264 
PEOPESSOE  OWEN  ON  THE  ilEGATHEETOI. 
ton,  is  thus  proved  to  be  well-founded.  The  true  connexions  of  the  sternal  ends  of  the 
clavicles  with  each  other  and  with  the  manubrium  stemi,  are  well  shown  in  the  view  of 
the  skeleton  in  the  British  Museum,  given  in  Plate  XVIII. 
Humerus. — The  humerus  (Plate  XX.  figs.  2 — 5)  is  remarkable  for  the  vast  expanse 
of  the  lower  fourth  part  of  the  bone ; but  this  is  limited  to  the  trans^'erse  direction ; so 
that,  viewed  sideways  (as  in  Plate  XVII.  Phil.  Trans.  1855),  the  humerus  of  the  Mega- 
therium appears  to  be  a comparatively  weak  and  slender  bone : the  whole  shaft,  how- 
ever, gives  indications  of  the  force  of  the  thick  muscular  masses  which  suiTounded  and 
operated  on  it. 
The  head  of  the  bone  (Plate  XX.  fig.  4)  presents  a smooth  convexity  of  an  ellip- 
tical form,  corresponding  with  that  of  the  glenoid  ca\ity  of  the  scapula ; the  long  axis 
of  the  ellipse  is  from  before  backward.  The  head  rises  clear  above  the  outer  and  inner 
tuberosities,  neither  of  which  are  so  developed  as  to  interfere,  as  in  ungulate  quadru- 
peds, with  the  free  rotation  of  the  bone. 
The  rugged  surface  of  the  inner  tuberosity  {ih.  figs.  2 and  3,  a)  slopes  downward  and 
inward  (ulnad)  from  the  peripheral  groove  of  the  head,  marking  the  attachment  of  the 
joint-capsule.  The  pectoral  ridge  is  continued  from  the  lower,  slightly  outstanding, 
part  of  this  tuberosity.  The  outer  tuberosity  (ib.  h)  projects  from  a lower  level,  is  larger 
and  more  prominent  than  the  inner  one ; and  is  divided  into  two  suhequal  rough  facets. 
The  outer  deltoid  ridge  begins  from  the  outer  side  of  the  tuberosity ; the  inner  deltoid 
ridge  (ib.  d)  some  inches  lower  down,  and  nearer  the  inner  side  of  the  shaft:  these 
ridges  converge,  strengthen  as  they  descend,  and  coalesce  at  the  beginning  of  the  lower 
third  of  the  shaft,  defining  a long  and  narrow  angular  tract  for  the  implantation  of  the 
powerful  deltoid  muscle.  A tuberosity  (ib.  c)  is  developed  fr’orn  the  outer  (radial)  side 
of  the  humerus,  one-third  down  the  bone,  from  which  a strong  ridge  descends  along  the 
same  side  of  the  middle  third : this  ridge  is  defined  below,  and  dfrided  fr’om  the  supi- 
nator ridge,  by  a deep  and  smooth  oblique  channel  (ib.  e)  for  the  passage  of  vessels  and 
nerves  from  the  back  to  the  fore-part  of  the  bone.  The  supinator  ridge  (ib.  f)  is  the 
most  prominent  feature  of  the  lower  third  of  the  humerus ; it  presents  a long,  triangular, 
rough  outer  facet,  widening  as  it  descends,  and  with  a secondary  ridge  fr’om  its  middle 
part.  The  longitudinal  contour  of  this  facet  forms  an  obtuse  angle  nfith  the  lower  half 
of  the  ridge,  extending  to  the  outer  condyle  of  the  humerus : the  outer  facet  of  this  half 
is  rough,  triangular,  with  the  base  upward.  The  pectoral  ridge  {ih.  fig.  3,  i)  terminates 
in  a low  tuberosity  (ib.  h)  on  the  inner  side  of  the  middle  of  the  shaft ; whence  a second 
ridge  is  continued  upward  upon  the  back  of  the  shaft.  This  surface  is  fiatter  than  the 
fore-part,  especially  at  its  lower  expanded  thu’d ; at  the  bottom  of  which,  midway  between 
the  outer  and  inner  supracondyloid  productions,  and  just  above  the  lower  ailicular  sur- 
face, is  a small  but  well-defined  olecranal  depression.  The  inner  supracondyloid  angular 
production  (ib.  k)  has  the  flat  rough  facet  only  upon  its  lower  half.  Owing  to  the  pro- 
duction of  these  ridges,  the  articular  condyles  themselves  (ib.  I)  appear  to  occupy  but 
a small  part  of  the  distal  end  of  the  bone ; for  the  extreme  breadth  of  this  end  being 
