The common opossum is about as big as a cat. But it looks much more like a very big rat, for its tail is long and scaly. It is found in North America. In South America there is a different species, called the crab-eating opossum, because it is so fond of the crabs and crayfishes which abound in the salt creeks and the great swamps of Brazil. Then Merian's opossum, in which the marsupial pouch is not developed, has a most curious way of carrying its young about, for the little ones stand in a row on their mother's back, with their tiny tails coiled tightly round hers, to prevent them from falling off. And the yapock opossum spends most of its life in the water, and lives upon fish, being such an excellent swimmer that it is able easily to overtake them. Last of all, we come to two most extraordinary animals, which differ from all other mammals in the fact that they lay eggs, while in some parts of their skeletons they closely resemble the reptiles. THE ECHIDNA The first of these creatures is called the echidna, and is also known as the spiny ant-eater. It is from fifteen to nineteen inches in length, and has the whole upper surface of the head and body covered with a mixture of stiff hairs and short sharp spines, something like those of a hedgehog. The head is drawn out into a very long, slender, beak-like snout, at the tip of which the nostrils are placed, and the tongue is long and worm-like and very sticky, just as it is in the true ant-eaters. The feet are furnished with enormous claws, which are used in tearing open the nests of the insects upon which the animal feeds, and those of the hind feet, strange to say, are turned backward in walking, so that they point toward the tail instead of the head. These claws are also used in digging, and can be used with such effect that if the animal is surprised when on sandy soil it sinks into the ground as if by magic. But if the ground is so hard that it cannot use its claws, it rolls itself up like a hedgehog, and trusts to its spiny coat for protection. The common echidna is found in Australia, Tasmania, and New Guinea. Besides this there is another species, called the three-toed echidna, which is found in New Guinea only. THE DUCKBILL Even more curious still is the duckbill, or duck-billed platypus, which not only lays eggs like a bird, but resembles a bird in several other ways as well. It has a bill, for example, just like that of a duck--broad and flat, with a number of grooves round the edges. And it feeds by taking a beakful of mud from the bottom of a pond or ditch, squirting out the mud itself through the grooves, and then swallowing the grubs and other small creatures which are left behind. Then its feet are like those of a duck, the toes being joined together by webbing, so that they can be used as paddles. And even the tail is rather like that of a duck, for it is very broad and flat, so that it can serve as a rudder when the animal is swimming.