62 
MR.  E.  T.  BENNETT  ON  THE  CHINCHILLID^. 
sides  other  minute  differences  in  these  organs,  exhibit  considerable  discrepancy  in  the 
form  of  the  skull  and  in  various  particulars  of  their  organization,  into  which  I  may 
perhaps  enter  more  fully  on  a  future  occasion. 
At  no  great  distance  from  these,  if  we  are  to  judge  alone  by  the  characters  of  its 
molar  teeth  as  figured  by  M.  F.  Cuvier,  is  placed  the  genus  Helamys,  of  which  unfor- 
tunately I  possess  neither  skull  nor  skeleton  for  comparison.  As  far,  however,  as  the 
stuffed  skin  can  be  relied  oil  for  such  a  purpose,  it  seems  in  many  of  its  characters  to 
approach  Lagostomus ;  while  the  differences  in  the  relative  proportion  of  its  limbs,  in 
the  elongated  claws  of  its  anterior  extremities,  in  the  character  of  its  fur,  and  above  all 
in  the  structure  of  its  teeth,  forbid  a  close  approximation.  With  no  better  materials 
than  I  can  at  present  refer  to,  it  would  be  hazardous  to  attempt  to  assign  its  true 
position. 
Equally  anomalous  appear,  in  the  present  state  of  our  knowledge  of  the  tribe,  two 
North  American  genera,  first  established  by  Dr.  Rafinesque,  Geomys  and  Diplostoma ; 
with  which  must  be  associated  Aplodontia  of  Dr.  Richardson.  The  teeth  of  Geomys  and 
Aplodontia,  as  figured  by  the  last-named  zoologist,  are  more  simple  in  their  structure 
than  those  of  any  other  genus  of  Herbivorous  Rodentia ;  and,  in  so  far,  they  seem  to 
approach  the  groups  of  which  I  have  just  been  speaking.  But  the  discrepancies  in 
other  respects  are  so  considerable,  that  further  information  must  still  be  considered 
necessary  to  determine  their  real  affinities. 
To  the  ArvicolidcB  succeed  the  Caviidce,  comprehending  Cavia,  Kerodon,  and  Doli- 
chotis  (Cavia  Patachonica,  Shaw)  ;  in  which  the  dental  triangles  are  more  distinct  and 
elongated,  and  form  but  one  series  instead  of  two,  their  acute  angles  projecting  ex- 
ternally in  the  lower  jaw  and  internally  in  the  upper:  they  are  also  fewer  in  number. 
The  number  of  the  molar  teeth  is  four  ;  and  here  again,  as  in  the  Hares  and  Rabbits, 
their  crowns  are  not  directly  opposed  to  those  of  the  opposite  jaw.  The  order  is  in 
these,  however,  the  reverse  of  that  which  occurs  in  the  genus  Lepus,  the  teeth  of  the 
upper  jaw  being  received  within  those  of  the  lower  ;  and  a  degree  of  obliquity  is  given 
to  the  insertion  as  well  as  to  the  surfaces  of  the  teeth,  sufficient  to  produce  the  effects 
of  a  more  perfect  opposition. 
The  series  is  closed  by  Hydrocheerus,  which  exhibits,  in  its  dental  character,  a  still 
further  complication  of  the  structure  observed  in  Arvicolidcs  and  Caviida,  with  much 
that  is  aberrant  in  the  order.  Some  valuable  observations  on  this  point  by  Mr.  Owen 
have  been  published  in  the  '  Proceedings  of  the  Committee  of  Science  and  Corre- 
spondence '  of  this  bodyi. 
The  mere  enumeration  of  the  groups  of  this  interesting  little  tribe  may  serve  as  an 
illustration  of  the  advances  which  the  science  of  Zoology  has  made  within  a  very  few 
years.  Of  the  nineteen  genera  named,  no  less  than  eleven  (including  the  whole  family 
of  Chinchillidcs,  Capromys,  Ctenodactylus,  Octodon,  Geomys,  Diplostoma,  Aplodontia, 
^  Part  II.  p.  187. 
