The duck-billed platypus and the echidna or spiny anteater,
indigenous to Australia, Tasmania, and New Guinea, are the only two species of
mammals that lay eggs (a non-mammalian feature) but suckle their young (a
mammalian feature). These mammals resemble reptiles in that they lay rubbery
shell-covered eggs that are incubated and hatched outside the mother's body. In
addition they resemble reptiles in their digestive, reproductive, and excretory
systems, and in a number of anatomical details (eye structure, presence of
certain skull bones, pectoral [shoulder] girdle, and rib and vertebral
structures). However they are classed as mammals because they have fur and a
four-chambered heart, nurse their young from gland milk, are warm-blooded, and
have some mammalian skeletal features.
(Adapted from 'The Handy Science Answer Book', compiled
by the Science and Technology department of the Carnegie Library of Pittsburgh)
