MR.  E.  T.  BENNETT  ON  THE  CHINCHILLIDiE. 
61 
excisure  of  the  posterior  margin  of  the  lower  jaw,  which  in  the  latter  animals  is  an  ex- 
tremely broad  and  rounded  bony  plate ;  and  many  other  equally  striking,  but  less  im- 
portant, modifications  in  the  bony  structure  of  the  head  alone.  In  the  possession  of 
perfect  clavicles,  and  the  substitution  of  a  large  open  suborbital  passage  for  the  perfo- 
rated bony  plate  found  below  the  orbit  in  the  Hares  and  Rabbits,  they  exhibit  a  more 
immediate  affinity  to  the  genus  Lagomys,  in  which  both  these  modifications  are  stated 
to  occur. 
Most  nearly  related  to  the  Chinchillidfs  on  the  opposite  side  is  the  genus  Capromys, 
Desm.,  {Isodon,  Say,)  in  which  the  molar  teeth  (also  four  in  number  on  each  side,  with 
flattened  crowns  and  surfaces  exactly  opposed  to  each  other,)  would  appear  on  a  super- 
ficial observation  to  be  subdivided  in  a  nearly  similar  manner.  On  looking  closer, 
however,  we  observe  that  the  processes  of  enamel  do  not,  in  Capromys,  entirely  traverse 
the  teeth,  but  form  alternate  indentations,  corresponding  with  external  sulci,  of  which 
two  are  seen  on  one  side  of  the  tooth,  and  one  only  on  the  other.  The  anterior  teeth 
are  also  more  elongated,  and  that  of  the  lower  jaw  projects  forwards  in  an  angular 
process,  which  has  on  its  inner  margin  a  third  denticular  fold  of  enamel.  The  cranium 
in  Capromys  is  very  narrow  and  much  elongated,  especially  in  its  frontal  and  parietal 
portions,  the  latter  having  a  slight  convexity  at  its  anterior  part ;  the  orbit  is  of  small 
capacity  ;  and  the  infra-orbital  foramen  is  large  and  open,  as  in  the  ChinchiUidee.  The 
number  of  the  ribs,  which  is  no  less  than  sixteen,  is  here  a  marked  peculiarity ;  and 
the  great  strength  of  the  limbs,  supported,  as  regards  the  anterior,  by  a  complete  cla- 
vicle, and  having  the  bones  of  the  fore-arm  nearly  equally  developed,  and  the  fibula 
strong  and  distinct,  affbrds  a  characteristic  difference,  connected  doubtless  with  the 
scandent  habits  of  the  animals. 
The  peculiarity  noticed  in  the  anterior  tooth  of  the  lower  jaw  in  Capromys  leads  us 
by  a  natural  transition  to  the  typical  Arvicolidce,  including  Arvicola,  Lemmus,  and  Fiber, 
in  which  only  the  three  anterior  molar  teeth  are  developed.  These  have  their  enamel 
so  folded  inwards  as  to  form  on  both  the  outer  and  inner  side  of  each  tooth  a  series  of 
distinct  triangles  alternating  with  each  other,  and  giving  rise  to  acute-angled  projections 
both  externally  and  internally.  In  the  genera  of  this  family  the  increasing  rounding  of 
the  anterior  surface  of  the  lower  incisors  renders  their  points,  when  obliquely  worn 
down,  either  rounded  or  acute,  instead  of  transversely  truncate,  as  in  the  preceding 
groups. 
Nearly  related  to  Arvicolida,  with  which  they  agree  in  general  appearance  and  in 
mode  of  life,  are  two  genera,  at  present  standing  in  some  degree  isolated,  Ctenodactylus^ 
and  Octodon^.  The  resemblance  in  the  form  of  the  molar  teeth  in  these  two  curious 
animals,  (both  of  which  have  been  very  recently  for  the  first  time  described,)  one  from 
Africa  and  the  other  from  Chih,  is  very  remarkable.  They  differ,  however,  in  number, 
the  latter  having  four  and  the  former  three  only  on  each  side  of  either  jaw ;  and,  be- 
'  Gray,  Spicilegia  Zoologica,  p.  10.  t.  10.  "  Bennett,  in  Proc.  Comm.  Sci.  Zool.  Soc,  Part  II.  p.  45. 
