MR.  E.  T.  BENNETT  ON  THE  CHINCHILLID^. 
57 
ticulars  with  Lagotis,  and  differing  from  Chinchilla.  The  sacral  vertebrae  are  stated  to 
be  three  in  number  ;  but  in  the  plate,  two  only  appear  to  be  united  to  the  ossa  ilii  by 
the  sacro-iliac  symphysis,  as  in  the  other  known  animals  of  the  family.  Making  this 
allowance,  the  number  of  caudal  vertebra  is  twenty-one.  The  anterior  extremities  have 
little  to  distinguish  them  from  those  of  Lagotis,  with  which  they  exactly  correspond  in 
the  number  of  the  toes  :  as  in  Lagotis,  there  is  no  vestige  of  a  thumb.  The  posterior 
limbs  bear  the  same  proportion  to  the  anterior,  being  just  double  their  length  ;  but  the 
number  of  the  metatarsal  bones,  and  consequently  of  the  toes,  is  only  three,  and  the 
claws,  especially  the  middle  one,  are  much  larger,  stronger,  and  more  produced.  In 
this  particular  the  figures  given  by  Mr.  Brookes  are  defective,  as  exhibiting  the  claws 
far  smaller  and  more  curved  than  is  natural. 
I  now  come  to  the  consideration  of  the  teeth,  which  I  have  purposely  reserved  to  the 
last.  With  a  general  agreement  in  number  and  composition,  these  important  organs 
offer,  in  the  three  animals  under  consideration,  differences  so  essential  as  to  justify  of 
themselves,  but  more  especially  when  considered  in  connexion  with  the  striking  modi- 
fications in  the  form  of  the  crania  and  in  the  organs  of  locomotion,  the  formation  of  a 
distinct  genus  for  the  reception  of  each.  In  all,  the  incisor  teeth  have  the  number  and 
form  which  are  common  to  nearly  the  whole  of  the  order :  they  are  two  in  each  jaw, 
chisel-shaped  at  the  apex,  and  those  of  the  upper  jaw  have  their  exserted  portion  nearly 
vertical,  while  those  of  the  lower  pass  obliquely  forwards  and  upwards.  The  diastematic 
space  between  them  and  the  molars  is  considerable ;  and  the  latter  are  four  in  number 
on  each  side  of  both  jaws.  They  are  all  constructed  nearly  upon  the  same  model, 
having  no  distinct  roots,  and  being  each  composed  of  either  two  or  three  parallel, 
ribband-like  lamincB  of  osseous  matter,  each  lamina  surrounded  by  its  own  proper  coat  of 
enamel,  and  united  to  its  fellow  by  an  intervening  cortical  substance.  In  Lagostomus 
the  lamincB  are  two  in  number  in  each  tooth,  with  the  exception  of  the  hinder  one  of 
the  upper  jaw,  which  has  a  third  but  smaller  lamina  superadded  posteriorly ;  and  the 
lamina  on  the  worn  surfaces  of  the  teeth  are  perfectly  straight,  and  nearly  equal.  In 
Lagotis  the  number  of  lamina  in  each  tooth  is  increased  to  three ;  the  teeth  of  the 
upper  jaw  have  the  posterior,  and  those  of  the  lower  the  anterior,  lamina  smaller  than 
the  others  ;  and  these  smaller  lamina  do  not,  in  the  former  case,  advance  to  the  inner, 
or  in  the  latter  case  to  the  outer  margin  of  their  respective  teeth.  In  consequence  of 
this  arrangement  the  teeth  of  the  lower  jaw  exhibit  an  appearance  in  some  degree  the 
reverse  of  those  of  the  upper,  and  this  effect  is  still  further  heightened  by  the  latter 
having  its  posterior,  and  the  former  its  anterior,  tooth  prolonged  into  a  triangular 
shape,  while  all  the  rest  are  square.  The  two  larger  lamina  form  on  the  worn  surfaces 
of  the  teeth  regular  curves  with  the  convexity  directed  forwards  in  the  upper  jaw  and 
backwards  in  the  lower ;  and  the  crowns  exhibit  in  the  former  two  grooves  externally 
and  one  internally,  marking  the  line  of  union  of  the  separate  lamella,  while  in  the  latter 
the  grooves  are,  like  the  lamina  themselves,  reversed.    In  Chinchilla  the  number  of 
VOL.  I.  I 
