The Insectivora are by many of their features recognisable as low types of placental mammals. They have a primitive type of skull, and frequently show a rather marked similarity between the several classes of teeth. The brain is relatively ill-developed, reminding one rather strongly of the Marsupials, and the hemispheres show little if any tendency to develop those wrinkles and fissures which always accompany the higher type of intelligence. Probably the most primitive member of the whole group is the Centetes from Madagascar, shown in Fig. 119. The hedgehog and the shrews are its best-known examples. An interesting point regarding the hedgehog is that, like few other mammals, it has persisted almost unchanged from early tertiary times. It is thus to be regarded as a very antique type, not only in its main features, but in its details. [Illustration: FIG. 121.--Vampires.] [Illustration: FIG. 122.--Porpoises.] [Illustration: FIG. 123.--Sea Lions.] [Illustration: FIG. 124.--Indian Leopard. Photo: Underwood.] The Lemurs are an interesting group, standing, as they do, midway between the primitive placental forms and the monkeys. Their special home, as already mentioned, is Madagascar, to which some thirty-six of the fifty known species are confined, but they occur also in Africa and in South-Eastern Asia. They are arboreal and mostly nocturnal in habits, and their food consists partly of fruit, etc., partly of insects. They were formerly much more widely distributed, and many fossils have been unearthed, for example, in North America. They show certain characters of a distinctly primitive kind, such, for instance, as their habit of hibernation. Their typical number of teeth is thirty-six, the same as in the lower monkeys, but fossil forms are known which possessed the full number of forty-four. In their general build they show marked adaptation to their arboreal life, and approach, some more and some less, the appearance of the monkeys. The fore-limbs are considerably modified from the condition in which they occur in ordinary mammals, in which they are placed vertically under the body. They are placed in a more lateral position, so that they can be moved through greater angles, and extended over the head. In common language, they are ceasing to be legs, and are becoming arms. As in the monkeys, the thumb and the great toe are opposed to the other digits so as to render the hand and foot more efficient as grasping organs. Hence the Lemurs may be included with the apes as 'Quadrumana,' or four-handed animals. The fingers and toes either bear claws, as in the lower animals, or flattened nails like those of the higher apes and man, many species possessing the two types of structures on different digits. The face is fox-like, and lacks the human expression that is seen in the monkeys. The brain shows considerable variation, being in some species far more primitive, in others rather more highly developed, than in the lower monkeys.