270 
PEOFESSOE  OWEX  OX  THE  IMEGATHEEIOI. 
border  with  the  flat  surface  for  the  unciforme.  The  concave  scaphoidal  surface  is  con- 
tinuous with  the  surface  for  the  trapezoides,  which  is  much  narrower  than  is  the  same 
surface  in  the  Mylodon,  the  scaphoidal  siu’face  being  broader  than  in  the  Mylodon. 
The  largest  articular  sm’face  is  that  for  the  base  of  the  great  middle  metacarpal,  which 
is  slightly  convex  except  at  its  fore-part,  which  is  produced  into  a cone. 
Unciforme. — Of  all  the  carpals  the  os  uncifonne  (ib.  u)  difiers  most  in  form  from  that 
of  the  Mylodon ; it  is  a thick  transversely  extended  bone,  the  free  tuberous  -smTace  on 
the  back  of  the  wrist  being  hexagonal,  with  the  two  inner  and  the  two  outer  sides  very' 
short ; one  of  the  outer  sides  is  formed  by  a protuberance  separating  the  articular  sur- 
faces for  the  os  cuneiforme  and  fifth  metacarpal,  which  meet  at  an  acute  angle  in  the 
Mylodon. 
The  proximal  oblong  concavo-convex  surface  for  the  cuneiforme  is  continuous,  at  the 
radial  side  of  the  bone,  with  the  surfaces  for  the  lunare  and  os  magnum ; and  the  latter 
with  the  broad  surface  along  the  distal  part  of  the  base  which  is  obscurely  di-rided  into 
three  facets,  the  middle  and  largest  for  the  base  of  the  fourth  metacarpal,  the  next  in 
size  for  the  outer  facet  or  the  base  of  the  third  metacarpal,  and  the  outermost  and 
smallest  facet  for  the  small  part  of  the  base  of  the  fifth  metacarpal  which  articulates 
with  the  carpus.  The  inner  or  palmar  rough  surface  of  the  unciforme  has  an  oblong 
tuberosity,  which  is  narrower  than  that  upon  the  dorsal  sm’face. 
Metacarpus. — The  innermost  or  first  metacarpal  (Plate  XXII.,  i,  ??i),  answei’ing  to 
that  of  the  pollex  or  thumb,  resolves,  by  its  rough  obtuse  distal  termination,  as  well  as 
by  its  diminutive  size,  one  of  the  points  considered  doubtful  by  Cuviee,  in  the  stractm’e 
of  the  fore-foot  of  the  Megatherium,  by  proving  that  the  pollex  was  absent,  or  repre- 
sented solely  by  the  rudimental  metacarpal,  which  must  have  been  concealed  beneath 
the  integument.  The  bone  is  of  an  irregular  subcubical  shape,  broader  than  it  is  long. 
At  its  base  are  two  separate  subcircular  flat  surfaces  articulating  uith  coiTesponding 
facets  on  the  trapezial  part  of  the  scapho-trapezium : on  the  side  next  the  second  meta- 
carpal is  a convex  articular  surface,  having  the  lower  part  obscm’ely  defined  for  articu- 
lation with  the  trapezoides,  and  the  rest  lodged  in  the  conca’vity,  partly  articular’,  partly 
rough,  upon  the  outside  of  the  base  of  the  second  metacarpal.  The  outer  side  of  the 
rudimental  first  metacarpal  is  obliquely  flattened,  the  surface  here  being  apparently  for 
the  insertion  of  a strong  tendon. 
The  metacarpal  of  the  second  digit  (ib.  m ii*)  has  a very  ii-regidar  exterior;  its 
comparatively  small  base  is  excavated  obhquely  to  receive  the  fore-part  of  the  trape- 
zoides ; on  the  outer  or  radial  side  it  shows  a triangular  excavation,  the  lower  half  of 
which  has  a smooth  articular  surface  for  the  rudimental  metacarpal,  m i.  On  the 
ulnar  side  of  the  base  there  is  a convex  articular  surface  dmded  into  a proximal  narrow 
tract  for  the  os  magnum,  and  a distal  broader  tract  for  the  contiguous  side  of  the  base  of 
the  middle  metacarpal.  Above  or  beyond  this  the  middle  third  of  the  bone  supports  a 
rugged  protuberance,  which  has  been  attached  by  ligament  to  a similar  rough  sm’face  on 
* Eeferred  to  the  ‘ annulaire’  or  fourth  digit  in  the  ‘ Ossemens  Eossiles,’  ed.  cit.  pi.  216,  fig.  8. 
