54 
MR.  E,  T.  BENNETT  ON  THE  CHINCHILLIDiE. 
apparent  capacity  of  the  cranium,  and  present  externally,  in  consequence  of  their  mag- 
nitude and  the  tenuity  of  their  parietes,  the  appearance  of  three  large  vesicular  pro- 
tuberances on  each  side  of  the  cranium.  Of  these  the  superior,  placed  immediately 
mesiad  of  the  upper  margin  of  the  external  meatus,  (which  is  a  large  open  cavity,  pene- 
trating deeply  from  above  downwards,)  is  nearly  hemispherical :  the  posterior,  situated 
behind  the  meatus,  is  oblong,  with  its  long  diameter  from  above  downwards :  and  the 
inferior,  which  with  its  fellow  occupies  nearly  the  whole  inferior  surface  of  the  cranial 
cavity,  is  pyriform  with  its  long  diameter  from  before  backwards,  and  its  narrowest 
portion  pointing  backwards  and  outwards.  At  the  point  of  junction  of  the  posterior 
and  inferior  of  these  protuberances,  which  have  bat  a  slight  appearance  of  separation 
from  each  other,  the  styloid  process  passes  down,  closely  applied  and  firmly  anchylosed 
to  their  substance. 
In  Lagotis,  on  the  contrary,  the  tympanic  cells  have  little  increased  development, 
and  none  of  the  vesicular  appearance.  Those  of  the  upper  surface  of  the  cranium  are 
placed  at  some  distance  mesiad  of  the  margins  of  the  meatus  externi,  and  are  flat  and 
scarcely  distinguishable :  the  posterior  are  long,  narrow,  and  flattened  :  and  the  inferior 
bear  no  comparison  to  the  size  of  the  same  parts  in  Chinchilla,  although  resembling 
them  in  shape.  The  external  meatus  is  formed  nearly  in  the  same  manner ;  but  the 
styloid  process  is  free  from  any  attachment  to  the  outer  parietes  of  the  cells.  The  whole 
tympanic  apparatus  of  Lagotis  does  not  equal  one  third  of  its  proportional  size  in  CMn- 
chilla. 
In  both  animals  the  rami  of  the  lower  jaw  posterior  to  their  union  are  remarkably 
thick  and  strong,  and  its  angular  plates  thin  and  delicate :  the  coronoid  process  is  but 
little  developed,  and  that  of  the  angle  is  much  prolonged,  especially  in  Chinchilla.  It 
terminates  in  a  point,  between  which  and  the  condyle  there  occurs  a  broad,  deep,  semi- 
lunar excision.  The  condyle  is  small  and  longitudinal,  and  the  glenoid  cavity  super- 
ficial, admitting  of  great  freedom  of  motion  in  the  antero-posterior  direction. 
In  both  animals  the  number  of  cervical  vertebra  is,  as  usual,  seven ;  the  dorsal  and 
lumbar  are  together  nineteen  ;  but  unless  a  rib  on  each  side  has  been  lost  in  the  prepa- 
ration of  Lagotis,  (and  of  this  I  can  perceive  no  proof  in  the  existence  of  an  articular 
surface,)  its  dorsal  vertebrae  are  only  twelve,  while  in  Chinchilla  they  are  certainly 
thirteen.  Two  anchylosed  vertebra  form  the  sacrum  in  each  ;  but  the  number  of  caudal 
vertebra  differs,  Lagotis  having  twenty-seven,  and  Chinchilla  only  twenty-three.  In 
both  the  atlas  is  broadly  developed,  and  there  is  a  considerable  spinous  process  on  the 
dentata ;  but  scarcely  any  elevation  exists  at  this  part  on  the  other  cervical  vertebra. 
The  spinous  processes  of  the  dorsal  vertebra,  from  the  third  to  the  ninth  inclusive,  are 
much  elongated,  and  directed  backwards ;  the  tenth  has  the  same  direction.  Those  of 
the  lumbar  vertebra  are  directed  forwards,  and  are  remarkably  strong  and  conspicuous 
on  the  three  last,  as  well  as  on  the  two  sacral,  where  these  processes  resume  a  vertical 
direction.    The  caudal  vertebra,  with  the  exception  of  the  first  seven,  are  long  and 
