PEOFESSOE  OWEN  ON  THE  MEOATHEEITJM. 
263 
The  confluence  of  the  acromion  with  the  coracoid  is  peculiar  to  the  Sloths  amongst 
existing  Mammals. 
Clavicle. — Before  the  discovery  of  the  Megatherium,  Man  was  usually  cited  in  Manuals 
of  Comparative  Anatomy  as  the  largest  animal  possessing  a ‘ collar-bone  ’ : he  is,  in  fact, 
the  largest  existing  animal  so  endowed.  But  whilst  the  length  of  the  human  clavicle 
averages  but  6 inches,  that  of  the  Megatherium  is  15  inches.  In  its  general  shape  and 
sigmoid  curve,  this  bone  (Plate  XX.  flg.  1)  singularly  resembles  the  human  clavicle, 
but  is  thicker  in  proportion  to  its  length.  No  single  bone  would  have  better  excused 
the  common  conclusion  of  the  mediaeval  anatomists  as  to  the  nature  of  large  fossil  bones, 
■viz.  that  they  were  those  of  human  giants,  than  the  collar-bone  of  the  Megatherium. 
The  sternal  end  is  expanded,  obliquely  truncate,  with  a rough,  irregular,  undulating, 
but  mainly  convex  articular  surface,  much  fitted  for  strong  ligamentous  attachment  to 
the  manubrium  sterni,  and  with  a narrow,  rather  flattened  facet  above  this  surface, 
where  it  abuts  against  the  same  part  in  its  fellow,  as  shown  at  Plate  XVIII.  sa. 
The  shaft  of  the  bone  is  most  contracted  at  its  sternal  third  part ; thence  it  gradually 
expands  to  the  acromial  end ; the  anterior  surface  is  moderately  smooth  and  convex : 
the  posterior  surface  (Plate  XX.  flg.  I)  is  rough  and  more  flattened,  and  is  traversed 
obliquely  by  a broad  ridge,  which  terminates  outwardly,  or  developes  the  rugged  upper 
border  of  the  acromial  half  of  the  bone.  This  expanded  end  bends  downward,  as  the 
sternal  end  bends  upward.  There  is  a large  tuberosity  on  the  outer  or  fore-part  of  the 
acromial  expansion,  which  terminates  in  an  oblong  convexity,  adapted  to  the  concavity 
beneath  the  expanded  end  of  the  acromion.  The  strong  aponeurotic  character  of  the 
periosteum  of  the  cla\icle  is  well  shown  by  the  linear  decussating  ridges  on  most  parts 
of  the  surface  of  the  shaft.  The  bone  is  solid. 
The  closer  affinity  of  the  Bradypus  didactyliis,  as  compared  with  the  Brad,  tridactylus., 
to  the  Megathenum,  is  illustrated  by  the  complete  clavicles  which  attach  the  scapulae 
to  the  sternum ; but  they  are  straighter,  relatively  more  slender,  and  more  suddenly 
expanded  at  the  sternal  end  than  in  the  Megatherium.  One  species  or  variety  of 
Thi’ee-toed  Sloth  has  only  a small  styliform  clavicular  bone,  appended  to  the  coracoid 
process.  The  clarfcle  in  Orycteropus  and  Myrmecophaga  is  complete,  but  has  a single 
curvatm-e.  The  clarfcle  of  the  Mylodon  is  intermediate,  in  its  form  and  proportions, 
between  that  of  the  Megatherium  and  that  of  the  Two-toed  Sloth. 
The  supposed  peculiarity  in  the  articulation  of  the  clarfcle  of  the  Megatherium  with 
the  flrst  rib  instead  of  the  sternum,  which  Cuviee  inferred  from  the  flgures  and  descrip- 
tions of  the  fossil  animal  which  had  been  published  in  his  time  is  shown  by  the  more 
perfect  specimens  since  received,  not  to  exist  in  nature ; and  the  suspicion  expressed  by  the 
gteat  anatomist,  viz.  that  it  might  be  due  to  some  misarticulation  in  the  Madrid  skele- 
* “ D’apres  les  figures  et  les  descriptions,  il  paraitrait  que  cette  clavicule  s’articulerait,  non  pas  avec  le 
sternum  comme  a 1’ ordinaire,  mais  avec  le  bas  de  la  premirfe  cote  qui  est  recourb^e,  et  presente  nne  concavite 
pour  la  recevoir.  Ce  serait  une  singularite  dont  je  ne  connais  pas  d’exemple.” — Ossemens  Fossiles,  Ed.  1835, 
8vo,  tom.  viii.  p.  349. 
2 N 2 
