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MR.  E.  T.  BENNETT  ON  THE  CHINCHILLID^. 
Lagostomus  trichodactylus,  Brookes,  in  Linn.  Trans.,  xvi,  p.  102.  t.  9.  (animal 
et  ejus  sceleton.) — Less.,  111.  Zool.,  livr.  3.  pi.  8. — Goldf.,  Naturh.  Atlas, 
Th.  m.  p.  262.  t.  289.  f.  2.  (figg.  Brookes.) 
Dipus  maximus,  Blainv. — Desm.,  in  Nouv.  Diet.  Hist.  Nat.,  xiii.  p.  117. — 
F.  Cuv.,  in  Diet.  Sci.  Nat.,  xviii.  p.  471. 
Marmot  Diana,  GrifF.,  Transl.  An.  Kingd.,  iii.  p.  170.  c.  fig. 
Callomys  Viscaccia,  Isid.  GeoiF.,  in  Ann.  Sci.  Nat.,  xxi.  p.  291. 
Quserendum  adhuc  est  ubi  referendum 
Callomys  aureus,  Isid.  Geoff.,  in  Ann.  Sci.  Nat.,  xxi.  p.  291. 
An  generis  Chinchillse  ? 
The  Herbivorous  subdivision  of  Rodentia,  as  proposed  by  M.  Frederic  Cuvier,  em- 
braces several  strongly  marked  groups  of  forms,  having  an  immediate  affinity  with  each 
other,  although  it  must  be  confessed  that  several  of  them  are  also  nearly  related  to 
genera  of  the  Omnivorous  tribe.  The  persistence  of  the  pulp  of  their  molar  teeth,  and 
the  consequent  unceasing  growth  of  those  teeth,  indicate,  however,  an  inferior  degree 
of  development  as  regards  those  most  essential  organs  ;  and  appear  to  me  to  oflfer  a 
sufficient  bond  of  connexion  between  them. 
Of  the  families  composing  this  tribe,  the  Leporidts,  including  Lepus  and  Lagomys, 
and  represented  by  the  Hares  and  Rabbits,  are  characterized  by  their  supplemental  in- 
cisors, and  by  the  tendency  to  still  further  subdivision  in  the  anterior  pair,  which  in 
Lagomys  especially  are  so  deeply  grooved  and  have  the  two  portions  so  different  in  size 
and  form,  as  to  simulate  two  distinct  teeth  on  each  side  of  the  symphysis  of  the  upper 
jaw.  The  molar  teeth  are  not  in  these  animals  opposed  crown  to  crown,  but  those  of 
the  lower  jaw  pass,  when  the  mouth  is  closed,  almost  entirely  within  those  of  the  upper, 
and  a  considerable  degree  of  lateral  motion  is  consequently  requisite  for  the  due  masti- 
cation of  the  food  ;  which  motion  is  much  facilitated  by  the  almost  hemispherical  form 
of  the  condyles  of  the  lower  jaw,  and  the  freedom  of  their  articulation  in  small  glenoid 
cavities.  By  means  of  this  organization  the  surfaces  of  the  molar  teeth  are  unequally 
worn,  and  offer  transverse  projecting  lines  of  enamel,  with  intermediate  depressions  of 
the  osseous  substance. 
To  this  family  the  ChinchillidcB  are  evidently  very  nearly  related,  in  the  lamellated 
composition  of  their  molar  teeth ;  in  the  general  form  of  the  body  ;  in  the  nature  of 
the  hairy  covering ;  and  in  habits  and  mode  of  life.  They  differ,  however,  in  many 
essential  particulars,  such  as  the  simplicity  of  their  incisor,  and  the  reduced  number  of 
their  molar,  teeth  ;  the  elongated  form  of  the  condyles  of  the  lower  jaw,  and  the  con- 
sequent limitation  of  the  process  of  mastication  to  a  motion  forwards  and  backwards, 
wearing  down  the  crowns  of  the  molars  (which  are  exactly  opposed  to  each  other)  in 
a  perfectly  equal  manner ;  the  depression  of  the  upper  surface  of  the  head,  which  in 
the  Hares  and  Rabbits  is  very  strongly  arched  throughout  its  whole  extent ;  the  deep 
