BIRDS. 3 
(4) Metatheria, Didelphia, or Marsupials—the prematurely bearing, 
usually pouch-possessing kangaroos, opossums, etc. 
{c) Prototheria, Ornithodelphia, or Monotremes—the egg-laying 
duckmole (Ornzthorhynchus), Echidna, and Proechidna. 
aS 
Fic. 2.—Phenacodus, a primitive extinct Mammal.—After Cope. 
Birds.— There can be 
no hesitation as to the 
class which ranks next to 
Mammals. For Birds are 
in most respects as highly 
developed as Mammals, 
though in a different direc- 
tion. They are character- 
ised by their feathers and 
wings, and many other 
adaptations for flight, by 
their high temperature, 
by the frequent spongi- 
ness and hollowness of 
their bones, by the tend- 
ency to fusion in many 
parts of the skeleton, 
by the absence of teeth 
in modern forms, by the 
fixedness of the lungs 
and their association with 
‘numerous air sacs, and so on. 
Fic. 3.—Extinct moa and modern 
er eed 
kiwi.—After Carus Sterne. 
But here again different grades must be distinguished—(1) There is 
the vast majority—the flying birds, with a breast-bone keel or carina, to 
