FIG. 350.—HORSE (front of jaw). ] [Illustration: FIG. 351.—GREENLAND WHALE. ] [Illustration: FIG. 352.—AMERICAN MONKEY. ] [Illustration: FIG. 353.—SLOTH (Fig. 363). ] [Illustration: FIG. 354.—ANT-EATER (Fig. 364). ] [Illustration: FIG. 355.—HORSE. ] [Illustration: FIG. 356.—DOG. Upper (_A_) and lower (_B_) jaw. ] [Illustration: FIG. 357.—HOG. ] [Illustration: FIG. 358.—SHEEP. ] [Illustration: FIG. 359.—DUCKBILL (_Ornithorhynchus paradoxus_). ] The =lowest order of mammals= contains only two species, the duckbill and the porcupine ant-eater, both living in the Australian region. Do you judge that the _duckbill_ of Tasmania (Fig. 359) lives chiefly in water or on land? Why? Is it probably active or slow in movement? It dabbles in mud and slime for worms and mussels, etc. How is it fitted for doing this? Which feet are markedly webbed? How far does the web extend? The web can be folded back when not in use. It lays two eggs in a nest of grass at the end of a burrow. Trace resemblances and differences between this animal and birds. [Illustration: FIG. 360.—SPINY ANT-EATER (_Echidna aculeata_). View of under surface to show pouch. (After Haacke.) ] The _porcupine ant-eater_ has numerous quill-like spines (Fig. 360) interspersed with its hairs. (Use?) Describe its claws. It has a long prehensile tongue. It rolls into a ball when attacked. Compare its jaws with a bird’s bill. It lays one egg, which is carried in a fold of the skin until hatched. Since it is pouched it could be classed with the pouched mammals (next order), but it is egg-laying. Suppose the two animals in this order did not nourish their young with milk after hatching, would they most resemble mammals, birds, or reptiles? Write the name of this _order_. —— (See Table, p. 193.) _Why_ do you place them in this order (——)? (See p. 193.) The name of the order comes from two Greek words meaning “one opening,” because the ducts from the bladder and egg glands unite with the large intestine and form a cloaca. What other classes of vertebrates are similar in this? [Illustration: FIG. 361.—OPOSSUM (_Didelphys Virginianus_). ] =Pouched Mammals.=—These animals, like the last, are numerous in the Australian region, but are also found in South America, thus indicating that a bridge of land once connected the two regions. The _opossum_ is the only species which has penetrated to North America (Fig. 361). Are its jaws slender or short? What kinship is thus suggested? As shown by its grinning, its lips are not well developed. Does this mean a low or a well-developed mammal? Where does it have a thumb? (Fig. 361.) Does the thumb have a nail? Is the tail hairy or bare? Why? Do you think it prefers the ground or the trees? State two reasons for your answer. It hides in a cave or bank or hollow tree all day, and seeks food at night. Can it run fast on the ground? It feigns death when captured, and watches for a chance for stealthy escape.