272 
PEOrESSOE  OWEX  OX  THE  ^klEGATHEEIOI. 
is  convex  from  above  downward,  whilst  in  the  Megatherium  it  is  straight,  or  rather 
concave,  and  it  joins  the  dorsal  suidace  at  an  acute  angle.  The  lateral  depressions  of 
the  pulley  are  narrower  in  the  Megatherium,  and  the  vertical  inflexions  of  the  phalanx 
must  have  been  more  limited  than  in  the  Mylodon. 
The  fourth  metacarpal  (ib.  wiv),  as  compared  with  the  third,  is  longer  and  more 
slender  in  the  Megatherium  than  in  the  Mylodon ; but  its  articulation  by  an  obliquely 
extended  base  with  the  third  and  fifth  metacarpals  and  the  unciform  bone,  closely 
corresponds  with  that  in  the  Mylodon 
The  two  oblique  metacarpal  surfaces  are  nearly  parallel,  the  radial  one  is  concave,  the 
ulnar  one  slightly  convex ; both  are  separated  by  a sharp  angle  from  the  intermediate  or 
carpal  surface,  which  is  nearly  square  and  is  slightly  concave.  The  proximal  half  of  the 
bone  is  bounded  by  four  flat  equal  sides,  with  inteiwening  angles,  shai-p  on  the  radial 
side.  The  upper  and  under  sides  are  nearly  smooth,  with  a broad  low  tuberosity  near 
the  proximal  end ; the  outer  and  inner  sides  are  rugged  for  close  syndesmosis  with  the 
adjoining  metacarpals.  Only  the  distal  half  of  the  bone  stands  freely  out ; it  expands 
in  vertical  breadth  to  the  articular  surface  for  the  fourth  finger.  The  angle  between 
the  upper  and  radial  sides  is  rounded  off ; that  between  the  upper  and  ulnar  sides  is,  in 
one  specimen,  developed  into  a sharp  ridge. 
The  distal  articular  surface  resembles  that  of  the  second  metacarpal,  but  occupies  a 
smaller  relative  proportion  of  the  whole  distal  surface ; the  vertical  prominence,  convex 
transversely,  is  broader,  more  concave  vertically,  and  extends  obliquely  from  the  upper 
and  radial  angle  to  the  lower  and  ulnar  one : the  flat  lateral  extension  of  the  articular 
surface  is  confined  to  the  radial  side  of  the  prominence.  A single  flat  surface  for  a sesa- 
moid bone  is  situated  below,  but  distinct  fr’om,  this  part  of  the  articulation.  A lai'ge 
and  strong  tuberosity  projects  below  the  prominent  part  of  the  joint ; an  oblique  chaimel 
divides  this  tuberosity  from  a longer  one  on  the  ulnar  side  of  the  distal  end  of  the  meta- 
carpal. 
The  distal  articular  surface  of  the  fourth  metacarpal  in  the  Mylodon  is  reduced  to  a 
small,  vertical,  oblong,  nearly  flat  surface ; this  difference  relating  to  the  stunted  deve- 
lopment and  limited  function  of  the  digit  it  has  to  support.  In  the  Megatherium  such 
simplification  of  the  distal  joint  of  the  metacarpal  is  limited  to  the  fifth  of  that  series  of 
bones  (ib.  m v) : this  metacarpal  f , of  the  same  length  as  the  fourth,  is  more  slerrder ; its 
proximal  end  is  wedge-shaped,  the  radial  articular  surface  and  the  flattened  orrter  facet 
converging  to  the  narrow  rough  tract  which  is  joined  by  ligamerrt  to  the  car-prrs.  The 
articular  surface  on  the  radial  side  has  a small  terminal  part  obscurely  marked  off  for  a 
facet  on  the  unciforme ; the  rest  receives  the  convexity  on  the  ulnar  side  of  the  fourth 
metacarpal : beyond  this  is  a rough  surface  for  the  syndesmotic  union  of  the  contiguous 
bones.  Rather  more  than  half  the  fifth  metacarpal  stands  fr’eely  out;  it  is  traversed 
above  by  a longitudinal  ridge  expanding  into  a broad  tuberosity  at  the  distal  end ; the 
* Op.  cit.  p.  92.  pi.  XV.  m 4. 
f Eeferred  to  the  index  or  second  digit  in  the  ‘ Ossemens  Eossiles,’  ed.  cit.  pi.  16.  fig.  11. 
