268 
PEOZESSOE  OAVEX  OX  THE  AIEGATHEEIOI. 
there  it  expands  in  all  directions  to  form  the  large  articular  caAity  (ib.  d and  fig.  6) 
for  the  major  part  of  the  carpus.  AboA'e  this  cavity,  on  the  ulnar  side,  is  the  rough 
and  shallow  depression  for  the  ligamentous  junction  of  the  corresponding  end  of  the 
ulna;  on  the  front  side  a broad  low  ridge  extends  obliquely  fi’om  the  suprastyloid 
tuberosity  to  the  border  of  the  cavity ; on  the  back  part  three  oblong,  short,  thick  ridges 
or  tubercles  divide  the  surface  into  four  channels,  for  tendons ; three  of  these  are  longi- 
tudinal and  parallel,  progressively  increasing  in  width  from  the  innermost  (or  one  next 
the  ulna)  to  the  third ; the  outermost  passes  obliquely  between  the  suprastyloid  tubercle 
and  the  styloid  process.  This  process  is  short  and  thick,  roimded  at  the  end ; flattened 
in  front,  with  the  smooth  articular  surface  continued  for  a few  lines  upon  the  lower 
border  of  this  surface,  from  the  general  articular  cavity  which  is  extended  over  the  lower 
end  of  the  radius  including  the  styloid  process.  This  cavity  presents  a triangular  form 
Avith  the  angles  rounded  off ; the  base  next  the  ulna  is  short  and  oblique ; the  anterior 
border  or  side  is  the  lorrgest,  the  opposite  border  is  the  thickest,  and  is  notched  near  the 
styloid  process : the  cavity  is  moderately  concave,  and  articulates  with  the  scaphoid  and 
lurrar  bones  of  the  carpus. 
Irr  the  Mylodon  the  radius  is  not  orrly  thicker  in  proportiorr  to  its  length,  brrt  is  more 
exterrsively  and  deeply  impressed  by  the  muscles  of  the  fore-arm,  especially  on  the  back 
part  of  the  borre.  The  tuberosity  for  the  insertion  of  the  biceps  is  further  from  the 
proxirrral  joint,  and  augments  the  poAver  of  the  muscle  in  the  same  degree : the  proximal 
articular  cavity  is  of  an  oval  form. 
Caf'pus. — The  carpus  (Plate  XXII.  s — u)  consists  of  seA'eir  bones,  forrr  in  the  proximal 
arrd  three  in  the  distal  row. 
Scapho-trapezium. — The  first  of  the  proximal  roAv  (ib.  s)  inclrrdes  the  bone  (ib.  t) 
arrswering  to  the  first  of  the  distal  roAV  in  Man  arrd  most  Marrrmals,  and  is  conseqrrently 
a ‘ scapho-trapezium ’ (s,  t)^,  as  it  is  also  irr  the  Sloths,  the  Mylodon  and  Scelido- 
therium.  It  is  of  arr  irregular  triangular  shape,  with  its  base  applied  to  the  ‘Irrnar'e’ 
(ib.  ^),  arrd  with  the  apex  somewhat  tAvisted.  It  preserrts  a broad  corrAex  ardicrrlar 
surface  [ib.  s)  for  the  outer  half  of  the  concavity  of  the  radirrs ; arrd  this  sruTace  is  corr- 
tinuous  with  a crescentic  one  of  about  orre  irrch  irr  breadth,  which  covers  the  proximal 
part  of  the  side  of  the  borre  next  the  os  lunare.  The  palmar  or  arrterior  horn  of  the 
crescent  is  corrtirruous  with  arr  oval  flat  articular  surface  joirrirrg  the  os  rrragrrrrm  (ib.  g) : 
the  opposite  or  dorsal  horn  is  separated  by  a rough  tract  fi’om  a corrr  ex  srrbqrradrute 
surface  Avhich  also  articulates  with  the  os  magnum.  On  the  orrter  side  of  this  srrrface, 
and,  like  it,  on  the  fore-part  of  the  bone,  is  a srrrface  corrcave  irr  one  dir'ectiorr  arrd  corrvex 
irr  the  opposite  direction,  for  articulation  Avith  the  small  trapezoides  (ib.  z).  Exterrral 
to  this,  the  fore-part  of  the  produced  arrd  twisted  apex  of  the  bone,  AAdrich  represerrts  the 
trapezium,  articulates  with  the  stunted  metacarpal  of  the  pollex  (ib.  in  i),  chiefly  by 
* This  is  the  bone  called  ‘ cuneiforme  ’ in  Cuyiee’s  chapter  on  the  Megatheriiun,  and  which  is  mai'ked  >• 
in  the  copy  of  Dr.  Pajstdee’s  figure  of  the  Madrid  skeleton  introduced  into  plate  217,  fig.  3,  of  the  edition  of 
the  ‘ Ossemens  Eossiles,’  8vo,  1835,  here  cited. 
