TEETH  OF  THE  PLACODES  LATICEPS. 
177 
The  dentine  presents,  under  an  adequate  magnifying  power,  extremely  fine,  numerous 
and  close-set  dentinal  tubuli,  without  admixture  of  medullary  canals : they  radiate  from 
the  mde  and  shallow  pulp-cavity,  with  a corresponding  feeble  divergence,  at  right  angles 
to  the  outer  sm-face  of  the  tooth.  The  tubuli  are,  at  first,  straight ; but  show  two  slight 
primary  bends  near  the  periphery  of  the  tooth.  They  present  a diameter  of  2 0,0'6'bl'^ 
of  an  inch,  with  interspaces  varying  between  two  and  three  times  that  diameter;  they 
di\ide  once  or  twice  in  their  course : a few  secondary  branches  were  discernible  near  the 
periphery  of  the  dentine.  The  difierence  between  this  dentinal  structure  and  that  of  true 
pycnodont  fishes  is  seen  in  the  larger  relative  size  and  much  closer  arrangement  of  the 
dentinal  tubuli  in  those  fishes,  and  their  apparent  composition  of  a closely  twisted  bundle 
of  smaller  tubes,  owing  to  the  oblique  direction  and  number  of  the  branches  sent  off 
into  the  intertubular  substance  *.  The  terminal  branches  into  which  the  tubuli  resolve 
themselves  penetrate,  in  Pycnodonts,  the  clear  substance  which  is  analogous  to  enamel, 
but  is  a continuation  and  slight  modification  of  the  intertubular  or  basement  tissue  of 
the  entire  tooth.  In  Placodus  the  layer  of  enamel  (fig.  1 e)  is  as  distinct  as  in  the 
Monitor  or  Crocodile.  It  is  a very  dense  and  compact  substance,  in  which  a structure 
of  fibres,  vertical  to  the  surface,  is  but  faintly  discernible  near  the  dentine. 
The  osseous  tissue  oi Placodus  (fig.  2)  exhibits  concentric 
layers  around  the  Haversian  canals,  the  area  of  one  of 
which  is  shown  in  fig.  2.  The  lacunae  or  bone-cells,  of  a 
size  and  shape  closely  resembling  those  of  the  Plesiosaurus 
and  Crocodile,  have  diverging  tubuli  larger  than  in  Mam- 
mals, with  a more  wavy  course  and  fewer  ramifications. 
The  tubuh  are  much  more  numerous  in  the  best-preserved 
and  prepared  slices  of  bone  than  in  the  spedmen  figured ; 
which  shows,  however,  the  characteristic  reptilian  size  and 
irregular  or  subangular  contour  of  the  bone-cell. 
We  cannot  contemplate  the  extreme  and  peculiar  modification  of  form  of  the  teeth  in 
the  genus  Placodus  without  a recognition  of  their  adaptation  to  the  pounding  and  crush- 
ing of  hard  substances,  and  a suspicion  that  the  association  of  the  fossils  with  shell-clad 
mollusks  in  such  multitudes  as  to  have  suggested  special  denominations  to  the  strata 
containing  Placodus  {e.  g.  Muschelkalk,  Terebratulitenkalk,  &c.),  is  indicative  of  the 
class  whence  the  Placodi  derived  their  chief  subsistence. 
No  doubt,  the  most  numerous  examples  of  similarly-shaped  teeth  for  a like  purpose 
are  afforded  by  the  class  of  Fishes,  as,  e.  g.,  by  the  extinct  Pycnodonts,  and  by  the  Wolf- 
fish {Anarrhichas  lupus)  and  the  Cestracion  of  the  existing  seas.  But  the  Reptilian 
class  is  not  without  its  instances  at  the  present  day  of  teeth  shaped  like  paving-stones, 
of  which  certain  Australian  lizards  exhibit  this  peculiarity  in  so  marked  a degree  that 
the  generic  name  Cyclodus  has  been  invented  to  express  that  peculiarity  f.  Amongst 
extinct  Reptiles,  also,  a species  of  lizard  from  the  tertiary  deposits  of  the  Limagne,  in 
* See  ‘ Odontography,’  pi.  33.  t Hid.  pi.  66.  fig.  7. 
2 B 
Osseous  tissue  of  jaw, 
Placodus  laticeps. 
MDCCCLVIII. 
