PEOFESSOE  OWEN  ON  THE  MEGATHEEIIJM. 
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A vascular  canal,  somewhat  larger  than  the  rest,  is  seen  near  the  fore-part  of  the  outer 
surface,  entering  the  bone  obliquely  upward,  A rising  of  the  surface,  with  a linear 
series  of  three  or  four  rough  tubercles,  marks  the  lower  fourth  of  the  inner  side  of  the 
bone;  a short  wide  longitudinal  channel  marks  the  back  surface  of  the  distal  end 
(fig.  3),  which  is  slightly  expanded  and  convex,  and  so  impressed  as  to  indicate  its 
ligamentous  junction  with  the  carpus. 
The  ulna  of  the  Megatherium  differs  from  that  of  the  Mylodon,  not  only  in  its  longer 
and  more  slender  proportions,  but  also  in  the  absolutely  as  well  as  relatively  minor 
height  or  length  of  the  olecranon ; in  the  much  less  relative  vertical  or  longitudinal 
extent  of  the  outer  division  of  the  ‘ sigmoid’  cavity;  and  in  the  ‘haversian’  fossa  on  the 
inner  division.  It  differs  in  the  much  narrower  channel  dividing  the  articular  cavity 
from  that  part  of  the  base  of  the  olecranon  which  is  continued  into  the  posterior  ridge 
or  border  of  the  shaft ; it  differs,  also,  in  the  convexity  of  the  distal  end  and  the  absence 
of  the  articular  facet,  which  is  distinctly  present  in  that  part  of  the  ulna  in  the  Mylo- 
don and  Scelidotherium. 
Badius. — The  radius  (Plate  XXI.  figs.,  4,  5,  6),  like  the  ulna,  of  the  Megatherium 
resembles  in  its  longer  and  more  slender  proportions  that  bone  in  the  Megalonyx,  and 
differs  from  the  proportionally  thicker  and  shorter  radius  of  both  the  Mylodon  and 
Scelidotherium.  The  proximal  end  is  circular,  and  is  occupied  by  a smooth,  moderately 
shallow,  articular  cavity  {ib.  fig.  5),  with  a well-defined  border,  over  which  the  articular 
surface  extends,  on  the  ulnar  side  of  the  head,  for  about  half  an  inch  down ; which  tract 
is  adapted  to  the  lower  portion  of  the  outer  division  of  the  sigmoid  cavity  of  the  ulna. 
The  articular  modification  of  the  head  of  the  radius  is  as  completely  adapted  for  the 
superadded  rotatory  movements  of  the  antibrachial  bones,  as  in  the  human  subject,  to 
the  head  of  the  radius  of  which  the  resemblance  of  that  of  the  Megatherium  is  strikingly 
close. 
The  shaft  of  the  radius  gradually  narrows,  in  the  antero-posterior  diameter,  along  the 
upper  fourth  part,  but  maintains  the  same  diameter  as  the  head,  transversely.  Three 
inches  below  the  head,  on  the  inner  and  fore-part  of  the  shaft,  is  the  tuberosity  (ib.  a) 
for  the  tendon  of  the  biceps,  which  measures  3 inches  in  long  diameter  and  If  inch 
across.  Here  the  bone  bends  a little  outward  (radiad) ; and  the  ridge  bounding  that 
side  is  developed  into  what  may  be  termed  a process  (ib.  Z»),  with  a low  angle,  whence  the 
ridge  is  continued  straight  down  the  lower  half  of  the  shaft  to  near  the  tuberosity  above 
the  styloid  process  (ib,  c),  where  it  curves  outwardly  to  terminate  in  that  tuberosity. 
The  fore-part  of  the  shaft  is  moderately  smooth  and  convex  across ; it  describes,  length- 
wise, a slight  concavity ; on  the  inner  side  of  the  bone,  a broad  and  very  rugged  tract 
begins,  about  an  inch  and  a half  below  the  bicipital  tuberosity,  and  extends  along  the 
middle  third  of  the  shaft ; a less  rough  tract  is  continued  thence,  gradually  expanding 
to  the  cavity  for  the  lower  end  of  the  ulna.  The  outer  side  of  the  shaft  of  the  radius  is 
smooth,  convex  across,  and  with  a slight  convexity  in  its  longitudinal  contour ; from  the 
external  process  downward  the  radius  maintains  an  equal  breadth  near  the  lower  end,  and 
