,  128.  t Hid.  p.  165. 
X The  NotJiosaurus  and  some  other  reptiles  of  the  Muschelkalk  seem  to  have  had  limbs  resembling  the 
paddles  of  the  Plesiosauri. — H.  v.  Metee,  op.  cit.  p.  33. 
