58 
MR.  E.  T.  BENNETT  ON  THE  CHINCHILLIDiE. 
distinct  lamince,  except  in  the  anterior  tooth  on  each  side  of  the  lower  jaw,  is  the  same 
as  in  Lagotis ;  and  the  same  apparent  reversal  of  the  teeth  in  the  two  jaws  is  manifest. 
The  posterior  lamina  of  the  teeth  in  the  upper  jaw,  and  the  anterior  in  those  of  the 
lower,  is  also  the  smallest  of  the  three,  and  these  smaller  lamina  fall  short  of  the 
respective  margins  of  the  teeth,  nearly  as  in  Lagotis,  giving  rise  to  a  similar  anomaly 
in  the  grooving  of  the  inner  and  outer  surfaces  of  the  crowns.  But  instead  of  the 
regular  curve  of  the  two  larger  lamina  which  takes  place  in  Lagotis,  those  of  Chinchilla 
are  nearly  straight,  with  the  exception  of  a  sharp  turn  backwards  of  the  inner  extremity 
of  the  intermediate  lamina  in  the  upper  jaw,  and  a  slight  prolongation  forwards  of  the 
outer  extremity  of  the  same  lamina  in  the  lower.  Add  to  this  that  a  still  more  marked 
difference  occurs  in  the  anterior  tooth  in  the  lower  jaw,  which,  instead  of  being  com- 
posed of  three  distinct  lamina  as  in  Lagotis,  has  the  line  of  enamel  subdividing  its  two 
anterior  portions  so  abbreviated  as  to  extend  little  more  than  half  way  across  the  tooth  ; 
which  consequently  consists  of  only  two  distinct  lamella,  the  anterior  bilobate  inter- 
nally, but  with  its  osseous  substance  externally  continuous.  In  all  the  family  the 
lamina  of  the  teeth  are  directed  obliquely  backwards  and  inwards,  and  still  more 
obliquely  so  in  the  lower  than  in  the  upper  jaw. 
From  this  account  of  the  dentition  of  the  three  animals,  the  differences  between 
Lagostomus  and  the  other  two  will  be  at  once  obvious :  those  which  distinguish  Lagotis 
and  Chinchilla  will  be  better  observed  in  the  figures  than  they  can  be  conveyed  in 
words.  The  following,  however,  is  a  summary  of  the  more  remarkable  points  of  dis- 
crepancy between  them.  These  consist,  first,  in  the  curvature  of  the  anterior  lamina 
of  the  teeth  of  the  upper  jaw  in  Lagotis,  as  compared  with  the  straightness  of  the  same 
lamella  in  Chinchilla ;  and  in  the  curvature  of  the  middle  lamella  taking  place  more 
gradually,  and  not  in  the  sudden  manner  in  which  it  occurs  in  Chinchilla :  secondly,  in 
the  greater  extent  of  the  anterior  lamella  of  the  three  posterior  teeth  of  the  lower  jaw  in 
Lagotis,  as  compared  with  Chinchilla :  thirdly,  in  the  complete  disjunction  of  the  two 
anterior  lamina  of  the  anterior  tooth  of  the  lower  jaw  in  Lagotis,  while  in  Chinchilla  the 
enamel  advancing  between  them  from  within,  extends  on  the  surface  of  the  crown  but 
little  more  than  half  across  the  tooth,  and  thus  leaves  a  space  in  which  the  osseous 
portions  of  the  two  lamella  run  into  and  are  continuous  with  each  other. 
I  shall  conclude  with  the  technical  characters  of  the  three  genera  which  constitute 
the  family  of  Chinchillida,  and  with  a  few  observations  on  its  place  in  the  tribe  to  which 
it  belongs. 
Ordo  RODENTIA. 
Tribus  Herbivora,  F.  Cuv. 
Dentes  raolares  eradicati,  per  totam  vitam  pulpa  persistente  crescentes. 
Fam.  Chinchillida. 
Dentes  incisores  | ,  superiores  simplices ;  molares  jEj,  e  lamellis  osseis  binis  ternisve 
