PEOFESSOE  OWEN  ON  THE  MEOATHEEIIJM. 
273 
bone  is  smooth  and  rounded  on  the  palmar  side.  The  small  terminal  articular  surface 
is  oval,  slightly  concave  vertically,  concave  transversely,  upon  the  radial  half  of  the  distal 
expansion.  There  is  no  articular  surface  for  a sesamoid. 
The  metacarpals  are  so  united  and  wedged  together  and  with  the  carpus,  as  to  trans- 
mit from  the  oblique  carpal  surface  which  sustains  the  radius  the  weight  of  the  fore 
part  of  the  Megatherium  to  the  fifth  digit,  the  stunted  extremity  of  which  was  imbedded 
in  the  marginal  hoof-like  callosity  on  which  the  ponderous  quadruped  trod,  with  the 
claw-bearing  toes  bent  inwards. 
From  the  scapho-trapezium  and  lunare  the  weight  was  transmitted  to  the  second  row 
of  carpal  bones ; and  by  the  oblique  production  of  the  base  of  the  second  metacarpal, 
and  especially  of  the  third  metacarpal,  it  was  concentrated  through  the  medium  of  the 
fourth  metacarpal  upon  the  fifth.  The  lateral  pressure  thus  occasioned  explains  the 
extent  of  syndesmotic  and  sutural  union  along  the  basal  part  of  the  metacarpals,  and  also 
the  squared  angular  shape  of  the  constituents  of  this  masonry  of  the  huge  fore-paw. 
On  comparing  the  structure  of  the  carpus  in  the  Megatherium  with  that  in  existing 
Mammals,  it  is  found  to  be  repeated  in  the  Unau  or  Two-toed  Sloth  {Bradypus  [Choice- 
pus)  didactylus),  and  in  that  species  or  subgenus  exclusively ; the  carpal  bones  being 
seven,  and  the  reduction  to  that  number  resulting,  also,  from  the  connation  of  the 
scaphoid  and  trapezium.  A scapho-trapezial  bone  exists  in  the  Ai  or  Three-toed  Sloth 
[Bradypus  [Achms)  tridactylus),  but  the  carpus  is  further  reduced  in  this  species  or 
subgenus  to  six  bones  by  the  confiuence  of  the  trapezoides  with  the  os  magnum.  The 
trapezius  is  a distinct  ossicle  in  the  Chlamyphorus,  as  in  most  other  Armadillos ; in  the 
Basypiis  sex-cinctus  it  coalesces  with  the  trapezoides.  In  the  Pangolins  [Manis)  the 
scaphoid  coalesces  with  the  lunare,  not  with  the  trapezium.  In  the  true  Anteaters 
[Myrmecophaga),  and  in  Orycteropus,  the  ordinary  eight  carpal  bones  retain  .thefr  indi- 
viduality. 
Digital  Phalanges. — The  stunted  metacarpal  of  the  pollex,  in  the  Megatherium,  bore 
no  rudiment  of  a digit.  They  were  powerfully  developed  and  unguiculate  in  the  three 
following  digits. 
The  index  digit  (Plate  XXII.,  ii)  has  three  phalanges : the  proximal  one  ( i)  is  almost 
twice  as  broad,  and  more  than  twice  as  deep  as  it  is  long ; the  metacarpal  surface  pre- 
sents a deep  and  wide,  vertically  elongated,  subangular  concavity,  fitting  the  vertical  pro- 
minence and  lateral  facets  of  the  distal  joint  of  the  metacarpal ; the  distal  surface  of  the 
proximal  phalanx  presents  a vertical  angular  fissure  dividing  two  oblong  convexities. 
The  mid-phalanx  [ih.  ii,  2}  has  a proximal  trochlea  playing  on  the  preceding,  the  median 
vertical  ridge  being  overhung  by  the  produced  upper  rough  surface ; the  distal  articula- 
tion repeats  that  of  the  proximal  phalanx,  but  the  median  fissure  is  less  deep,  and  the 
lateral  convexities  are  more  regularly  rounded  and  prominent.  The  ungual  phalanx 
[ih.  II,  3)  exceeds  the  length  of  both  preceding  phalanges ; the  upper  or  dorsal  side  of 
its  base  is  produced  backwards  into  an  obtuse  point;  the  rough  sheath  of  the  claw 
extends  along  three-fourths  of  the  phalanx ; it  is  convex  radiad,  vertical  and  fiat  ulnad ; 
