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XV.  On  the  Megatherium  (Megatherium  Americanum,  Cuvier  and  Blumenbach). 
Part  lY. — Bones  of  the  Anterior  Extremities.  By  Professor  Owen, 
Superintendent  of  the  Natural  History  Departments  in  the  British  Museum. 
Received  May  6,  1851 , — Bead  May  8,  1851. 
The  bones  of  the  limbs  of  the  Megatherium  are  not  less  fraught  with  interest  to  the 
Comparative  xlnatomist  and  Physiologist,  than  are  those  of  the  trunk  and  head,  by  reason 
of  their  peculiar  proportions  and  configurations,  and  more  especially  as  the  unguiculate 
type  on  which  they  are  constructed  is  exemplified  in  a quadruped  of  such  enormous 
bulk.  The  anterior  extremities  (Plate  XVIII.)  exceed  the  posterior  ones  in  length : in 
their  bony  structm’e  they  include  a complete  clavicle  (ss)  with  the  scapula  (si),  a hume- 
rus (53),  an  antibrachium,  consisting  of  fully  developed  and  reciprocally  rotating  radius  (ss) 
and  ulna  (54),  carpus,  metacarpus,  and  four  digits ; they  manifest,  in  short,  all  the  main 
perfections  of  brachial  structure,  save  the  opposable  thumb,  observable  in  the  mammalian 
class.  These  perfections,  moreover,  are  associated  with  proportions  and  processes  indi- 
cative of  enormous  strength,  and  bespeak  a limb  fitted  not  only  to  take  its  full  share  in 
the  support  of  the  body,  but  to  be  employed  on  operations  in  which  unusual  resistance 
had  manifestly  to  be  overcome.  In  no  respect,  perhaps,  does  the  Megatherium  more 
strikingly  differ  in  its  osseous  structure  from  the  existing  quadrupeds  of  corresponding 
bulk,  than  in  the  bony  fulcra  of  the  anterior  extremity. 
Scapula. — The  scapula  (Plate  XVIII.  51,  and  Plate  XIX.  figs.  I and  2)  is  a vast 
expanse  of  bone,  with  a double  spinous  process ; the  normal  one  expanding  into  a large 
acromion,  which  is  continued  into,  and  is  confluent  with,  the  coracoid  process.  The 
scapula  usually  presents  an  inequilateral  triangular  form  (Plate  XIX.  fig.  I),  of  which 
the  acromion  [k)  is  the  apex.  The  upper  border  (^,  c)  is  the  shortest;  but,  in  one 
specimen,  owing  to  the  greater  development  of  the  basal  border,  as  indicated  by  the 
dotted  outline  in  Plate  XIX.,  the  upper  border  appeared  to  begin  at  the  part  of  the 
base  marked  «,  and  to  form  a low  angle,  as  if  continued  about  one-fourth  of  the  distance 
from  the  base  parallel  with  the  lower  border,  whilst  the  rest  of  the  costa  inclines  down- 
ward towards  the  coracoid  (c),  with  a slight  concave  outline.  The  upper  border  increases 
in  thickness  as  it  passes  into  the  origin  of  the  coracoid.  The  base  of  the  scapula,  from 
the  point  «,  is  straight  as  far  as  the  origin  of  the  spine,  a';  it  then  bends,  with  a convex 
curve,  and  increases  in  thickness  to  the  inferior  angle  of  the  scapula  (d),  close  to  which 
commences  the  second  or  lower  spine.  The  inferior  costa  of  the  scapula  extends  forward, 
straight  and  parallel  with  the  lower  spine,  for  some  way,  and  then  is  lost  upon  the  inner 
MDCCCLVIII.  2 N 
