276 
PEOFESSOE  OWEX  OJf  THE  ^lEGATHEEIOI. 
describes  a deeper  curve  towards  the  palmar  aspect,  and  the  trapezial  portion  (described 
by  De  Blainville  as  the  sesamoid  of  the  pollex*)  is  relatively  longer  than  in  the  Mega- 
therium. The  base  of  the  stunted  metacarpal  of  the  poUex  is  expanded,  and  abuts  by 
one  part  against  the  trapezium  and  by  another  against  the  base  of  the  second  meta- 
carpal. The  trapezoides  is  a small  bone  articulated,  as  in  the  Megatherium,  with  the 
scapho-trapezial,  the  os  magnum,  and  the  second  metacarpal ; the  os  magnum  presents 
almost  the  same  pentagonal  contour,  dorsally,  as  in  the  Megatherium,  the  anterior  facet 
being  also  partly  convex  for  adaptation  to  a concavity  in  the  base  of  the  middle  meta- 
carpal, which  likewise  is  so  extended  as  to  interpose  itself  between  the  fourth  meta- 
carpal and  the  carpus. 
The  atrophy  of  the  fifth  finger,  which  has  proceeded  in  the  Megatherium  to  cause  the 
abseirce  of  the  ungual  phalanx,  and  which  atrophy  similarly  affects  both  the  fifth  and 
fourth  digits  in  the  Mylodon  and  Scelidotherium,  has  proceeded  in  the  Unau  to  the 
removal  of  all  the  bones  of  those  digits,  save  the  metacar*pal  of  the  fomih,  which  is 
reduced  to  a rudiment  of  even  smaller  size  than  that  which  forms  the  vestige  of  the 
thumb  on  the  radial  side  of  the  hand : it  rests,  as  a great  part  of  the  fourth  metacar’pal 
in  the  Megatherium  does,  upon  the  expanded  base  of  the  third  metacarpal. 
In  the  Ai  {Bradypus  tridactylus)  the  metacarpal  rudiment  of  the  pollex  is  anchylosed 
at  its  lateral  joint  to  the  base  of  the  metacarpal  of  the  index,  but  it  retains  its  free  arti- 
culation with  the  scapho-trapezium.  The  chief  modifications  of  both  hand  and  foot  in 
the  Three-toed  Sloth  are  the  extensive  anchyloses  of  different  bones : this  character  is 
shown  by  the  coalescence  of  the  trapezoides  with  the  os  magnum,  such  compound  bone 
supporting  the  base  of  the  second  metacarpal  and  a great  part  of  that  of  the  middle 
metacarpal;  thus  fulfilling  the  same  relations  to  the  metacarpus  as  do  the  separated 
borres  in  the  Urrau  and  Megatherium. 
The  great  extent  to  which  the  metacarpals  are  sutm-ally  united  to  each  other  in  the 
Megatherium,  is  a character  repeated  in  those  of  the  Ai,  but  the  suture  is  speedily,  in 
the  living  Sloth,  converted  into  bony  union,  and  the  three  metacarpals,  like  the  three 
metatarsals,  thus  form  one  compound  bone,  as  in  Birds.  The  unguiculate  digits  which 
this  bone  supports  in  the  fore-paw,  are  the  homologues  of  the  three  claw-bearing  toes  in 
the  Megatherium.  The  rudiment  of  the  fifth  finger  appears  as  a mere  process  from  the 
outside  of  the  base  of  the  metacarpal  of  the  fourth : the  huge  terrestrial  predecessor  of 
the  small  leaf-eating  and  tree-dwelling  quadrupeds  retained  the  fifth  toe,  mmus  its  ter- 
minal phalanx,  yet  of  great  size  and  strength,  and  modified  expressly  for  the  piu'pose  of 
supporting  the  ponderous  body  in  terrestrial  progression. 
The  fore-foot  of  the  Mylodon  more  closely  conforms,  in  its  essentials,  to  the  type  of 
that  of  the  Unau,  inasmuch  as  the  two  outer  digits  (fourth  and  fifth)  were  mutilated 
and  clawless ; they  were,  however,  developed  to  the  same  degree  as  the  fifth  digit  is  in 
the  Megatherium,  and  for  the  same  end,  but  probably  made  little  show,  externally,  in 
the  entire  foot.  The  pollex,  however,  instead  of  being  rudimental,  was  fully  developed, 
* Osteographie  de  Paresseux,  4to,  p.  22. 
