228   MR.  R.  OWEN  ON  THE  YOUNG  OF  THE  ORNITHORHYNCHUS  PARADOXUS. 
Meckel'.  The  ductus  arteriosus  was  here  very  evident,  and  formed  a  filamentary  chord 
in  the  usual  situation  between  the  aorta  and  pulmonary  artery,  but  proportionately 
longer  than  in  the  true  viviparous  Mammalia.  Here  also  we  have  the  indication  of 
a  more  prolonged  foetal  existence  than  in  the  Marsupial  animals,  there  being  no  trace 
of  a  ductus  arteriosus  either  in  the  uterine  or  mammary  foetus  of  the  Kangaroo. 
The  Ornithorhynchus  also  deviates  from  the  ordinary  Marsupiata  in  having  the  thymus 
gland.  This  is  situated  in  front  of  the  great  vessels  of  the  heart,  and  consists  of  two 
lobes,  of  which  the  right  is  the  largest. 
PLATE  XXXII. 
Fig.  1 .  The  smaller  specimen  of  Ornithorhynchus  paradoxus. 
2.  The  larger  specimen. 
3.  The  same  in  another  view. 
4.  Front  view  of  the  mandibles  of  the  same,  a  little  open  to  show  the  tongue. 
In  each  of  these  figures,  a,  is  the  nostrils  ;  &,  the  prominence  on  the  upper 
beak  ;  c,  the  eyes  ;  d,  the  ears  ;  e,  the  vent ;  /,  the  orifice  and  rudimentary 
spur  of  the  hind  foot ;  g,  the  membrane  at  the  base  of  the  mandibles ; 
h,  the  tongue. 
5.  A  magnified  figure  of  the  hind  foot  of  the  female,  showing  the  rudimentary 
spur  projecting  from  the  socket. 
PLATE  XXXIII. 
Fig.  1 .  Abdominal  viscera  of  the  smaller  specimen,  principally  to  show  the  remains 
of  g,  the  umbilical  vein,  and  h,  the  allantois  ;  i,  is  the  liver  ;  k,  the  stomach. 
2.  The  stomach ;  and  I,  the  spleen  of  the  same  specimen. 
3.  The  stomach  and  spleen  of  the  larger  specimen. 
4.  The  heart,  showing  the  single  ductus  arteriosus. 
5.  m.  The  kidneys  ;  n,  the  supra-renal  glands  ;  o,  the  testes  ;  and  p,  the  urinary 
bladder  of  the  younger  specimen. 
6.  The  corresponding  parts  in  situoi  the  female  specimen  :  g,  the  ovaries;  r,  the 
uteri  and  oviducts. 
7.  The  urinary  bladder,  umbilical  arteries,  and  allantois  Qi)  of  the  smaller  spe- 
cimen, magnified. 
8.  The  upper  mandible  of  the  smaller  specimen,  magnified,  to  show  the  caruncle 
(&)  between  the  nostrils.    The  figure  marked  thus  *  is  of  the  natural  size. 
[All  the  figures,  except  where  otherwise  indicated,  are  of  the  natural  size.  Those  in  Plate  XXXH.  were 
drawn  by  ray  friend  T.  Rymer  Jones,  Esq.,  whose  valuable  assistance  I  am  happy  in  acknowledging.] 
'  Omithorhynchi- paradoxi  Ana,tomia,- tab.  vii.  fig.  1. 
