PEOrESSOE  OWEN  ON  THE  MEOATHEEIUM. 
275 
of  which  the  ulnar  one  is  the  larger.  The  ungual  phalanx  (^5.  iv,  3)  is  long,  triedral, 
with  the  radial  side  of  the  sheath  vertical,  the  under  surface  rough  and  flattened,  and 
the  ulnar  surface  sloping  from  above  downward  and  outward  to  join  the  under  surface. 
The  upper  posterior  tuberosity,  overhanging  the  joint,  inclines  radiad,  and  is  somewhat 
flattened ; the  proximal  surface  presents  a median  vertical  rising  for  the  channel  in  the 
preceding  joint. 
The  small  eUiptical  surface  on  the  radial  half  of  the  distal  end  of  the  fifth  metacarpal 
supports  a very  short  lamelliform  phalanx,  produced  outward  some  way  beyond  the 
joint ; and  to  this  is  articulated  a stunted  second  phalanx  with  a rough,  obtuse,  non- 
articular  end.  In  some  instances  the  above  two  phalanges  are  blended  together. 
Amongst  existing  Mammals  th.e  Sloths  alone  present  the  connation  of  the  scaphoid 
with  the  trapezium,  the  Two-toed  Anteater  and  the  Armadillo  that  of  the  first  with  the 
second  phalanx.  This  latter  character,  peculiar  to  the  middle  digit  in  the  Megathe- 
rium, is  limited  also  to  the  same  digit  in  the  Myrmecophaga  didactyla ; in  Dasypus  it 
aflects  the  thu-d,  fourth  and  flfth  digits. 
The  Bradypus  tridactyliis  has  but  two  flexible  phalanges  in  each  of  the  three  ungui- 
culate  toes,  but  this  reduction  is  due  to  the  early  anchylosis  of  the  proximal  phalanx 
with  the  metacarpal,  not,  as  in  Megatherium,  with  the  second  phalanx.  In  the  Bra- 
dypiis  didactylus  the  unguiculate  digits  preserve  the  normal  number  of  free  phalanges. 
The  pollex  is  atrophied  in  the  Megatherium,  as  it  is  in  both  existing  species  of  Sloth ; 
and,  as  in  the  Bradypus  tridactylus,  only  the  second,  third  and  fourth  digits  support 
claws ; but  the  flfth  digit,  instead  of  being  wanting,  as  in  the  Ai,  is  developed,  so  far  as 
was  needed  for  the  purposes  of  terrestrial  progression,  in  the  Megatherium.  The  small 
existing  arboreal  Sloths  are  seldom  obliged  to  walk  on  the  ground,  and  there  can  only 
crawl  along  with  difficulty.  In  the  Mylodon  the  pollex  was  developed  and  unguiculate, 
hut  both  the  foui’th  and  flfth  digits  were  terminated  by  a stunted  second  phalanx.  In  the 
Unau,  not  only  the  fourth  and  fifth  digits,  but  also  the  first  are  suppressed  in  the  fore- 
foot. Yet  this  is  the  Sloth,  as  already  remarked,  which  so  peculiarly  illustrates  the  bra- 
dypodal  affinities  of  the  Megatherium  in  the  structure  of  the  carpus,  notwithstanding 
the  degi’ee  to  which  the  adaptive  modifications  of  the  Megatherioid  type  of  fore-foot 
are  carried  in  relation  to  the  exclusively  arboreal  life  of  this  small  existing  tardigrade. 
The  coalescence  of  the  scaphoid  and  trapezium,  which  Cuvier  was  the  first  to  recog- 
nize in  the  existing  Sloths,  he  continued  to  affirm  in  the  latest  edition  of  the  ‘ Ossemens 
Fossiles’  to  be  peculiar  to  them.  The  bony  structure  of  the  fore-foot  of  the  Megathe- 
rium he  regarded  as  most  resembling  that  of  the  Basypus  gigas.  M.  Laurillard,  after 
the  subsequent  reception  of  casts  of  the  carpal  bones  of  the  Megatherium,  which  had 
been  transmitted  to  England,  with  other  bones  of  the  Megatherium,  by  Sir  Woodbine 
Parish,  K.H.,  inferred  that  the  fore-foot  of  the  Megatherium  had  a greater  analogy  with 
that  of  the  MyrmecopTiaga  juhata,  but  he  did  not  detect  the  connation  of  the  scaphoid 
with  the  trapezium.  The  form  of  the  scapho-trapezial  bone  in  both  existing  Sloths 
bears  an  unmistakeable  resemblance  to  that  in  the  Megatherium,  but  in  the  Unau  it 
