PEOrESSOE  OWEN  ON  THE  MEOATHEEIIJM. 
265 
13  inches,  that  of  the  articular  sui’face  is  but  7^  inches.  It  consists  of  two  convexities, 
side  by  side,  di\ided  by  a narrow  and  deep  channel,  continued  from  the  front  non-arti- 
cular  sui’face  half-ivay  towards  the  back  part  of  the  bone.  Both  convexities  have  a full 
elliptic  periphery ; the  outer  one  [ih.  figs.  2,  3,  5,  g)  with  the  long  axis  from  before 
backward,  the  inner  one  (ib.  1)  with  the  long  axis  from  side  to  side : the  outer  condyle  is 
the  larger  and  more  prominent  of  the  two ; it  forms  more  than  a hemisphere,  the  antero- 
posterior contour  describing  full  three-fourths  of  a circle.  The  extent  of  flexion  and 
extension  of  the  fore-arm  on  the  arm  is  thus  shown  to  be  considerable.  The  articular 
surface  continued  from  one  condyle  to  the  other  is  concave  transversely. 
The  centre  of  the  shaft  of  the  humerus  is  occupied  throughout  by  a coarse  cancellous 
structure.  A very  small  medullary  artery  penetrates  the  back  part  of  the  bone,  below 
the  pectoral  tuberosity,  the  canal  extending  obliquely  downward  and  outward. 
The  Myrmecophaga  didactyla,  amongst  existing  Bruta,  most  resembles  the  Megathe- 
rium in  the  development  of  the  supinator  or  outer  supracondyloid  ridge. 
In  its  general  proportions  the  humerus  of  the  Megatherium  resembles  that  of  the 
Megalonyx ; and  is  more  slender,  in  proportion  to  its  length,  than  in  the  Mylodon  or 
Scelido there.  The  articular  head  forms  a larger  proportion  of  a sphere,  and  projects 
more  freely  beyond  the  tuberosities : these  are  relatively  smaller,  and  are  more  equal 
than  in  the  Mylodon  or  Scelidothere : the  external  tuberosity,  in  particular,  is  more 
developed  in  these  smaller  Megatherioids.  In  them  also  the  external  ridge  is  continued 
from  above  the  ‘ musculo-spiral ' groove  mwards,  along  the  front  of  the  humerus  to  the 
apex  of  the  deltoidal  tract,  forming  its  outer  boundary : in  the  Megatherium,  a smooth 
concave  surface  dirides  the  outer  ridge  fr’om  the  deltoidal  elevation,  which  is  absolutely 
naiTOwer.  The  vertical  outline  of  the  back  part  of  the  shaft  of  the  humerus  in  the 
Megatherium  is  almost  straight,  being  but  a little  bent  forwards  at  its  lower  third,  as  it 
is  likewise  in  the  Megalonyx ; and,  in  both,  the  olecranal  depression  is  well  defined : in 
the  Mylodon  and  Scelidotherinm,  the  same  outline  of  the  humerus  is  slightly  concave ; 
the  lower  third  of  the  bone  being,  as  it  were,  a little  bent  back  below  the  deltoidal 
platform ; and  the  olecranal  fossa  is  not  defined.  The  inner  supracondyloid  plate  is 
produced  at  its  upper  part  into  a strong  tuberosity,  in  both  the  Mylodon  and  Scelido- 
therium,  but  not  in  the  Megatherium. 
In  the  existing  Sloths,  the  humerus  at  this  part,  viz.  above  the  inner  condyle,  is  per- 
forated in  one  genus  {Cholcepus)  and  not  in  another  [Adieus,  F.  Cuvier):  and  the  same 
difference  occurs  in  the  great  extinct  Sloths.  In  the  Megatherium  the  humerus  is 
imperforate,  as  it  is  in  the  Mylodon : in  the  Megalonyx  and  Scelidotherinm  it  is  per- 
forated above  the  inner  condyle.  Yet  the  Megalonyx  most  resembles  the  Megathe- 
rium, not  only  in  the  general  proportions  of  the  humerus,  but  in  the  configuration  of 
its  two  articular  ends.  The  inner  or  ulnar  condyle,  e.  g.,  is  convex  in  every  direction  in 
the  MegalonjTc  as  in  the  Megatherium : in  the  Mylodon  and  Scelidotherinm  it  is  convex 
only  from  before  backwards,  but  is  concave  from  side  to  side.  In  the  more  robust  pro- 
portions, in  the  shape  of  the  articulations  and  the  development  of  the  processes  of  the 
