116 
LOWER MAMMAL GALLERY. 
depressed, with a large rounded brain-case, the walls of which 
are thin, as in Birds. There are no true teeth in adult life, but 
in the Platypus the young are provided with three pairs of 
peculiar flattened saucer-like teeth in each jaw, which are 
afterwards shed and replaced by horny plates. There are 19 
vertebrae in the trunk, well-marked sternal ribs, and a pair of 
large marsupial bones placed on the pelvis. 
The two families of the order differ in many important 
respects, especially in the shape of the skull. The Duck-billed 
Platypus, or Platypus, O^mitliorliyiivlius, has a broad, flat ex- 
pansion, forked in front, which supports the beak, and in which 
first the teeth and then the horny plates are implanted ; while 
in the Spiny Anteaters, Eclridna (fig. 62), the snout is long, 
narrow, and toothless, and forms merely a long tube for the 
'Die of a Jspiny Anteater, oi- l^'chidna, Nat. .size. 
lodgment of the tongue, as in the true Anteaters. In 
Proealddna hruijmi from New Guinea, of which a skeleton is 
mounted, the snout is nearly twice as long as the brain-case, 
and very much curved downwards, but in the Common Echidna 
it is shorter and curved upwards. 
In both families the fore-limbs are more powerfully developed 
than the hind pair, the humerus especially being exceedingly 
thick, and provided with large ridges for the attachment of 
muscles. 
Monotremes lay eggs and the Echidnas have a breeding- 
pouch, but the mode of incubation is not yet satisfactorily 
known. They are without true nipples, the milk exuding from 
groups of pores in the skin. The males are provided with 
horny spurs on the heels, connected with a small gland on the 
back of the thigh. The temperature of the blood is lower than 
