PEOFESSOE  OWEN  ON  THE  MEGATHEEIUM. 
271 
the  middle  metacarpal ; so  that  little  more  than  one-third  of  the  bone  projects  freely 
from  the  metacarpus.  This  free  part  is  subcompressed  and  expands  in  the  direction 
from  the  back  to  the  palm  of  the  hand,  so  as  to  form  the  surface  for  a trochlear  articu- 
lation of  great  extent  in  that  sense.  A ridge  from  the  back  surface  of  the  metacarpal 
expands  into  a tuberosity  near  the  dorsal  end  of  that  articulation;  and  a similar 
but  smaller  tuberosity  projects  from  the  palmar  end  of  the  same  articulation,  along  each 
side  of  which  there  runs  a thickish  edge.  The  distal  surface  itself  presents  a median 
vertical  or  longitudinal  prominence,  beyond  the  lower  half  of  which  the  articular  surface 
is  produced  laterally  so  as  to  make  the  surface  concave  transversely  on  each  side  the 
median  ridge ; whilst  at  the  upper  half  the  whole  surface  is  convex  transversely,  as  it  is, 
in  a minor  degree,  longitudinally. 
The  middle  metacai’pal  (ib.  m iii)  is  a little  longer  than  the  second,  but  is  twice  as 
thick,  with  a quadrate  transverse  section,  the  four  sides  of  the  shaft  being  flat  and  sharply 
defined.  The  base  of  the  bone  is  produced  ‘ ulnad  ’ and  ‘ proximad,’  so  as  to  be  wedged 
between  the  fourth  metacarpal,  the  magnum  and  the  unciforme.  The  articular  surface 
on  the  ulnar  side  of  this  production  for  the  fourth  metacarpal  is  extensive,  and  for  the 
most  part  slightly  convex ; the  surface  on  what  may  be  termed  the  truncate  end  of  the 
production,  for  articulation  with  the  unciforme,  is  slightly  convex  on  the  dorsal  half, 
slightly  concave  on  the  palmar  half.  The  surface  for  the  magnum  on  the  proper  base  of 
the  metacarpal  sinks  into  a conical  cavity  near  its  dorsal  end ; the  rest  being  nearly  flat. 
On  the  radial  side  of  the  base  is  an  oblong  articular  tract  for  the  second  metacarpal, 
and  beyond  this  the  extensive  rough  surface  for  the  ligamentous  connexion  with  the 
same  bone.  The  radial  surface  is  grooved  above  the  rough  facet,  obliquely  by  a wide  and 
moderately  deep  canal ; apparently  for  the  passage  of  a tendon  to  the  digit.  The 
dorsal  sui’face  shows  an  obhque  broad  tuberosity,  extending  from  above  the  tendinal 
groove  to  the  upper  or  dorsal  end  of  the  distal  articulation.  The  smooth  and  almost 
flat  dorsal  smdace  gradually  deepens  into  a broad  and  shallow  oblique  channel  on  the 
ulnar  side  of  the  oblique  tuberosity.  The  ulnar  side  of  the  bone,  beyond  the  articular 
siudace  for  the  fourth  metacarpal,  is  occupied  by  a rugged  flat  tract  for  ligamentous 
connexion  with  the  same  bone.  The  palmar  surface  is  pretty  smooth,  flat  transversely, 
slightly  concave  lengthwise ; produced  into  a tubercle  below  the  middle  prominence 
of  the  distal  joint.  The  articular  surface  of  this  joint  does  not  cover  the  whole  distal 
end  of  the  bone ; it  is  long  and  rather  narrow,  extending  obliquely  from  the  palmar 
forward  to  the  dorsal  sui’face ; the  dorsal  side  of  the  bone  being  longer  than  the  palmar 
one.  It  is  traversed  lengthwise  by  a median  prominence,  convex  transversely,  almost 
straight  lengthwise ; and  the  surface  is  continued  upon  a flat  tract  on  each  side  of  the 
prominence,  that  on  the  radial  side  of  the  prominence  being  the  broadest. 
The  general  form  of  this  bone  is  more  like  that  of  a brick  or  of  an  ashlar  stone  for  a 
strong  wall,  than  like  that  of  the  usual  support  of  a flexible  digit  of  a fore-paw  or  hand. 
The  chief  difference  between  the  middle  metacarpals  of  the  Megatherium  and  Mylo- 
don  is  in  the  form  of  the  distal  articulation.  This  surface,  in  the  smaller  Megatherioid, 
2 o 2 
