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PEOFESSOE  OWEN  02s  THE  iMEOATHEETOI. 
humerus,  the  Mylodon  and  Scelidotherium  as  closely  resemble  each  other,  as  the  Mega- 
therium resembles  the  Megalonyx  in  the  same  characters ; yet  the  humerus  of  the  Sceli- 
dotherium has  the  inner  perforation,  and  that  of  the  Mylodon  a groove  merely,  for  the 
brachial  artery  and  nerve.  This  variety,  and  the  corresponding  one  above  noticed 
between  the  Megatherium  and  Megalonyx,  show  the  inapplicability  of  the  final  cause 
commonly  assigned  to  the  ento-condyloid  hole,  the  existence  or  otheinvise  of  which 
depends  merely  on  the  ossification  or  non-ossification  of  the  aponeurosis  extending  from 
the  shaft  of  the  humerus  to  the  ento-condyloid  process. 
Ulna. — The  bones  of  the  fore-arm  in  the  Megatherium,  as  in  the  Megalonyx,  ex- 
emplify by  their  greater  length,  as  compared  with  those  of  the  same  segment  in  the 
hind  limb,  the  Bradypodal  affinities  of  these  huge  extinct  quadi-upeds. 
The  ulna  in  the  Megatherium  (Plate  XXI.  figs.  1,  2,  3)  is,  however,  peculiar  for  the 
vast  expanse  of  its  proximal  end,  in  connexion  ufith  its  long  and  slender  shaft.  The 
olecranon  (ib.  a)  is  twice  as  broad  as  it  is  long ; its  inner  border,  springing  from  the  notch 
which  penetrates  the  inner  and  back  part  of  the  humeral  articular  fossa,  extends  obliquely 
upward  and  inward  for  3^  inches,  the  ridge  or  edge  of  the  plate  being  about  an  inch 
in  thickness : the  plate  from  this  ridge  sweeps  round  the  back  part  of  the  bone,  subsiding 
and  increasing  in  breadth  as  it  approaches  the  radial  side,  where  it  terminates  in  a 
tuberosity,  divided  by  a groove  from  the  radial  articular  ca'vity  (ib.  b),  and  prolonged 
downward  into  the  ridge  bounding  the  radial  side  of  the  upper  half  of  the  ulna.  The 
olecranon  projects  rather  backward  than  upward,  and  is  strengthened  and  supported  at 
its  highest  part  by  a strong  angular  buttress,  which  gradually  subsides  upon  the  back  of 
the  ulna. 
The  great  ‘ sigmoid’  articular  surface  is  divided  into  two  facets  by  a median  portion, 
Avhich  is  produced  forward  in  the  longitudinal  or  vertical  direction,  and  is  convex  from 
side  to  side ; the  divisions  so  defined  are  concave.  The  inner  one  (ib.  c),  for  the  inner 
humeral  condyle,  describes  a semicircle  from  behind  forwards ; it  has  little  more  than 
half  that  extent  from  side  to  side,  and  is  encroached  upon  by  a narrow,  rather  deep, 
rough  channel,  continued  from  the  inner  origin  of  the  olecranon,  expanding,  to  neai‘ 
the  middle  of  the  cavity.  The  outer  division  of  the  sigmoid  fossa  (ib.  b)  has  reverse 
proportions,  the  transverse  being  nearly  double  the  extent  of  the  longitudinal  diameter, 
and  it  is  less  concave  than  the  inner  division ; about  half  an  inch  of  its  lower  border 
bends  a little  back  for  the  articular  margin  of  the  head  of  the  radius,  just  above  the 
rough  fossa,  for  the  reception  of  the  non-articular  part  of  the  same  head : the  rest  of  the 
outer  division  of  the  sigmoid  cavity  receives  the  back  part  of  the  outer  condyle  of  the 
humerus.  Below  the  outer  division  the  radial  fossa  (ib.  d)  presents  a triangrdar  form, 
bounded  by  a rough  ridge  externally  and  by  a tuberosity  internally.  The  ridge  is  con- 
tinued upon  the  radial  side  of  the  shaft  of  the  ulna  to  within  a foui’th  paid  of  its  distal 
end ; the  surface  of  this  side  of  the  bone  is  irregularly  sculptured,  indicating  the  strong 
ligamentous  connexion  between  the  ulna  and  radius  at  this  part.  The  back  and  inner 
sides  of  the  shaft  of  the  ulna  are  comparatively  smooth. 
