PEOFESSOE  OWEN  ON  THE  MEOATHEEIUM. 
277 
though  small,  in  the  Myloclon.  In  the  Megatherium  this  digit  is  rudimental,  as  in  both 
forms  of  existing  Sloth ; but  the  bones  of  the  fore-foot  correspond  more  closely  with 
the  type  of  the  manus  in  the  Ai ; there  being,  indeed,  as  regards  the  digits,  only  this 
essential  difference,  that  the  fifth,  instead  of  being,  like  the  first,  a mere  rudiment,  was 
developed  to  be  adapted  to  progression  on  the  ground.  It  is  most  interesting,  however, 
to  trace  the  interchangeable  relations  between  the  two  above-cited  great  extinct  Mega- 
therioids  and  the  two  existing  forms  of  Sloth,  respectively. 
In  regard  to  other  existing  Edentata,  the  Myrmecophaga  jichata,  by  reason  of  the 
clawless  condition  of  its  fifth  digit,  and  the  Myrmecophaga  didactyla,  by  that  of  the 
rudimental  poUex  as  well  as  fourth  and  fifth  digits,  ought  to  succeed  the  Sloths  as  next 
of  kin  to  the  Megatherioid  quadrupeds,  the  interval  being  due  to  the  difference  of  carpal 
structure. 
CuTiEE  has  observed  that  the  fore-foot  of  the  Dasypus  gigas  is  one  of  the  most  extra- 
ordinary among  quadrupeds ; and,  he  adds,  that  it  alone  would  give  the  key  to  all  the 
anomalies  in  that  of  the  Megatherium  But  this  could  only  have  been  affirmed  under 
a misconception  of  the  real  nature  of  those  anomalies.  In  the  Dasypus  gigas  the  fore- 
foot is  pentadactyle ; all  the  digits  are  unguiculate,  and,  in  three  of  them,  the  claw- 
phalanges  fui'nish  a bony  sheath  as  well  as  core  to  the  claws ; but  these  belong  to  the 
third,  fourth  and  fifth  toes,  not,  as  in  the  Mylodon,  to  the  first,  second  and  third,  or,  as 
in  the  Megatherium,  to  the  second,  third  and  fourth  toes ; they  moreover  successively 
decrease  in  size  from  the  radial  to  the  ulnar  aspect,  instead  of  the  reverse  proportions 
which  they  present  in  the  Mylodon.  No  doubt  the  claw  on  the  middle  digit  is  the  most 
developed,  as  in  the  Megatherium,  and  the  first  and  second  phalanges  of  this  digit  haye 
coalesced ; but  here  ends  the  particular  resemblance  between  the  Megatherium  and  the 
great  Armadillo,  in  regard  to  the  bony  structure  of  the  fore-foot. 
We  can  state  with  confidence,  what  M.  Laurillaed  suggests  f,  viz.  that  the  fore-feet 
of  the  Megatherium,  as  represented  by  the  skeleton  in  the  Madrid  Museum,  are  not 
transposed,  the  right  being  on  the  left  and  the  left  on  the  right  side,  as  Cuvier  was  led 
to  suspect ; but  that  the  articulation  of  those  complex  parts  by  the  laborious  Prosector 
and  Curator  Bru,  was  in  the  main  coiTect. 
The  bony  structure  of  the  fore-limb  of  the  Megatherium  is  now,  indeed,  as  com- 
pletely understood,  and  the  homologies  of  every  constituent  bone  as  exactly  defined,  as 
in  any  existing  species  of  quadruped.  And,  to  the  degree  in  which  so  important  a part 
of  the  frame  throws  light  on  the  whole,  the  Naturalist  may  thereby  trace  the  affinities, 
and  the  Physiologist  infer  the  habits,  of  the  great  extinct  beast. 
* “ La  main  du  tatou  (jeant  eat  une  dea  plus  extraordinaires  qu’il  y ait  parmi  lea  quadrupkies,  et  a elle 
aeule  elle  expliquerait  toutes  lea  anomalies  que  nous  verrons  dans  celle  du  Megatherium. ’’-^Ossemens  Fos- 
siles,  ed.  cit.  tom.  viii.  p.  242.  No  qualifying  note  is  appended  by  the  editors  to  this  statement. 
t See  the  note  (1)  appended  by  that  able  anatomist  to  the  chapter  on  the  Megatherium,  in  the  posthu- 
mous edition  of  the  ‘ Ossemens  Fossiles,’  8vo,  t.  viii.  p.  355. 
MDCCCLVIII.  2 P 
